The sheep which we have reared is induced to follow us, as it would be

led to follow the flock among which it was brought up; and, when

individuals of gregarious species have been accustomed to one master, it

is he alone whom they acknowledge as their chief--he only whom they

obey. "The elephant allows himself to be directed only by the carnac

whom he has adopted; the dog itself, reared in solitude with its master,

manifests a hostile disposition towards all others; and every body knows

how dangerous it is to be in the midst of a herd of cows, in pasturages

that are little frequented, when they have not at their head the keeper

who takes care of them.

"Every thing, therefore, tends to convince us, that formerly men were

only with regard to the domestic animals, what those who are

particularly charged with the care of them still are--namely, members of

the society which these animals form among themselves; and, that they

are only distinguished, in the general mass, by the authority which they

have been enabled to assume from their superiority of intellect. Thus,

every social animal which recognizes man as a member, and as the chief

of its herd, is a domestic animal. It might even be said, that, from the

moment when such an animal admits man as a member of its society, it is

domesticated, as man could not enter into such society without becoming

the chief of it."[820]

But the ingenious author whose observations I have here cited, admits

that the obedience which the individuals of many domestic species yield

indifferently to every person, is without analogy in any state of things

which could exist previously to their subjugation by man. Each troop of

wild horses, it is true, has some stallion for its chief, who draws

after him all the individuals of which the herd is composed; but when a

domesticated horse has passed from hand to hand, and has served several

masters, he becomes equally docile towards _any person_, and is

subjected to the whole human race. It seems fair to presume that the

capability in the instinct of the horse to be thus modified, was given

to enable the species to render greater services to man; and, perhaps,

the facility with which many other acquired characters become hereditary

in various races of the horse, may be explicable only on a like

supposition. The amble, for example, a pace to which the domestic races

in some parts of Spanish America are exclusively trained, has, in the

course of several generations, become hereditary, and is assumed by all

the young colts before they are broken in.[821]

It seems, also, reasonable to conclude, that the power bestowed on the

horse, the dog, the ox, the sheep, the cat, and many species of domestic

fowls, of supporting almost every climate, was given expressly to enable

them to follow man throughout all parts of the globe, in order that we

might obtain their services, and they our protection. If it be objected

that the elephant which, by the union of strength, intelligence, and

docility, can render the greatest services to mankind, is incapable of

living in any but the warmest latitudes, we may observe that the

quantity of vegetable food required by this quadruped would render its

maintenance in the temperate zones too costly, and in the arctic

impossible.

Among the changes superinduced by man, none appear, at first sight, more

remarkable than the perfect tameness of certain domestic races. It is

well known that, at however early an age we obtain possession of the

young of many unreclaimed races, they will retain, throughout life, a

considerable timidity and apprehensiveness of danger; whereas, after one

or two generations, the descendants of the same stock will habitually

place the most implicit confidence in man. There is good reason,

however, to suspect that such changes are not without analogy in a state

of nature; or, to speak more correctly, in situations where man has not

interfered.

We learn from Mr. Darwin, that in the Galapagos archipelago, placed

directly under the equator, and nearly 600 miles west of the American

continent, all the terrestrial birds, as the finches, doves, hawks, and

others, are so tame, that they may be killed with a switch. One day,

says this author, "a mocking bird alighted on the edge of a pitcher

which I held in my hand, and began quietly to sip the water, and allowed

me to lift it with the vessel from the ground." Yet formerly, when the

first Europeans landed, and found no inhabitants in these islands, the

birds were even tamer than now: already they are beginning to acquire

that salutary dread of man which in countries long settled is natural

even to young birds which have never received any injury. So in the

Falkland Islands, both the birds and foxes are entirely without fear of

man; whereas, in the adjoining mainland of South America, many of the

same species of birds are extremely wild; for there they have for ages

been persecuted by the natives.[822]

Dr. Richardson informs us, in his able history of the habits of the

North American animals, that, "in the retired parts of the mountains

where the hunters had seldom penetrated, there is no difficulty in

approaching the Rocky Mountain sheep, which there exhibit _the

simplicity of character so remarkable in the domestic species_; but

where they have been often fired at, they are exceedingly wild, alarm

their companions, on the approach of danger, by a hissing noise, and

scale the rocks with a speed and agility that baffle pursuit."[823]

It is probable, therefore, that as man, in diffusing himself over the

globe, has tamed many wild races, so, also, he has made many tame races

wild. Had some of the larger carnivorous beasts, capable of scaling the

rocks, made their way into the North American mountains before our

hunters, a similar alteration in the instincts of the sheep would

doubtless have been brought about.

_Wild elephants domesticated in a few years._--No animal affords a more

striking illustration of the principal points which I have been

endeavouring to establish than the elephant; for, in the first place,

the wonderful sagacity with which he accommodates himself to the society

of man, and the new habits which he contracts, are not the result of

time, nor of modifications produced in the course of many generations.

These animals will breed in captivity, as is now ascertained, in

opposition to the vulgar opinion of many modern naturalists, and in

conformity to that of the ancients AElian and Columella[824]: yet it has

always been the custom, as the least expensive mode of obtaining them,

to capture wild individuals in the forests, usually when full grown;

and, in a few years after they are taken--sometimes, it is said, in the

space of a few months--their education is completed.

Had the whole species been domesticated from an early period in the

history of man, like the camel, their superior intelligence would,

doubtless, have been attributed to their long and familiar intercourse

with the lord of the creation; but we know that a few years is

sufficient to bring about this wonderful change of habits; and although

the same individual may continue to receive tuition for a century

afterwards, yet it makes no farther progress in the general development

of its faculties. Were it otherwise, indeed, the animal would soon

deserve more than the poet's epithet of "half-reasoning."

From the authority of our countrymen employed in the late Burmese war,

it appears, in corroboration of older accounts, that when elephants are

required to execute extraordinary tasks, they may be made to understand

that they will receive unusual rewards. Some favourite dainty is shown

to them, in the hope of acquiring which the work is done; and so

perfectly does the nature of the contract appear to be understood, that

the breach of it, on the part of the master, is often attended with

danger. In this case, a power has been given to the species to adapt

their social instincts to new circumstances with surprising rapidity;

but the extent of this change is defined by strict and arbitrary limits.

There is no indication of a tendency to continued divergence from

certain attributes with which the elephant was originally endued--no

ground whatever for anticipating that, in thousands of centuries, any

material alteration could ever be effected. All that we can infer from

analogy is, that some more useful and peculiar races might probably be

formed, if the experiment were fairly tried; and that some individual

characteristic, now only casual and temporary, might be perpetuated by

generation.

In all cases, therefore, where the domestic qualities exist in animals,

they seem to require no lengthened process for their developement; and

they appear to have been wholly denied to some classes, which, from

their strength and social disposition, might have rendered great

services to man; as, for example, the greater part of the quadrumana.

The orang-outang, indeed, which, for its resemblance in form to man, and

apparently for no other good reason, has been assumed by Lamarck to be

the most perfect of the inferior animals, has been tamed by the savages

of Borneo, and made to climb lofty trees, and to bring down the fruit.

But he is said to yield to his masters an unwilling obedience, and to be

held in subjection only by severe discipline. We know nothing of the

faculties of this animal which can suggest the idea that it rivals the

elephant in intelligence; much less anything which can countenance the

dreams of those who have fancied that it might have been transmuted into

the "dominant race." One of the baboons of Sumatra (_Simia carpolegus_)

appears to be more docile, and is frequently trained by the inhabitants

to ascend trees, for the purpose of gathering cocoa-nuts; a service in

which the animal is very expert. He selects, says Sir Stamford Raffles,

the ripe nuts, with great judgment, and pulls no more than he is

ordered.[825] The capuchin and cacajao monkeys are, according to

Humboldt, taught to ascend trees in the same manner, and to throw down

fruit on the banks of the lower Orinoco.[826]

It is for the Lamarckians to explain how it happens that those same

savages of Borneo have not themselves acquired, by dint of longing, for

many generations, for the power of climbing trees, the elongated arms of

the ourang, or even the prehensile tails of some American monkeys:

Instead of being reduced to the necessity of subjugating stubborn and

untractable brutes, we should naturally have anticipated "that their

wants would have excited them to efforts, and that continued efforts

would have given rise to new organs;" or rather to the re-acquisition of

organs which, in a manner irreconcileable with the principle of the

_progressive_ system, have grown obsolete in tribes of men which have

such constant need of them.

_Recapitulation._--It follows, then, from the different facts which have

been considered in this chapter, that a short period of time is

generally sufficient to effect nearly the whole change which an

alteration of external circumstances can bring about in the habits of a

species, and that such capacity of accommodation to new circumstances is

enjoyed in very different degrees, by different species.

Certain qualities appear to be bestowed exclusively with a view to the

relations which are destined to exist between different species, and,

among others, between certain species and man; but these latter are

always so nearly connected with the original habits and propensities of

each species in a wild state, that they imply no indefinite capacity of

varying from the original type. The acquired habits derived from human

tuition are rarely transmitted to the offspring; and when this happens,

it is almost universally the case with those merely which have some

obvious connexion with the attributes of the species when in a state of

independence.

CHAPTER XXXVI.

WHETHER SPECIES HAVE A REAL EXISTENCE IN NATURE--_continued_.

Phenomena of hybrids--Hunter's opinions--Mules not strictly

intermediate between parent species--Hybrid plants--Experiments of

Kolreuter and Wiegmann--Vegetable hybrids prolific throughout

several generations--Why rare in a wild state--Decundolle on hybrid

plants--The phenomena of hybrids confirm the distinctness of

species--Theory of the gradation in the intelligence of animals as

indicated by the facial angle--Doctrine that certain organs of the

foetus in mammalia assume successively the forms of fish, reptile,

and bird--Recapitulation.

_Phenomena of hybrids._--We have yet to consider another class of

phenomena, those relating to the production of hybrids, which have been

regarded in a very different light with reference to their bearing on

the question of the permanent distinctness of species; some naturalists

considering them as affording the strongest of all proofs in favor of

the reality of species; others, on the contrary, appealing to them as

countenancing the opposite doctrine, that all the varieties of

organization and instinct now exhibited in the animal and vegetable

kingdoms may have been propagated from a small number of original types.

In regard to the mammifers and birds it is found that no sexual union

will take place between races which are remote from each other in their

habits and organization; and it is only in species that are very nearly

allied that such unions produce offspring. It may be laid down as a

general rule, admitting of very few exceptions among quadrupeds, that

the hybrid progeny is sterile; and there seem to be no well

authenticated examples of the continuance of the mule race beyond one

generation. The principal number of observations and experiments relate

to the mixed offspring of the horse and the ass; and in this case it is

well established that the he-mule can generate, and the she-mule

produce. Such cases occur in Spain and Italy, and much more frequently

in the West Indies and New Holland; but these mules have never bred in

cold climates, seldom in warm regions, and still more rarely in

temperate countries.

The hybrid offspring of the she-ass and the stallion, the [Greek:

ginnos] of Aristotle, and the hinnus of Pliny, differs from the mule, or

the offspring of the ass and mare. In both cases, says Buffon, these

animals retain more of the dam than of the sire, not only in the

magnitude, but in the figure of the body: whereas, in the form of the

head, limbs, and tail, they bear a greater resemblance to the sire. The

same naturalist infers, from various experiments respecting cross-breeds

between the he-goat and ewe, the dog and she-wolf, the goldfinch and

canary-bird, that the male transmits his sex to the greatest number, and

that the preponderance of males over females exceeds that which prevails

where the parents are of the same species.

_Hunter's opinion._--The celebrated John Hunter has observed, that the

true distinction of species must ultimately be gathered from their

incapacity of propagating with each other, and producing offspring

capable of again continuing itself. He was unwilling, however, to admit

that the horse and the ass were of the same species, because some rare

instances had been adduced of the breeding of mules, although he

maintained that the wolf, the dog, and the jackal were all of one

species; because he had found, by two experiments, that the dog would

breed both with the wolf and the jackal; and that the mule, in each

case, would breed again with the dog. In these cases, however, it may be

observed, that there was always one parent at least of pure breed, and

no proof was obtained that a true hybrid race could be perpetuated; a

fact of which I believe no examples are yet recorded, either in regard

to mixtures of the horse and ass, or any other of the mammalia.

Should the fact be hereafter ascertained, that two mules can propagate

their kind, we must still inquire whether the offspring may not be

regarded in the light of a monstrous birth, proceeding from some

accidental cause, or, rather, to speak more philosophically, from some

general law not yet understood, but which may not be permitted

permanently to interfere with those laws of generation by which species

may, in general, be prevented from becoming blended. If, for example, we

discovered that the progeny of a mule race degenerated greatly, in the

first generation, in force, sagacity, or any attribute necessary for its

preservation in a state of nature, we might infer that, like a monster,

it is a mere temporary and fortuitous variety. Nor does it seem probable

that the greater number of such monsters could ever occur unless

obtained by art; for, in Hunter's experiments, stratagem or force was,

in most instances, employed to bring about the irregular connexion.[827]

_Mules not strictly intermediate between the parent species._--It seems

rarely to happen that the mule offspring is truly intermediate in

character between the two parents. Thus Hunter mentions that, in his

experiments, one of the hybrid pups resembled the wolf much more than

the rest of the litter; and we are informed by Wiegmann, that, in a

litter lately obtained in the Royal Menagerie at Berlin, from a white

pointer and a she-wolf, two of the cubs resembled the common wolf-dog,

but the third was like a pointer with hanging ears.

There is undoubtedly a very close analogy between these phenomena and

those presented by the intermixture of distinct races of the same

species, both in the inferior animals and in man. Dr. Prichard, in his

"Physical History of Mankind," cites examples where the peculiarities of

the parents have been transmitted very unequally to the offspring; as

where children, entirely white, or perfectly black, have sprung from the

union of the European and the negro. Sometimes the colour or other

peculiarities of one parent, after having failed to show themselves in

the immediate progeny, reappear in a subsequent generation; as where a

white child is born of two black parents, the grandfather having been a

white.[828]

The same author judiciously observes that, if different species mixed

their breed, and hybrid races were often propagated, the animal world

would soon present a scene of confusion; its tribes would be every where

blended together, and we should perhaps find more hybrid creatures than

genuine and uncorrupted races.[829]

_Hybrid plants._--_Kolreuter's experiments_.--The history of the

vegetable kingdom has been thought to afford more decisive evidence in

favour of the theory of the formation of new and permanent species from

hybrid stocks. The first accurate experiments in illustration of this

curious subject appear to have been made by Kolreuter, who obtained a

hybrid from two species of tobacco, _Nicotiana rustica_ and _N.

paniculata_, which differ greatly in the shape of their leaves, the

colour of the corolla, and the height of the stem. The stigma of a plant

of _N. rustica_ was impregnated with the pollen of a plant of _N.

paniculata_. The seed ripened, and produced a hybrid which was

intermediate between the two parents, and which, like all the hybrids

which this botanist brought up, had imperfect stamens. He afterwards

impregnated this hybrid with the pollen of _N. paniculata_, and obtained

plants which much more resembled the last. This he continued through

several generations, until, by due perseverance, he actually changed the

_Nicotiana rustica_ into the _Nicotiana paniculata_.

The plan of impregnation adopted, was the cutting off of the anthers of

the plant intended for fructification before they had shed pollen, and

then laying on foreign pollen upon the stigma.

_Wiegmann's experiments._--The same experiment has since been repeated

with success by Wiegmann, who found that he could bring back the hybrids

to the exact likeness of either parent, by crossing them a sufficient

number of times.

The blending of the characters of the parent stocks, in many other of

Wiegmann's experiments, was complete; the colour and shape of the leaves

and flowers, and even the scent, being intermediate, as in the offspring

of the two species of verbascum. An intermarriage, also, between the

common onion and the leek (_Allium cepa_ and _A. porrum_) gave a mule

plant, which, in the character of its leaves and flowers, approached

most nearly to the garden onion, but had the elongated bulbous root and

smell of the leek.

The same botanist remarks, that vegetable hybrids, when not strictly

intermediate, more frequently approach the female than the male parent

species; _but they never exhibit characters foreign to both_. A re-cross

with one of the original stocks generally causes the mule plant to

revert towards that stock; but this is not always the case, the

offspring sometimes continuing to exhibit the character of a full

hybrid.

In general, the success attending the production and perpetuity of

hybrids among plants depends, as in the animal kingdom, on the degree of

proximity between the species intermarried. If their organization be

very remote, impregnation never takes place; if somewhat less distant,

seeds are formed, but always imperfect and sterile. The next degree of

relationship yields hybrid seedlings, but these are barren; and it is

only when the parent species are very nearly allied that the hybrid race

may be perpetuated for several generations. Even in this case the best

authenticated examples seem confined to the crossing of hybrids with

individuals of pure breed. In none of the experiments most accurately

detailed does it appear that both the parents were mules.

Wiegmann diversified as much as possible his mode of bringing about

these irregular unions among plants. He often sowed parallel rows, near

to each other, of the species from which he desired to breed; and,

instead of mutilating, after Kolreuter's fashion, the plants of one of

the parent stocks, he merely washed the pollen off their anthers. The

branches of the plants in each row were then gently bent towards each

other and intertwined; so that the wind, and numerous insects, as they

passed from the flowers of one to those of the other species, carried

the pollen and produced fecundation.

_Vegetable hybrids why rare in a wild slate._--The same observer saw a

good exemplification of the manner in which hybrids may be formed in a

state of nature. Some wallflowers and pinks had been growing in a

garden, in a dry sunny situation, and their stigmas had been ripened so

as to be moist, and to absorb pollen with avidity, although their

anthers were not yet developed. These stigmas became impregnated by

pollen blown from some other adjacent plants of the same species; but

had they been of different species, and not too remote in their

organization, mule races must have resulted.

When, indeed, we consider how busily some insects have been shown to be

engaged in conveying anther-dust from flower to flower, especially bees,

flower-eating beetles, and the like, it seems a most enigmatical problem

how it can happen that promiscuous alliances between distinct species

are not perpetually occurring.

How continually do we observe the bees diligently employed in collecting

the red and yellow powder by which the stamens of flowers are covered,

loading it on their hind legs, and carrying it to their hive for the

purpose of feeding their young! In thus providing for their own progeny,

these insects assist materially the process of fructification.[830] Few

persons need be reminded that the stamens in certain plants grow on

different blossoms from the pistils; and unless the summit of the pistil

be touched with the fertilizing dust, the fruit does not swell, nor the

seed arrive at maturity. It is by the help of bees chiefly, that the

development of the fruit of many such species is secured, the powder

which they have collected from the stamens being unconsciously left by

them in visiting the pistils.

How often, during the heat of a summer's day, do we see the males of

dioecious plants, such as the yew-tree, standing separate from the

females, and sending off into the air, upon the slightest breath of

wind, clouds of buoyant pollen! That the zephyr should so rarely

intervene to fecundate the plants of one species with the anther-dust of

others, seems almost to realize the converse of the miracle believed by

the credulous herdsmen of the Lusitanian mares--

Ore omnes versae in Zephyrum, stant rupibus altis

Exceptantque leves auras: et saepe sine ullis

Conjugiis, vento gravidae, mirabile dictu.[831]

But, in the first place, it appears that there is a natural aversion in

plants, as well as in animals, to irregular sexual unions; and in most

of the successful experiments in the animal and vegetable world, some

violence has been used in order to procure impregnation. The stigma

imbibes, slowly and reluctantly, the granules of the pollen of another

species, even when it is abundantly covered with it; and if it happen

that, during this period, ever so slight a quantity of the anther-dust

of its own species alight upon it, this is instantly absorbed, and the

effect of the foreign pollen destroyed. Besides, it does not often

happen that the male and female organs of fructification, in different

species, arrive at a state of maturity at precisely the same time. Even

where such synchronism does prevail, so that a cross impregnation is

effected, the chances are very numerous against the establishment of a

hybrid race.

If we consider the vegetable kingdom generally, it must be recollected

that even of the seeds which are well ripened, a great part are either

eaten by insects, birds, and other animals, or decay for want of room

and opportunity to germinate. Unhealthy plants are the first which are

cut off by causes prejudicial to the species, being usually stifled by

more vigorous individuals of their own kind. If, therefore, the relative

fecundity or hardiness of hybrids be in the least degree inferior, they

cannot maintain their footing for many generations, even if they were

ever produced beyond one generation in a wild state. In the universal

struggle for existence, the right of the strongest eventually prevails;

and the strength and durability of a race depend mainly on its

prolificness, in which hybrids are acknowledged to be deficient.

_Centaurea hybrida_, a plant which never bears seed, and is supposed to

be produced by the frequent intermixture of two well-known species of

Centaurea, grows wild upon a hill near Turin. _Ranunculus lacerus_, also

sterile, has been produced accidentally at Grenoble, and near Paris, by

the union of two Ranunculi; but this occurred in gardens.[832]

_Mr. Herbert's experiments._--Mr. Herbert, in one of his ingenious

papers on mule plants, endeavors to account for their non-occurrence in

a state of nature, from the circumstance that all the combinations that

were likely to occur have already been made many centuries ago, and have

formed the various species of botanists; but in our gardens, he says,

whenever species, having a certain degree of affinity to each other, are

transported from different countries, and brought for the first time

into contact, they give rise to hybrid species.[833] But we have no

data, as yet, to warrant the conclusion, that a single permanent hybrid

race has ever been formed, even in gardens, by the intermarriage of two

allied species brought from distant habitations. Until some fact of this

kind is fairly established, and a new species, capable of perpetuating

itself in a state of perfect independence of man, can be pointed out, it

seems reasonable to call in question entirely this hypothetical source

of new species. That varieties do sometimes spring up from

cross-breeds, in a natural way, can hardly be doubted; but they

probably die out even more rapidly than races propagated by grafts or

layers.

_Opinion of De Candolle._--De Candolle, whose opinion on a philosophical

question of this kind deserves the greatest attention, has observed, in

his Essay on Botanical Geography, that the _varieties_ of plants range

themselves under two general heads: those produced by external

circumstances, and those formed by hybridity. After adducing various

arguments to show that neither of these causes can explain the permanent

diversity of plants indigenous in different regions, he says, in regard

to the crossing of races, "I can perfectly comprehend without altogether

sharing the opinion, that, where many species of the same genera occur

near together, hybrid species may be formed, and I am aware that the

great number of species of certain genera which are found in particular

regions may be explained in this manner; but I am unable to conceive how

any one can regard the same explanation as applicable to species

which live naturally at great distances. If the three larches, for

example, now known in the world, lived in the same localities, I might

then believe that one of them was the produce of the crossing of the two

others; but I never could admit that the Siberian species has been

produced by the crossing of those of Europe and America. I see, then,

that there exist in organized beings, permanent differences which cannot

be referred to any one of the actual causes of variation, and these

differences are what constitute species."[834]

_Reality of species confirmed by the phenomena of hybrids._--The most

decisive arguments perhaps, amongst many others, against the probability

of the derivation of permanent species from cross-breeds, are to be

drawn from the fact alluded to by De Candolle, of species having a close

affinity to each other occurring in distinct botanical provinces, or

countries inhabited by groups of distinct species of indigenous plants;

for in this case naturalists, who are not prepared to go the whole

length of the transmutationists, are under the necessity of admitting

that, in some cases, species which approach very near to each other in

their characters, were so created from their origin; an admission fatal

to the idea of its being a general law of nature that a few original

types only should be formed, and that all intermediate races should

spring from the intermixture of those stocks.

This notion, indeed, is wholly at variance with all that we know of

hybrid generation; for the phenomena entitle us to affirm, that had the

types been at first somewhat distinct, _no cross-breeds would ever have

been produced_, much less those prolific races which we now recognize as

distinct species.

In regard, moreover, to the permanent propagation of hybrid races among

animals, insuperable difficulties present themselves, when we endeavor

to conceive the blending together of the different instincts and

propensities of two species, so as to insure the preservation of the

intermediate race. The common mule, when obtained by human art, may be

protected by the power of man; but, in a wild state, it would not have

precisely the same wants either as the horse or the ass; and if in

consequence of some difference of this kind, it strayed from the herd,

it would soon be hunted down by beasts of prey, and destroyed.

If we take some genus of insects, such as the bee, we find that each of

the numerous species has some difference in its habits, its mode of

collecting honey, or constructing its dwelling, or providing for its

young, and other particulars. In the case of the common hive bee, the

workers are described, by Kirby and Spence, as being endowed with no

less than thirty distinct instincts.[835] So also we find that, amongst

a most numerous class of spiders, there are nearly as many different

modes of spinning their webs as there are species. When we recollect how

complicated are the relations of these instincts with co-existing

species, both of the animal and vegetable kingdoms, it is scarcely

possible to imagine that a bastard race could spring from the union of

two of these species, and retain just so much of the qualities of each

parent stock as to preserve its ground in spite of the dangers which

surround it.

We might also ask, if a few generic types alone have been _created_

among insects, and the intermediate species have proceeded from

hybridity, where are those original types, combining, as they ought to

do, the elements of all the instincts which have made their appearance

in the numerous derivative races? So also in regard to animals of all

classes, and of plants; if species are in general of hybrid origin,

where are the stocks which combine in themselves the habits, properties,

and organs, of which all the intervening species ought to afford us mere

modifications?

_Recapitulation of the arguments from hybrids._--I shall now conclude

this subject by summing up, in a few words, the results to which I have

been led by the consideration of the phenomena of hybrids. It appears

that the aversion of individuals of distinct species to the sexual union

is common to animals and plants; and that it is only when the species

approach near to each other in their organization and habits, that any

offspring are produced from their connexion. Mules are of extremely rare

occurrence in a state of nature, and no examples are yet known of their

having procreated in a wild state. But it has been proved, that hybrids

are not universally sterile, provided the parent stocks have a near

affinity to each other, although the continuation of the mixed race, for

several generations, appears hitherto to have been obtained only by

crossing the hybrids with individuals of pure species; an experiment

which by no means bears out the hypothesis that a true hybrid race could

ever be permanently established.

Hence we may infer, that aversion to sexual intercourse is, in general,

a good test of the distinctness of original stocks, or of _species_; and

the procreation of hybrids is a proof of the near affinity of species.

Perhaps, hereafter, the number of generations for which hybrids may be

continued, before the race dies out (for it seems usually to degenerate

rapidly), may afford the zoologist and botanist an experimental test of

the difference in the degree of affinity of allied species.

I may also remark, that if it could have been shown that a single

permanent species had ever been produced by hybridity (of which there is

no satisfactory proof), it might certainly have lent some countenance to

the notions of the ancients respecting the gradual deterioration of

created things, but none whatever to Lamarck's theory of their

progressive perfectibility, for observations have hitherto shown that

there is a tendency in mule animals and plants to degenerate in

organization.

It was before remarked, that the theory of progressive development arose

partly from an attempt to ingraft the doctrines of the transmutationists

upon one of the most popular generalizations in geology. But we have

seen in the ninth chapter, that the modern researches of geologists have

broken at many points the chain of evidence once supposed to exist in

favor of the doctrine, that, at each successive period in the earth's

history, animals and plants of a higher grade, or more complex

organization, have been created. The recent origin of man, and the

absence of all signs of any rational being holding an analogous relation

to former states of the animate world, affords one, and perhaps in the

present state of science the only argument of much weight in support of

the hypothesis of a progressive scheme; but none whatever in favor of

the fancied evolution of one species out of another.

_Theory of the gradation of intellect as shown by the facial

angle._--When the celebrated anatomist, Camper, first attempted to

estimate the degrees of sagacity of different animals, and of the races

of man, by the measurement of the facial angle, some speculators were

bold enough to affirm that certain Simiae, or apes, differed as little

from the more savage races of men, as those do from the human race in

general; and that a scale might be traced from "apes with foreheads

villanous low" to the African variety of the human species, and from

that to the European. The facial angle was measured by drawing a line

from the prominent centre of the forehead to the most advanced part of

the lower jaw-bone, and observing the angle which it made with the

horizontal line; and it was affirmed, that there was a regular series of

such angles from birds to the mammalia.

The gradation from the dog to the monkey was said to be perfect, and

from that again to man. One of the ape tribe has a facial angle of 42

degrees; and another, which approximated nearest to man in figure, an

angle of 50 degrees. To this succeeds (longo sed proximus intervallo)

the head of the African negro, which, as well as that of the Calmuck,

forms an angle of 70 degrees; while that of the European contains 80

degrees. The Roman painters preferred the angle of 95 degrees; and the

character of beauty and sublimity so striking in some works of Grecian

sculpture, as in the head of the Apollo, and in the Medusa of Sisocles,

is given by an angle which amounts to 100 degrees.[836]

A great number of valuable facts and curious analogies in comparative

anatomy were brought to light during the investigations which were made

by Camper, John Hunter, and others, to illustrate this scale of

organization; and their facts and generalizations must not be confounded

with the fanciful systems which White and others deduced from them.[837]

That there is some connexion between an elevated and capacious forehead,

in certain races of men, and a large developement of the intellectual

faculties, seems highly probable; and that a low facial angle is

frequently accompanied with inferiority of mental powers, is certain;

but the attempt to trace a gradual scale of intelligence through the

different species of animals accompanying the modifications of the form

of the scull, is a mere visionary speculation. It has been found

necessary to exaggerate the sagacity of the ape tribe at the expense of

the dog; and strange contradictions have arisen in the conclusions

deduced from the structure of the elephant; some anatomists being

disposed to deny the quadruped the intelligence which he really

possesses, because they found that the volume of his brain was small in

comparison to that of the other mammalia; while others were inclined to

magnify extravagantly the superiority of his intellect, because the

vertical height of his skull is so great when compared to its horizontal

length.

_Different races of men are all of one species._--It would be irrelevant

to our subject if we were to enter into a farther discussion on these

topics; because, even if a graduated scale of organization and

intelligence could have been established, it would prove nothing in

favor of a tendency, in each species, to attain a higher state of

perfection. I may refer the reader to the writings of Blumenbach,

Prichard, Lawrence, and more recently Latham[838], for convincing proofs

that the varieties of form, color, and organization of different races

of men, are perfectly consistent with the generally received opinion,

that all the individuals of the species have originated from a single

pair; and, while they exhibit in man as many diversities of a

physiological nature as appear in any other species, they confirm also

the opinion of the slight deviation from a common standard of which

species are capable.

The power of existing and multiplying in every latitude, and in every

variety of situation and climate, which has enabled the great human

family to extend itself over the habitable globe, is partly, says

Lawrence, the result of physical constitution, and partly of the mental

prerogative of man. If he did not possess the most enduring and flexible

corporeal frame, his arts would not enable him to be the inhabitant of

all climates, and to brave the extremes of heat and cold, and the other

destructive influences of local situation.[839] Yet, notwithstanding

this flexibility of bodily frame, we find no signs of indefinite

departure from a common standard, and the intermarriages of individuals

of the most remote varieties are not less fruitful than between those of

the same tribe.

_Tiedemann on the brain of the foetus in vertebrated animals._--There

is yet another department of anatomical discovery to which I must

allude, because it has appeared to some persons to afford a distant

analogy, at least, to that progressive development by which some of the

inferior species may have been gradually perfected into those of more

complex organization. Tiedemann found, and his discoveries have been

most fully confirmed and elucidated by M. Serres, that the brain of the

foetus, in the highest class of vertebrated animals, assumes, in

succession, forms, bearing a certain degree of resemblance to those

which belong to fishes, reptiles, and birds, before it acquires the

additions and modifications which are peculiar to the mammiferous tribe;

so that, in the passage from the embryo to the perfect mammifer, there

is a typical representation, it is said, of all those transformations

which the primitive species are supposed to have undergone, during a

long series of generations, between the present period and the remotest

geological era.

"If you examine the brain of the mammalia," says M. Serres, "at an early

stage of uterine life, you perceive the cerebral hemispheres

consolidated, as in fish, in two vesicles, isolated one from the other;

at a later period, you see them affect the configuration of the cerebral

hemispheres of reptiles; still later again, they present you with the

forms of those of birds; finally they acquire, at the era of birth, and

sometimes later, the permanent forms which the adult mammalia present.

"The cerebral hemispheres, then, arrive at the state which we observe in

the higher animals only by a series of successive metamorphoses. If we

reduce the whole of these evolutions to four periods, we shall see, that

in the first are born the cerebral lobes of fishes; and this takes place

homogeneously in all classes. The second period will give us the

organization of reptiles; the third, the brain of birds; and the fourth,

the complex hemispheres of mammalia.

"If we could develop the different parts of the brain of the inferior

classes, we should make, in succession, a reptile out of a fish, a bird

out of a reptile, and a mammiferous quadruped out of a bird. If, on the

contrary, we could starve this organ in the mammalia, we might reduce it

successively to the condition of the brain of the three inferior

classes.

"Nature often presents us with this last phenomenon in monsters, but

never exhibits the first. Among the various deformities which organized

beings may experience, they never pass the limits of their own classes

to put on the forms of the class above them. Never does a fish elevate

itself so as to assume the form of the brain of a reptile; nor does the

latter ever attain that of birds; nor the bird that of the mammifer. It

may happen that a monster may have two heads; but the conformation of

the brain always remains circumscribed narrowly within the limits of its

class."[840]

Dr. Clark of Cambridge, in a memoir on "Foetal Development" (1845),

has shown that the concurrent labours of Valentin, Ratka, and Bischoff

disprove the reality of the supposed anatomical analogy between the

embryo condition of certain organs in the higher orders, and the perfect

structure of the same organs in animals of an inferior class. The hearts

and brains, for example, of birds and mammals do not pass through forms

which are permanent in fishes and reptiles; there is only just so much

resemblance as may point to a unity of plan running through the

organization of the whole series of vertebrated animals; but which lends

no support whatever to the notion of a gradual transmutation of one

species into another; least of all of the passage, in the course of many

generations, from an animal of a more simple to one of a more complex

structure.

_Recapitulation._--For the reasons, therefore, detailed in this and the

two preceding chapters, we may draw the following inferences in regard

to the reality of _species_ in nature:--

1st. That there is a capacity in all species to accommodate themselves,

to a certain extent, to a change of external circumstances, this extent

varying greatly, according to the species.

2ndly. When the change of situation which they can endure is great, it

is usually attended by some modifications of the form, colour, size,

structure, or other particulars; but the mutations thus superinduced are

governed by constant laws, and the capability of so varying, forms part

of the permanent specific character.

3dly. Some acquired peculiarities, of form, structure, and instinct, are

transmissible to the offspring; but these consist of such qualities and

attributes only as are intimately related to the natural wants and

propensities of the species.

4thly. The entire variation from the original type, which any given kind

of change can produce, may usually be effected in a brief period of

time, after which no farther deviation can be obtained by continuing to

alter the circumstances, though ever so gradually; indefinite

divergence, either in the way of improvement or deterioration, being

prevented, and the least possible excess beyond the defined limits being

fatal to the existence of the individual.

5thly. The intermixture of distinct species is guarded against by the

aversion of the individuals composing them to sexual union, or by the

sterility of the mule offspring. It does not appear that true hybrid

races have ever been perpetuated for several generations, even by the

assistance of man; for the cases usually cited relate to the crossing of

mules with individuals of pure species, and not to the intermixture of

hybrid with hybrid.

6thly. From the above considerations, it appears that species have a

real existence in nature; and that each was endowed, at the time of its

creation, with the attributes and organization by which it is now

distinguished.

CHAPTER XXXVII.

LAWS WHICH REGULATE THE GEOGRAPHICAL DISTRIBUTION OF SPECIES.

Analogy of climate not attended with identity of species--Botanical

geography--Stations--Habitations--Distinct provinces of indigenous

plants--Vegetation of islands--Marine vegetation--In what manner

plants become diffused--Effects of wind, rivers, marine

currents--Agency of animals--Many seeds pass through the stomachs of

animals and birds undigested--Agency of man in the dispersion of

plants, both voluntary and involuntary--Its analogy to that of the

inferior animals.

Next to determining the question whether species have a real existence,

the consideration of the laws which regulate their geographical

distribution is a subject of primary importance to the geologist. It is

only by studying these laws with attention, by observing the positions

which groups of species occupy at present, and inquiring how these may

be varied in the course of time by migrations, by changes in physical

geography, and other causes, that we can hope to learn whether the

duration of species be limited, or in what manner the state of the

animate world is affected by the endless vicissitudes of the inanimate.

_Different regions inhabited by distinct species._--That different

regions of the globe are inhabited by entirely distinct animals and

plants, is a fact which has been familiar to all naturalists since

Buffon first pointed out the want of _specific_ identity between the

land quadrupeds of America and those of the Old World. The same

phenomenon has, in later times, been forced in a striking manner upon

our attention, by the examination of New Holland, where the indigenous

species of animals and plants were found to be, almost without

exception, distinct from those known in other parts of the world.

But the extent of this parcelling out of the globe amongst different

_nations_, as they have been termed, of plants and animals--the

universality of a phenomenon so extraordinary and unexpected, may be

considered as one of the most interesting facts clearly established by

the advance of modern science.

Scarcely fourteen hundred species of plants appear to have been known

and described by the Greeks, Romans, and Arabians. At present, more than

three thousand species are enumerated, as natives of our own

island.[841] In other parts of the world there have been now collected

(1846) upwards of 100,000 species, specimens of which are preserved in

European herbariums. It was not to be supposed, therefore, that the

ancients should have acquired any correct notions respecting what may be

called the geography of plants, although the influence of climate on the

character of the vegetation could hardly have escaped their observation.

Antecedently to investigation, there was no reason for presuming that

the vegetable productions, growing wild in the eastern hemisphere,

should be unlike those of the western, in the same latitude; nor that

the plants of the Cape of Good Hope should be unlike those of the south

of Europe; situations where the climate is little dissimilar. The

contrary supposition would have seemed more probable, and we might have

anticipated an almost perfect identity in the animals and plants which

inhabit corresponding parallels of latitude. The discovery, therefore,

that each separate region of the globe, both of the land and water, is

occupied by distinct groups of species, and that most of the exceptions

to this general rule may be referred to disseminating causes now in

operation, is eminently calculated to excite curiosity, and to stimulate

us to seek some hypothesis respecting the first introduction of species

which may be reconcileable with such phenomena.

_Botanical geography._--A comparison of the _plants_ of different

regions of the globe affords results more to be depended upon in the

present state of our knowledge than those relating to the animal

kingdom, because the science of botany is more advanced, and probably

comprehends a great proportion of the total number of the vegetable

productions of the whole earth. Humboldt, in several eloquent passages

of his Personal Narrative, was among the first to promulgate

philosophical views on this subject. Every hemisphere, says this

traveller, produces plants of different species; and it is not by the

diversity of climates that we can attempt to explain why equinoctial

Africa has no Laurinae, and the New World no Heaths; why the Calceolariae

are found only in the southern hemisphere; why the birds of the

continent of India glow with colors less splendid than the birds of the

hot parts of America: finally, why the tiger is peculiar to Asia, and

the ornithorhynchus to New Holland.[842]

"We can conceive," he adds, "that a small number of the families of

plants, for instance, the Musaceae and the Palms, cannot belong to very

cold regions, on account of their internal structure and the importance

of certain organs; but we cannot explain why no one of the family of

Melastomas vegetates north of the parallel of thirty degrees; or why no

rose-tree belongs to the southern hemisphere. Analogy of climates is

often found in the two continents without identity of productions."[843]

The luminous essay of De Candolle on "Botanical Geography" presents us

with the fruits of his own researches and those of Humboldt, Brown, and

other eminent botanists, so arranged, that the principal phenomena of

the distribution of plants are exhibited in connexion with the causes to

which they are chiefly referrible.[844] "It might not, perhaps, be

difficult," observes this writer, "to find two points, in the United

States and in Europe, or in Equinoctial America and Africa, which

present all the same circumstances: as, for example, the same

temperature, the same height above the sea, a similar soil, an equal

dose of humidity; yet nearly all, _perhaps all_, the plants in these two

similar localities shall be distinct. A certain degree of analogy,

indeed, of aspect, and even of structure, might very possibly be

discoverable between the plants of the two localities in question; but

the _species_ would in general be different. Circumstances, therefore,

different from those which now determine the _stations_, have had an

influence on the _habitations_ of plants."

_Stations and habitations of plants._--As I shall frequently have

occasion to speak of the _stations_ and _habitations_ of plants in the

technical sense in which the terms are used in the above passage, I may

remind the geologist that station indicates the peculiar nature of the

locality where each species is accustomed to grow, and has reference to

climate, soil, humidity, light, elevation above the sea, and other

analogous circumstances; whereas, by habitation is meant a general

indication of the country where a plant grows wild. Thus the _station_

of a plant may be a salt-marsh, a hill-side, the bed of the sea, or a

stagnant pool. Its _habitation_ may be Europe, North America, or New

Holland, between the tropics. The study of stations has been styled the

topography, that of habitations the geography, of botany. The terms thus

defined, express each a distinct class of ideas, which have been often

confounded together, and which are equally applicable in zoology.

In farther illustration of the principle above alluded to, that

difference of longitude, independently of any influence of temperature,

is accompanied by a great, and sometimes a complete, diversity in the

species of plants, De Candolle observes, that, out of 2891 species of

phaenogamous plants described by Pursh, in the United States, there are

only 385 which are found in northern or temperate Europe. MM. Humboldt

and Bonpland, in all their travels through equinoctial America, found

only twenty-four species (these being all Cyperaceae and Gramineae) common

to America and any part of the Old World. They collected, it is true,

chiefly on the mountains, or the proportion would have been larger; for

Dr. J. Hooker informs me that many tropical plants of the New World are

identical with African species. Nevertheless, the general discordance of

these Floras is very striking. On comparing New Holland with Europe, Mr.

Brown ascertained that, out of 4100 species, discovered in Australia,

there were only 166 common to Europe, and of this small number there

were some few which may have been transported thither by man. Almost all

of the 166 species were cryptogamic, and the rest consist, in nearly

every case, of phaenogamous plants which also inhabit intervening

regions.

But what is still more remarkable, in the more widely separated parts of

the ancient continent, notwithstanding the existence of an uninterrupted

land-communication, the diversity in the specific character of the

respective vegetations is almost as striking. Thus there is found one

assemblage of species in China, another in the countries bordering the

Black Sea and the Caspian, a third in those surrounding the

Mediterranean, a fourth in the great platforms of Siberia and Tartary,

and so forth.

The distinctness of the groups of indigenous plants, in the same

parallel of latitude, is greatest where continents are disjoined by a

wide expanse of ocean. In the northern hemisphere, near the pole, where

the extremities of Europe, Asia, and America unite or approach near to

one another, a considerable number of the same species of plants are

found, common to the three continents. But it has been remarked, that

these plants, which are thus so widely diffused in the arctic regions,

are also found in the chain of the Aleutian islands, which stretch

almost across from America to Asia, and which may probably have served

as the channel of communication for the partial blending of the Floras

of the adjoining regions. It has, indeed, been observed to be a general

rule, that plants found at two points very remote from each other occur

also in places intermediate.

Dr. J. Hooker informs me that in high latitudes in the southern ocean,

in spite of the great extent of the sea, Floras of widely disconnected

islands contain many species in common. Perhaps icebergs, transporting

to vast distances not only stones, but soil with the seeds of plants,

may explain this unusually wide diffusion of insular plants.

In islands very distant from continents the total number of plants is

comparatively small; but a large proportion of the species are such as

occur nowhere else. In so far as the Flora of such islands is not

peculiar to them, it contains, in general, species common to the nearest

main lands.[845] The islands of the great southern ocean exemplify these

rules; the easternmost containing more American, and the western more

Indian plants.[846] Madeira and Teneriffe contain many species, and even

entire genera, peculiar to them; but they have also plants in common

with Portugal, Spain, the Azores, and the north-west coast of

Africa.[847]

In the Canaries, out of 533 species of phaenogamous plants, it is said

that 310 are peculiar to these islands, and the rest identical with

those of the African continent; but in the Flora of St. Helena, which is

so far distant even from the western shores of Africa, there have been

found, out of thirty native species of the phaenogamous class, only _one

or two_ which are to be found in any other part of the globe. On the

other hand, of sixty cryptogamic plants, collected by Dr. J. Hooker in

the same island, twelve only were peculiar.

The natural history of the Galapagos archipelago, described by Mr.

Darwin, affords another very instructive illustration of the laws

governing the geographical distribution of plants and animals in

islands. This group consists of ten principal islands, situated in the

Pacific Ocean, under the equator, about 600 miles westward of the coast

of South America. As they are all formed of volcanic rocks, many of the

craters, of which there are about 2000 in number, having a very fresh

aspect, we may regard the whole as much more modern in origin than the

mass of the adjoining continent; yet neither has the Flora nor Fauna

been derived from South America, but consist of species for the most

part indigenous, yet stamped with a character decidedly South American.

What is still more singular, there is a difference between the species

inhabiting the different islands. Of flowering plants, for example,

there are 185 species at present known, and forty cryptogamic, making

together 225. One hundred of the former class are new species, probably

confined to this archipelago; and of the rest, ten at least have been

introduced by man. Of twenty-one species of _Compositae_, all but one are

peculiar, and they belong to twelve genera, no less than ten of which

genera are confined to the Galapagos. Dr. Hooker observes, that the type

of this Flora has an undoubted relation to that of the western side of

South America, and he detects in it no affinity with that of the

numerous islands scattered over other parts of the Pacific. So in regard

to the birds, reptiles, land-shells, and insects, this archipelago,

standing as it does in the Pacific Ocean, is zoologically part of

America. Although each small island is not more than fifty or sixty

miles apart, and most of them are in sight of each other, formed of

precisely the same rocks, rising nearly to an equal height, and placed

under a similar climate, they are tenanted each by a different set of

beings, the tortoises, mocking-thrushes, finches, beetles, scarcely any

of them ever ranging over the whole, and often not even common to any

two of the islands.

"The archipelago," says Mr. Darwin, "is a little world within itself, or

rather a satellite attached to America; whence it has derived a few

stray colonists, and has received the general character of its

indigenous productions. One is astonished," he adds, "at the amount of

creative force displayed on so many small, barren, and rocky islands,

and still more so, at its diverse, yet analogous action on points so

near each other. I have said that the Galapagos archipelago might be

called a satellite attached to America, but it should rather be called a

group of satellites physically similar, organically distinct, yet

intimately related to each other, and all related in a marked, though

much lesser degree, to the great American continent."[848]

_Number of botanical provinces._--De Candolle has enumerated twenty

great botanical provinces inhabited by indigenous or aboriginal plants;

and although many of these contain a variety of species which are common

to several others, and sometimes to places very remote, yet the lines of

demarcation are, upon the whole, astonishingly well defined.[849] Nor is

it likely that the bearing of the evidence on which these general views

are founded will ever be materially affected, since they are already

confirmed by the examination of nearly one hundred thousand species of

plants.

The entire change of opinion which the contemplation of those phenomena

has brought about is worthy of remark. The first travellers were

persuaded that they should find, in distant regions, the plants of their

own country, and they took a pleasure in giving them the same names. It

was some time before this illusion was dissipated; but so fully sensible

did botanists at last become of the extreme smallness of the number of

phaenogamous plants common to different continents, that the ancient

Floras fell into disrepute. All grew diffident of the pretended

identifications; and we now find that every naturalist is inclined to

examine each supposed exception with scrupulous severity.[850] If they

admit the fact, they begin to speculate on the mode whereby the seeds

may have been transported from one country into the other, or enquire on

which of two continents the plant was indigenous, assuming that a

species, like an individual, cannot have two birthplaces.

_Marine vegetation._--The marine vegetation is divisible into different

systems, like those prevailing on the land; but they are much fewer, as

we might have expected, the temperature of the ocean being more uniform

than that of the atmosphere, and consequently the dispersion of species

from one zone to another being less frequently checked by the

intervention of uncongenial climates. The proportion also of land to sea

throughout the globe being small, the migration of marine plants is not

so often stopped by barriers of land, as is that of the terrestrial

species by the ocean. The number of hydrophytes, as they are termed, is

very considerable, and their stations are found to be infinitely more

varied than could have been anticipated; for while some plants are

covered and uncovered daily by the tide, others live at the depth of

several hundred feet. Among the known provinces of Algae, we may mention,

1st, The north circumpolar, from lat 60 degrees N. to the pole; 2dly,

The North Atlantic or the region of Fucus proper and Delesseriae,

extending from lat. 40 degrees N. to lat. 60 degrees N.; 3dly, That of

the Mediterranean, which may be regarded as a sub-region of the _fourth_

or warmer temperate zone of the Atlantic, between lat. 23 degrees N. and

lat. 40 degrees N.; 5thly, The Tropical Atlantic, in which Sargassum,

Rhodomelia, Corallinea, and Siphonia abound; 6thly, The South Atlantic,

where the Fucus reappears; 7thly, The Antarctic American, comprehending

from Chili to Cape Horn, the Falkland Islands, and thence round the

world south of latitude 50 degrees S.; 8thly, The Australian and New

Zealand, which is very peculiar, being characterized, among other

generic forms, by Cystoseiriae and Fuceae; 9thly, The Indian Ocean and

Red Sea; and, 10thly, The Chinese and Japanese seas.[851] In addition to

the above provinces, there are several others not yet well determined in

the Pacific Ocean and elsewhere. There are, however, many species which

range through several of these geographical regions of subaqueous

vegetation, being common to very remote countries; as, for example, to

the coasts of

Europe and the United States, and others, to Cape Horn and Van Diemen's

Land, the same plants extending also for the most part to the New

Zealand sea. Of the _species_ strictly antarctic (excluding the New

Zealand and Tasmanian groups) Dr. Hooker has identified not less than a

fifth part of the whole with British Algae! Yet is there a much smaller

proportion of cosmopolite species among the Algae than among the

terrestrial cellular plants, such as lichens, mosses, and Hepaticae.

It must always be borne in mind, that the distinctness alluded to

between the provinces, whether of subaqueous or terrestrial plants,

relates strictly to _species_, and not to forms. In regard to the

numerical preponderance of certain forms, and many peculiarities of

internal structure, there is usually a marked agreement in the vegetable

productions of districts placed in corresponding latitudes, and under

similar physical circumstances, however remote their position. Thus

there are innumerable points of analogy between the vegetation of the

Brazils, equinoctial Africa, and India; and there are also points of

difference wherein the plants of these regions are distinguishable from

all extra-tropical groups. But there is a very small proportion of the

entire number of species common to the three continents. The same may be

said, if we compare the plants of the United States with that of the

middle of Europe; the species are distinct, but the forms are often so

analogous, as to have been styled "geographical representatives." There

are very few _species_ of phaenogamous plants, says Dr. J. Hooker, common

to Van Diemen's Land, New Zealand, and Fuegia, but a great many

_genera_, and some of them are confined to those three distant regions

of the southern hemisphere, being in many instances each severally

represented by a single species. The same naturalist also observes that

the southern temperate as well as the antarctic regions, possess each of

them representatives of some of the genera of the analogous climates of

the opposite hemisphere; but very few of the species are identical

unless they be such as are equally diffused over other countries, or

which inhabit the Andes, by the aid of which they have evidently

effected their passage southwards.

_Manner in which plants become diffused.--Winds._--Let us now consider

what means of diffusion, independently of the agency of man, are

possessed by plants, whereby, in the course of ages, they may be enabled

to stray from one of the botanical provinces above mentioned to another,

and to establish new colonies at a great distance from their birthplace.

The principal of the inanimate agents provided by nature for scattering

the seeds of plants over the globe, are the movements of the atmosphere

and of the ocean, and the constant flow of water from the mountains to

the sea. To begin with the winds: a great number of seeds, are furnished

with downy and feathery appendages, enabling them, when ripe, to float

in the air, and to be wafted easily to great distances by the most

gentle breeze. Other plants are fitted for dispersion by means of an

attached wing, as in the case of the fir tree, so that they are caught

up by the wind as they fall from the cone, and are carried to a

distance. Amongst the comparatively small number of plants known to

Linnaeus, no less than 138 genera are enumerated as having winged seeds.

As winds often prevail for days, weeks, or even months together, in the

same direction, these means of transportation may sometimes be without

limits; and even the heavier grains may be borne through considerable

spaces, in a very short time, during ordinary tempests; for strong

gales, which can sweep along grains of sand, often move at the rate of

about forty miles an hour, and if the storm be very violent, at the rate

of fifty-six miles.[852] The hurricanes of tropical regions, which root

up trees and throw down buildings, sweep along at the rate of ninety

miles an hour; so that, for however short a time they prevail, they may

carry even the heavier fruits and seeds over friths and seas of

considerable width, and doubtless are often the means of introducing

into islands the vegetation of adjoining continents. Whirlwinds are also

instrumental in bearing along heavy vegetable substances to considerable

distances. Slight ones may frequently be observed in our fields, in

summer carrying up haycocks into the air, and then letting fall small

tufts of hay far and wide over the country; but they are sometimes so

powerful as to dry up lakes and ponds, and to break off the boughs of

trees, and carry them up in a whirling column of air.

Franklin tells us, in one of his letters, that he saw, in Maryland, a

whirlwind which began by taking up the dust which lay in the road, in

the form of a sugar loaf with the pointed end downwards, and soon after

grew to the height of forty or fifty feet, being twenty or thirty in

diameter. It advanced in a direction contrary to the wind; and although

the rotary motion of the column was surprisingly rapid, its onward

progress was sufficiently slow to allow a man to keep pace with it on

foot. Franklin followed it on horseback, accompanied by his son, for

three quarters of a mile, and saw it enter a wood, where it twisted and

turned round large trees with surprising force. These were carried up in

a spiral line, and were seen flying in the air, together with boughs and

innumerable leaves, which, from their height, appeared reduced to the

apparent size of flies. As this cause operates at different intervals of

time throughout a great portion of the earth's surface, it may be the

means of bearing not only plants but insects, land testacea and their

eggs, with many other species of animals, to points which they could

never otherwise have reached, and from which they may then begin to

propagate themselves again as from a new centre.

_Distribution of cryptogamous plants._--It has been found that a great

numerical proportion of the exceptions to the limitation of species to

certain quarters of the globe occur in the various tribes of cryptogamic

plants. Linnaeus observed that, as the germs of plants of this class,

such as mosses, fungi, and lichens, consist of an impalpable powder, the

particles of which are scarcely visible to the naked eye, there is no

difficulty to account for their being dispersed throughout the

atmosphere, and carried to every point of the globe, where there is a

station fitted for them.

Lichens in particular ascend to great elevations, sometimes growing two

thousand feet above the line of perpetual snow, at the utmost limits of

vegetation, and where the mean temperature is nearly at the freezing

point. This elevated position must contribute greatly to facilitate the

dispersion of those buoyant particles of which their fructification

consists.[853]

Some have inferred, from the springing up of mushrooms whenever

particular soils and decomposed organic matter are mixed together, that

the production of fungi is accidental, and not analogous to that of

perfect plants. But Fries, whose authority on these questions is

entitled to the highest respect, has shown the fallacy of this argument

in favor of the old doctrine of equivocal generation. "The sporules of

fungi," says this naturalist, "are so infinite, that in a single

individual of _Reticularia maxima_, I have counted above ten millions,

and so subtile as to be scarcely visible, often resembling thin smoke;

so light that they may be raised perhaps by evaporation into the

atmosphere, and dispersed in so many ways by the attraction of the sun,

by insects, wind, elasticity, adhesion, &c., that it is difficult to

conceive a place from which they may be excluded."[854]

The club-moss called _Lycopodium cernuum_ affords a striking example of

a cryptogamous plant universally distributed over all equinoctial

countries. It scarcely ever passes beyond the northern tropic, except in

one instance, where it appears around the hot-springs in the Azores,

although it is neither an inhabitant of the Canaries nor Madeira.

Doubtless its microscopic sporules are everywhere present, ready to

germinate on any spot where they can enjoy throughout the year the

proper quantity of warmth, moisture, light, and other conditions

essential to the species.

Almost every lichen brought home from the southern hemisphere by the

antarctic expedition under Sir James Ross, amounting to no less than 200

species, was ascertained to be also an inhabitant of the northern

hemisphere, and almost all of them European.

_Agency of rivers and currents._--In considering, in the next place, the

instrumentality of the aqueous agents of dispersion, I cannot do better

than cite the words of one of our ablest botanical writers. "The

mountain stream or torrent," observes Keith, "washes down to the valley

the seeds which may accidentally fall into it, or which it may happen to

sweep from its banks when it suddenly overflows them. The broad and

majestic river, winding along the extensive plain, and traversing the

continents of the world, conveys to the distance of many hundreds of

miles the seeds that may have vegetated at its source. Thus the southern

shores of the Baltic are visited by seeds which grew in the interior of

Germany, and the western shores of the Atlantic by seeds that have been

generated in the interior of America."[855] Fruits, moreover, indigenous

to America and the West Indies, such as that of the _Mimosa scandens_,

the cashewnut and others, have been known to be drifted across the

Atlantic by the Gulf stream, on the western coasts of Europe, in such a

state that they might have vegetated had the climate and soil been

favourable. Among these the _Guilandina Bonduc_, a leguminous plant, is

particularly mentioned, as having been raised from a seed found on the

west coast of Ireland.[856]

Sir Hans Sloane states, that several kinds of beans cast ashore on the

Orkney Isles, and Ireland, but none of which appear to have naturalized

themselves, are derived from trees which grow in the West Indies, and

many of them in Jamaica. He conjectures that they might have been

conveyed by rivers into the sea, and then by the Gulf stream to greater

distances, in the same manner as the sea-weed called _Lenticula marina_,

or Sargasso, which grows on the rocks about Jamaica, is known to be

"carried by the winds and current towards the coast of Florida, and

thence into the North American ocean, where it lies very thick on the

surface of the sea."[857]

The absence of liquid matter in the composition of seeds renders them

comparatively insensible to heat and cold, so that they may be carried

without detriment through climates where the plants themselves would

instantly perish. Such is their power of resisting the effects of heat,

that Spallanzani mentions some seeds that germinated after having been

boiled in water.[858] Sir John Herschel informs me that he has sown at

the Cape of Good Hope the seeds of the _Acacia lophanta_ after they had

remained for twelve hours in water of 140 degrees Fahrenheit, and they

germinated far more rapidly than unboiled seeds. He also states that an

eminent botanist, Baron Ludwig, could not get the seeds of a species of

cedar to grow at the Cape till they were thoroughly boiled.

When therefore, a strong gale, after blowing violently off the land for

a time, dies away, and the seeds alight upon the surface of the waters,

or wherever the ocean, by eating away the sea-cliffs, throws down into

its waves plants which would never otherwise reach the shores, the tides

and currents become active instruments in assisting the dissemination of

almost all classes of the vegetable kingdom. The pandanus and many other

plants have been distributed in this way over the islands of the

Pacific. I have before called attention (p. 618.) to the interesting

fact that one-fifth of all the algae found in the antarctic regions in

1841-3, by Dr. J. Hooker, were of species common to the British seas. He

has suggested that cold currents which prevail from Cape Horn to the

equator, and are there met by other cold water, may by their direct

influence, as well as by their temperature, facilitate the passage of

antarctic species to the Arctic Ocean. In like manner the migration of

certain marine animals from the southern to the northern hemisphere may

have been brought about by the same cause.

In a collection of six hundred plants from the neighborhood of the river

Zaire, in Africa, Mr. Brown found that thirteen species were also met

with on the opposite shores of Guiana and Brazil. He remarked that most

of these plants were found only on the lower parts of the river Zaire,

and were chiefly such as produced seeds capable of retaining their

vitality a long time in the currents of the ocean. Dr. J. Hooker informs

me that after an examination of a great many insular floras, he has

found that no one of the large natural orders is so rich in species

common to other countries, as the Leguminosae. The seeds in this order,

which comprises the largest proportion of widely diffused littoral

species, are better adapted than those of any other plants for

water-carriage.

_The migration of plants aided by islands._--Islands, moreover, and even

the smallest rocks, play an important part in aiding such migrations;

for when seeds alight upon them from the atmosphere, or are thrown up by

the surf, they often vegetate, and supply the winds and waves with a

repetition of new and uninjured crops of fruit and seeds. These may

afterwards pursue their course through the atmosphere, or along the

surface of the sea, in the same direction. The number of plants found at

any given time on an islet affords us no test whatever of the extent to

which it may have co-operated towards this end, since a variety of

species may first thrive there and then perish, and be followed by other

chance-comers like themselves. If neither St. Helena nor Ascension have

promoted the botanical intercourse between the Old and New Worlds, we

may easily account for the fact by remembering that they are not only

extremely minute and isolated spots, but are also bounded by lofty and

precipitous shores without beaches, where the seeds of foreign species

could readily establish themselves.

Currents and winds in the arctic regions drift along icebergs covered

with an alluvial soil, on which herbs and pine-saplings are seen

growing, which may often continue to vegetate on some distant shore

where the ice-island is stranded.

_Dispersion of marine plants._--With respect to marine vegetation, the

seeds, being in their native element, may remain immersed in water

without injury for indefinite periods, so that there is no difficulty in

conceiving the diffusion of species wherever uncongenial climates,

contrary currents, and other causes do not interfere. All are familiar

with the sight of the floating sea-weed,

"Flung from the rock on ocean's foam to sail,

Where'er the surge may sweep, the tempest's breath prevail."

Remarkable accumulations of that species of sea-weed generally known as

gulf-weed, or sargasso, occur on each side of the equator in the

Atlantic, Pacific, and Indian Oceans. Columbus and other navigators, who

first encountered these banks of algae in the Northern Atlantic, compared

them to vast inundated meadows, and state that they retarded the

progress of their vessels. The most extensive bank is a little west of

the meridian of Fayal, one of the Azores, between latitudes 35 degrees

and 36 degrees: violent north-winds sometimes prevail in this space, and

drive the sea-weed to low latitudes, as far as the 24th or even the 20th

degree.[859] Along the northern edge of the Gulf stream Dr. Hooker found

_Fucus nodosus_, and _F. serratus_, which he traced all the way from

lat. 36 degrees N. to England.

The hollow pod-like receptacle in which the seeds of many algae are

lodged, and the filaments attached to the seed-vessels of others, seem

intended to give buoyancy; and I may observe that these hydrophytes are

in general _proliferous_, so that the smallest fragment of a branch can

be developed into a perfect plant. The seeds, moreover, of the greater

number of species are enveloped with a mucous matter like that which

surrounds the eggs of some fish, and which not only protects them from

injury, but serves to attach them to floating bodies or to rocks.

_Agency of animals in the distribution of plants._--But we have as yet

considered part only of the fertile resources of nature for conveying

seeds to a distance from their place of growth. The various tribes of

animals are busily engaged in furthering an object whence they derive

such important advantages. Sometimes an express provision is found in

the structure of seeds to enable them to adhere firmly by prickles,

hooks, and hairs, to the coats of animals, or feathers of the winged

tribe, to which they remain attached for weeks, or even months, and are

borne along into every region whither birds or quadrupeds may migrate.

Linnaeus enumerates fifty genera of plants, and the number now known to

botanists is much greater, which are armed with hooks, by which, when

ripe, they adhere to the coats of animals. Most of these vegetables, he

remarks, require a soil enriched with dung. Few have failed to mark the

locks of wool hanging on the thorn-bushes, wherever the sheep pass, and

it is probable that the wolf or lion never give-chase to herbivorous

animals without being unconsciously subservient to this part of the

vegetable economy.

A deer has strayed from the herd when browsing on some rich pasture,

when he is suddenly alarmed by the approach of his foe. He instantly

takes to flight, dashing through many a thicket, and swimming across

many a river and lake. The seeds of the herbs and shrubs which have

adhered to his smoking flanks are washed off again by the waters. The

thorny spray is torn off, and fixes itself in its hairy coat, until

brushed off again in other thickets and copses. Even on the spot where

the victim is devoured many of the seeds which he had swallowed

immediately before the chase may be left on the ground uninjured, and

ready to spring up in a new soil.

The passage, indeed, of undigested seeds through the stomachs of animals

is one of the most efficient causes of the dissemination of plants, and

is of all others, perhaps, the most likely to be overlooked. Few are

ignorant that a portion of the oats eaten by a horse preserve their

germinating faculty in the dung. The fact of their being still

nutritious is not lost on the sagacious rook. To many, says Linnaeus, it

seems extraordinary, and something of a prodigy, that when a field is

well tilled and sown with the best wheat, it frequently produces darnel

or the wild oat, especially if it be manured with new dung; they do not

consider that the fertility of the smaller seeds is not destroyed in the

stomachs of animals.[860]

_Agency of birds._--Some birds of the order Passeres devour the seeds of

plants in great quantities, which they eject again in very distant

places, without destroying its faculty of vegetation: thus a flight of

larks will fill the cleanest field with a great quantity of various

kinds of plants, as the melilot trefoil (_Medicago lupulina_), and

others whose seeds are so heavy that the wind is not able to scatter

them to any distance.[861] In like manner, the blackbird and

misselthrush, when they devour berries in too great quantities, are

known to consign them to the earth undigested in their excrement.[862]

Pulpy fruits serve quadrupeds and birds as food, while their seeds,

often hard and indigestible, pass uninjured through the intestines, and

are deposited far from their original place of growth in a condition

peculiarly fit for vegetation.[863] So well are the farmers, in some

parts of England, aware of this fact, that when they desire to raise a

quickset hedge in the shortest possible time, they feed turkeys with the

haws of the common white-thorn (_Crataegus Oxyacantha_), and then sow the

stones which are ejected in their excrement, whereby they gain an entire

year in the growth of the plant.[864] Birds, when they pluck cherries,

sloes, and haws, fly away with them to some convenient place; and when

they have devoured the fruit, drop the stone into the ground. Captain

Cook, in his account of the volcanic island of Tanna, one of the New

Hebrides, which he visited in his second voyage, makes the following

interesting observation:--"Mr. Forster, in his botanical excursion this

day, shot a pigeon, in the craw of which was a wild nutmeg."[865] It is

easy, therefore, to perceive, that birds in their migrations to great

distances, and even across seas, may transport seeds to new isles and

continents.

The sudden deaths to which great numbers of frugivorous birds are

annually exposed must not be omitted as auxiliary to the transportation

of seeds to new habitations. When the sea retires from the shore, and

leaves fruits and seeds on the beach, or in the mud of estuaries, it

might, by the returning tide, wash them away again, or destroy them by

long immersion; but when they are gathered by land birds which frequent

the sea side, or by waders and water-fowl, they are often borne inland;

and if the bird to whose crop they have been consigned is killed, they

may be left to grow up far from the sea. Let such an accident happen but

once in a century, or a thousand years, it will be sufficient to spread

many of the plants from one continent to another; for in estimating the

activity of these causes, we must not consider whether they act slowly

in relation to the period of our observation, but in reference to the

duration of species in general.

Let us trace the operation of this cause in connection with others. A

tempestuous wind bears the seeds of a plant many miles through the air,

and then delivers them to the ocean; the oceanic current drifts them to

a distant continent; by the fall of the tide they become the food of

numerous birds, and one of these is seized by a hawk or eagle, which,

soaring across hill and dale to a place of retreat, leaves, after

devouring its prey, the unpalatable seeds to spring up and flourish in a

new soil.

The machinery before adverted to, is so capable of disseminating seeds

over almost unbounded spaces, that were we more intimately acquainted

with the economy of nature, we might probably explain all the instances

which occur of the aberration of plants to great distances from their

native countries. The real difficulty which must present itself to every

one who contemplates the present geographical distribution of species,

is the small number of exceptions to the rule of the non-intermixture of

different groups of plants. Why have they not, supposing them to have

been ever so distinct originally, become more blended and confounded

together in the lapse of ages?

_Agency of man in the dispersion of plants._--But in addition to all the

agents already enumerated as instrumental in diffusing plants over the

globe, we have still to consider man--one of the most important of all.

He transports with him, into every region, the vegetables which he

cultivates for his wants, and is the involuntary means of spreading a

still greater number which are useless to him, or even noxious. "When

the introduction of cultivated plants," says De Candolle, "is of recent

date, there is no difficulty in tracing their origin; but when it is of

high antiquity, we are often ignorant of the true country of the plants

on which we feed. No one contests the American origin of the maize or

the potatoe; nor the origin, in the Old World, of the coffee-tree, and

of wheat. But there are certain objects of culture, of very ancient

date, between the tropics, such for example as the banana, of which the

origin cannot be verified. Armies, in modern times, have been known to

carry, in all directions, grain and cultivated vegetables from one

extremity of Europe to the other; and thus have shown us how, in more

ancient times, the conquests of Alexander, the distant expeditions of

the Romans, and afterwards the crusades, may have transported many

plants from one part of the world to the other."[866]

But, besides the plants used in agriculture, the numbers which have been

naturalized by accident, or which man has spread unintentionally, is

considerable. One of our old authors, Josselyn, gives a catalogue of

such plants as had, in his time, sprung up in the colony since the

English planted and kept cattle in New England. They were two-and-twenty

in number. The common nettle was the first which the settlers noticed;

and the plantain was called by the Indians "Englishman's foot," as if it

sprung from their footsteps.[867]

"We have introduced every where," observes De Candolle, "some weeds

which grow among our various kinds of wheat, and which have been

received, perhaps, originally from Asia along with them. Thus, together

with the Barbary wheat, the inhabitants of the south of Europe have

sown, for many ages, the plants of Algiers and Tunis. With the wools and

cottons of the East, or of Barbary, there are often brought into France

the grains of exotic plants, some of which naturalize themselves. Of

this I will cite a striking example. There is, at the gate of

Montpellier, a meadow set apart for drying foreign wool, _after it has

been washed_. There hardly passes a year without foreign plants being

found naturalized in this drying-ground. I have gathered there

_Centaurea parviflora_, _Psoralea palaestina_, and _Hypericum crispum_."

This fact is not only illustrative of the aid which man lends

inadvertently to the propagation of plants, but it also demonstrates the

multiplicity of seeds which are borne about in the woolly and hairy

coats of wild animals.

The same botanist mentions instances of plants naturalized in seaports

by the ballast of ships; and several examples of others which have

spread through Europe from botanical gardens, so as to have become more

common than many indigenous species.

It is scarcely a century, says Linnaeus, since the Canadian erigeron, or

flea-bane, was brought from America to the botanical garden at Paris;

and already the seeds have been carried by the winds so that it is

diffused over France, the British islands, Italy, Sicily, Holland, and

Germany.[868] Several others are mentioned by the Swedish naturalist, as

having been dispersed by similar means. The common thorn-apple (_Datura

Stramonium_), observes Willdenow, now grows as a noxious weed throughout

all Europe, with the exception of Sweden, Lapland and Russia. It came

from the East Indies and Abyssinia to us, and was thus universally

spread by certain quacks, who used its seeds as an emetic.[869] The same

plant is now abundant throughout the greater part of the United States,

along road-sides and about farm-yards. The yellow monkey-flower,

_Mimulus luteus_, a plant from the north-west region of America, has now

established itself in various parts of England, and is spreading

rapidly.

In hot and ill-cultivated countries, such naturalization takes place

more easily. Thus the _Chenopodium ambrosioides_, sown by Mr. Burchell

on a point of St. Helena, multiplied so fast in four years as to become

one of the commonest weeds in the island, and it has maintained its

ground ever since 1845.[870]

The most remarkable proof, says De Candolle, of the extent to which man

is unconsciously the instrument of dispersing and naturalizing species,

is found in the fact, that in New Holland, America, and the Cape of Good

Hope, the aboriginal European species exceed in number all the others

which have come from any distant regions; so that, in this instance, the

influence of man has surpassed that of all the other causes which tend

to disseminate plants to remote districts. Of nearly 1600 British

flowering plants, it is supposed that about 300 species are naturalized;

but a large proportion of these would perish with the discontinuance of

agriculture.

Although we are but slightly acquainted, as yet, with the extent of our

instrumentality in naturalizing species, yet the facts ascertained

afford no small reason to suspect that the number which we introduce

unintentionally exceeds all those transported by design. Nor is it

unnatural to suppose that the functions, which the inferior beings,

extirpated by man, once discharged in the economy of nature, should

devolve upon the human race. If we drive many birds of passage from

different countries, we are probably required to fulfil their office of

carrying seeds, eggs of fish, insects, mollusks, and other creatures, to

distant regions: if we extirpate quadrupeds, we must replace them not

merely as consumers of the animal and vegetable substances which they

devour, but as disseminators of plants, and of the inferior classes of

the animal kingdom. I do not mean to insinuate that the very same

changes which man brings about, would have taken place by means of the

agency of other species, but merely that he supersedes a certain number

of agents; and so far as he disperses plants unintentionally, or against

his will, his intervention is strictly analogous to that of the species

so extirpated.

I may observe, moreover, that if, at former periods, the animals

inhabiting any given district have been partially altered by the

extinction of some species, and the introduction of others, whether by

new creations or by immigration, a change must have taken place in

regard to the particular plants conveyed about with them to foreign

countries. As, for example, when one set of migratory birds is

substituted for another, the countries from and to which seeds are

transported are immediately changed. Vicissitudes, therefore, analogous

to those which man has occasioned, may have previously attended the

springing up of new relations between species in the vegetable and

animal worlds.

It may also be remarked, that if man is the most active agent in

enlarging, so also is he in circumscribing the geographical boundaries

of particular plants. He promotes the migration of some, he retards that

of other species; so that, while in many respects he appears to be

exerting his power to blend and confound the various provinces of

indigenous species, he is, in other ways, instrumental in obstructing

the fusion into one group of the inhabitants of contiguous provinces.

Thus, for example, when two botanical regions exist in the same great

continent, such as _the European region_, comprehending the central

parts of Europe, and those surrounding the Mediterranean, and _the

Oriental region_, as it has been termed, embracing the countries

adjoining the Black Sea and the Caspian, the interposition between these

of thousands of square miles of cultivated lands, opposes a new and

powerful barrier against the mutual interchange of indigenous plants.

Botanists are well aware that garden plants naturalize and diffuse

themselves with great facility in comparatively unreclaimed countries,

but spread themselves slowly and with difficulty in districts highly

cultivated. There are many obvious causes for this difference; by

drainage and culture the natural variety of stations is diminished, and

those stray individuals by which the passage of a species from one fit

station to another is effected, are no sooner detected by the

agriculturist, than they are uprooted as weeds. The larger shrubs and

trees, in particular, can scarcely ever escape observation, when they

have attained a certain size, and will rarely fail to be cut down if

unprofitable.

The same observations are applicable to the interchange of the insects,

birds, and quadrupeds of two regions situated like those above alluded

to. No beasts of prey are permitted to make their way across the

intervening arable tracts. Many birds, and hundreds of insects, which

would have found some palatable food amongst the various herbs and trees

of the primeval wilderness, are unable to subsist on the olive, the

vine, the wheat, and a few trees and grasses favored by man. In

addition, therefore, to his direct intervention, man, in this case,

operates indirectly to impede the dissemination of plants, by

intercepting the migration of animals, many of which would otherwise

have been active in transporting seeds from one province to another.

Whether, in the vegetable kingdom, the influence of man will tend, after

a considerable lapse of ages, to render the geographical range of

_species in general_ more extended, as De Candolle seems to anticipate,

or whether the compensating agency above alluded to will not

counterbalance the exceptions caused by our naturalizations, admits at

least of some doubt. In the attempt to form an estimate on this subject,

we must be careful not to underrate, or almost overlook, as some appear

to have done, the influence of man in checking the diffusion of plants,

and restricting their distribution to narrower limits.

CHAPTER XXXVIII.

LAWS WHICH REGULATE THE GEOGRAPHICAL DISTRIBUTION OF

SPECIES--_continued_.

Geographical distribution of animals--Buffon on specific

distinctness of quadrupeds of Old and New World--Doctrine of

"natural barriers"--Different regions of indigenous

mammalia--Europe--Africa--India, and Indian

Archipelago--Australia--North and South America--Quadrupeds in

islands--Range of the Cetacea--Dispersion of quadrupeds--Their

powers of swimming--Migratory instincts--Drifting of animals on

ice-floes--On floating islands of drift-timber--Migrations of

Cetacea--Habitations of birds--Their migrations and facilities of

diffusion--Distribution of reptiles, and their power of

dissemination.

_Geographical distribution of animals._--Although in speculating on

"philosophical possibilities," said Buffon, "the same temperature might

have been expected, all other circumstances being equal, to produce the

same beings in different parts of the globe, both in the animal and

vegetable kingdoms, yet it is an undoubted fact, that when America was

discovered, its indigenous quadrupeds were all dissimilar to those

previously known in the Old World. The elephant, the rhinoceros, the

hippopotamus, the camelopard, the camel, the dromedary, the buffalo, the

horse, the ass, the lion, the tiger, the apes, the baboons, and a number

of other mammalia, were nowhere to be met with on the new continent;

while in the old, the American species, of the same great class, were

nowhere to be seen--the tapir, the lama, the pecari, the jaguar, the

couguar, the agouti, the paca, the coati, and the sloth."

These phenomena, although few in number relatively to the whole animate

creation, were so striking and so positive in their nature, that the

great French naturalist caught sight at once of a general law in the

geographical distribution of organic beings, namely, the limitation of

groups of distinct species to regions separated from the rest of the

globe by certain natural barriers. It was, therefore, in a truly

philosophical spirit that, relying on the clearness of the evidence

obtained respecting the larger quadrupeds, he ventured to call in

question the identifications announced by some contemporary naturalists

of species of animals said to be common to the southern extremities of

America and Africa.[871]

The migration of quadrupeds from one part of the globe to another,

observes Dr. Prichard, is prevented by uncongenial climates and the

branches of the ocean which intersect continents. "Hence, by a

reference to the geographical site of countries, we may divide the

earth into a certain number of regions fitted to become the abodes of

particular groups of animals, and we shall find, on inquiry, that each

of these provinces, thus conjecturally marked out, is actually inhabited

by a distinct nation of quadrupeds."[872] It will be observed that the

language of Buffon respecting "natural barriers," which has since been

so popular, would be wholly without meaning if the geographical

distribution of organic beings had not led naturalists to adopt very

generally _the doctrine of specific centres_, or, in other words, to

believe that each species, whether of plant or animal, originated in a

single birth-place. Reject this view, and the fact that not a single

native quadruped is common to Australia, the Cape of Good Hope, and

South America, can in no ways be explained by adverting to the wide

extent of intervening ocean, or to the sterile deserts, or the great

heat or cold of the climates, through which each species must have

passed, before it could migrate from one of those distant regions to

another. It might fairly be asked of one who talked of impassable

barriers, why the same kangaroos, rhinoceroses, or lamas, should not

have been created simultaneously in Australia, Africa, and South

America! The horse, the ox, and the dog, although foreign to these

countries until introduced by man, are now able to support themselves

there in a wild state, and we can scarcely doubt that many of the

quadrupeds at present peculiar to Australia, Africa, and South America,

might have continued in like manner to inhabit each of the three

continents had they been indigenous or could they once have got a

footing there as new colonists.

At the same time every zoologist will be willing to concede, that even

if the departure of each species from a single centre had not appeared

to be part of the plan of Nature, the range of species in general must

have become limited, under the influence of a variety of causes,

especially in the class of terrestrial mammalia. Scarcely any one of

these could be expected to retain as fair a claim to the title of

cosmopolite as man, although even the human race, fitted as it is by its

bodily constitution and intellectual resources to spread very widely

over the earth, is far from being strictly cosmopolite. It is excluded

both from the arctic and antarctic circles, from many a wide desert and

the summits of many mountain-chains; and lastly, from three-fourths of

the globe covered by water, where there are large areas very prolific in

animal life, even in the highest order of the vertebrate class. But the

_habitations_ of species are, as before stated, in reference to plants

(see above, p. 614), circumscribed by causes different from those which

determine their _stations_, and these causes are clearly connected with

the time and place of the original creation of each species.

As the names and characters of land quadrupeds are much better known to

the general reader than those of other great families of the animal

kingdom, I shall select this class to exemplify the zoological provinces

into which species are divisible, confining myself, however, to those

facts which may help to elucidate some principle, or rule apparently

followed by the Author of Nature, in regard to that "mystery of

mysteries," the first peopling of the earth with living beings.[873]

First, then, the _European region_ comprehends, besides Europe, the

borders of the Mediterranean, and even the north of Africa, and extends

into Asia, beyond the Oural mountains and the Caspian. Although the

species are almost all peculiar, the number of characteristic _genera_

is remarkably small. The bear, the fox, the hare, the rabbit, the deer,

and almost every European form is found equally in several of the other

large provinces of mammalia, where the species are distinct. Even the

mole (_Talpa_), although confined to the northern parts of the old

world, ranges eastwards, as far as the Himalaya mountains.

2dly. The _African_ Fauna, on the other hand, is singularly rich in

generic forms, not met with in a living state in any other region. The

hippopotamus, for example, of which two very distinct species are known,

the giraffe, the Chimpanzee, the blue-faced baboon, the four-fingered

monkeys (_Colubus_), many carnivora, such as _Proteles_, allied to the

hyaena, and a multitude of other forms, are exclusively African. A few of

the species inhabiting the northern confines of this continent, such as

the dromedary, lion, and jackall, are also common to Asia; and a much

larger number of _forms_ belong equally to the great Asiatic province,

the species being distinct. The elephant, for example, of Africa is

smaller, has a rounder head, and larger ears than the Indian one, and

has only three instead of four nails on each hind foot. In like manner,

not one of three African species of Rhinoceros agrees with one of the

three Indian kinds.

3dly. The _Southern_ region of _Africa_, where that continent extends

into the temperate zone, constitutes another separate zoological

province, surrounded as it is on three sides by the ocean, and cut off

from the countries of milder climate in the northern hemisphere, by the

intervening torrid zone. In many instances, this region contains the

same genera which are found in temperate climates to the northward of

the line: but then the southern are different from the northern species.

Thus, in the south we find the quagga and the zebra; in the north, the

horse, the ass, and the jiggetai of Asia.

The south of Africa is spread out into fine level plains from the tropic

to the Cape. In this region, says Pennant, besides the horse genus, of

which five species have been found, there are also peculiar species of

rhinoceros, the hog, and the hyrax, among pachydermatous races; and

amongst the ruminating, the Cape buffalo, and a variety of remarkable

antelopes, as the springbok, the oryx, the gnou, the leucophoa, the

pygarga, and several others.[874]

4thly. The assemblage of quadrupeds in _Madagascar_ affords a striking

illustration of the laws before alluded to, as governing the

distribution of species in islands. Separated from Africa by the

Mozambique channel, which is 300 miles wide, Madagascar forms, with two

or three small islands in its immediate vicinity, a zoological province

by itself, all the species except one, and nearly all the genera, being

peculiar. The only exception consists of a small insectivorous quadruped

(_Centetes_), found also in the Mauritius, to which place it is supposed

to have been taken in ships. The most characteristic feature of this

remarkable fauna consists in the number of quadrumana of the Lemur

family, no less than six genera of these monkeys being exclusively met

with in this island, and a seventh genus of the same, called _Galago_,

which alone has any foreign representative, being found, as we might

from analogy have anticipated, in the nearest main land. Had the species

of quadrupeds in Madagascar agreed with those of the contiguous parts of

Africa, as do those of England with the rest of Europe, the naturalist

would have inferred that there had been a land communication since the

period of the coming in of the existing quadrupeds, whereas we may now

conclude that the Mozambique channel has constituted an insuperable

barrier to the fusion of the continental fauna with that of the great

island during the whole period that has elapsed since the living species

were created.

5thly. Another of the great nations of terrestrial mammalia is that of

_India_, containing a great variety of peculiar forms, such as the

sloth-bear (_Prochilus_), the musk-deer (_Moscus_), the nylghau, the

gibbon or long-armed ape, and many others.

6thly. A portion of the islands of the _Indian archipelago_ might,

perhaps, be considered by some geologists as an appendage of the same

province. In fact, we find in the large islands of Java, Sumatra, and

Borneo, the same genera, for the most part, as on the continent of

India, and some of the same species, _e. g._ the tapir (_Tapirus

Malayanus_), the rhinoceros of Sumatra, and some others. Most of the

species, however, are distinct, and each island has many, and even a few

genera, peculiar to itself. Between eighty and ninety species are known

to inhabit Java, and nearly the same number occur in Sumatra. Of these,

more than half are common to the two islands. Borneo, which is much less

explored, has yielded already upwards of sixty species, more than half

of which are met with either in Java or Sumatra. Of the species

inhabiting Sumatra and not found in Java, Borneo contains the greater

portion. Upon the whole, if these three large islands were united, and a

fusion of their respective indigenous mammalia should take place, they

would present a fauna related to that of continental India, and

comprising about as many species as we might expect from analogy to

discover in an area of equal extent. The Philippine Islands are peopled

with another assemblage of species generically related to the great

Indian type.

7thly. But the islands of Celebes, Amboina, Timor, and _New Guinea_,

constitute a different region of mammalia more allied to the Australian

type, as having an intermixture of marsupial quadrupeds, yet showing an

affinity also to the Indian in such forms as the deer (_Cervus_), the

weasel (_Viverra_), the pig (_Sus_), the Macaque monkey

(_Cercopithecus_), and others. As we proceed in a south-westerly

direction, from Celebes to Amboina and thence to New Guinea, we find the

Indian types diminishing in number, and the Australian (_i. e._

marsupial forms) increasing. Thus in New Guinea seven species of pouched

quadrupeds have been detected, and among them two singular

tree-kangaroos; yet only one species of the whole seven, viz. the flying

opossum (_Petauris ariel_), is common to the Indian archipelago and the

main land of Australia. The greater the zoological affinity, therefore,

between the latter and the New Guinea fauna, although it seems in some

way connected with geographical proximity, is not to be explained simply

by the mutual migration of species from the one to the other.

8thly. When _Australia_ was discovered, its land quadrupeds, belonging

almost exclusively to the marsupial or pouched tribe, such as the

kangaroos, wombats, flying opossums, kangaroo-rats, and others, some

feeding on herbs and fruits, others carnivorous, were so novel in their

structure and aspect, that they appeared to the naturalist almost as

strange as if they were the inhabitants of some other planet. We learn

from the recent investigations of Mr. Waterhouse,[875] that no less than

170 species of marsupial quadrupeds have now been determined, and of the

whole number all but thirty-two are exclusively restricted to Australia.

Of these thirty-two, nine belong to the islands in the Indian

archipelago before mentioned, and the other twenty-three are all species

of opossum inhabiting the tropical parts of South America, or a few of

them extending into Mexico and California, and one, the Virginian

opossum, into the United States.

9thly. It only remains for me to say something of the mammiferous fauna

of _North_ and _South America_. It has often been said that, where the

three continents of Asia, Europe, and North America, approach very near

to each other towards the pole, the whole arctic region forms one

zoological and botanical province. The narrow straits which separate the

old and new world are frozen over in winter, and the distance is farther

lessened by intervening islands. Many plants and animals of various

classes have accordingly spread over all the arctic lands, being

sometimes carried in the same manner as the polar bear, when it is

drifted on floating ice from Greenland to Iceland. But on a close

inspection of the arctic mammalia, it has been found of late years that

a very small number of the American species are identical with those of

Europe or Asia. The genera are, in great part, the same or nearly

allied; but the species are rarely identical, and are often very unlike,

as in the case of the American badger and that of Europe. Some of the

_genera_ of arctic America, such as the musk ox (_Ovibos_), are quite

peculiar, and the distinctness of the fauna of the great continents goes

on increasing in proportion as we trace them southwards, or as they

recede farther from each other, and become more and more separated by

the ocean. At length we find that the three groups of tropical mammalia,

belonging severally to America, Africa, and India, have not a single

species in common.

The predominant influence of climate over all the other causes which

limit the range of species in the mammalia is perhaps nowhere so

conspicuously displayed as in North America. The arctic fauna, so

admirably described by Sir John Richardson, has scarcely any species in

common with the fauna of the state of New York, which is 600 miles

farther south, and comprises about forty distinct mammifers. If again we

travel farther south about 600 miles, and enter another zone, running

east and west, in South Carolina, Georgia, Alabama, and the contiguous

states, we again meet with a new assemblage of land quadrupeds, and this

again differs from the fauna of Texas, where frosts are unknown. It will

be observed that on this continent there are no great geographical

barriers running east and west, such as high snow-clad mountains, barren

deserts, or wide arms of the sea, capable of checking the free migration

of species from north to south. But notwithstanding the distinctness of

those zones of indigenous mammalia, there are some species, such as the

buffalo (_Bison Americanus_), the racoon (_Procyon lotor_), and the

Virginian opossum (_Didelphis Virginiana_), which have a wider

habitation, ranging almost from Canada to the Gulf of Mexico; but they

form exceptions to the general rule. The opossum of Texas (_Didelphis

carnivora_) is different from that of Virginia, and other species of the

same genus inhabit westward of the Rocky Mountains, in California, for

example, where almost all the mammalia differ from those of the United

States.

10thly. The _West Indian_ land quadrupeds are not numerous, but several

of them are peculiar; and 11thly, _South America_ is the most distinct,

with the exception of Australia, of all the provinces into which the

mammalia can be classed geographically. The various genera of monkeys,

for example, belong to the family Platyrrhini, a large natural division

of the quadrumana, so named from their widely separated nostrils. They

have a peculiar dentition, and many of them prehensile tails, and are

entirely unknown in other quarters of the globe. The sloths and

armadillos, the true blood-sucking bats or vampyres (_Phyllostomidae_),

the capybara, the largest of the rodents, the carnivorous coatimondi

(_Nasua_), and a great many other forms, are also exclusively

characteristic of South America.

"In Peru and Chili," says Humboldt, "the region of the grasses, which is

at an elevation of from 12,300 to 15,400 feet, is inhabited by crowds

of lama, guanaco, and alpaca. These quadrupeds, which here represent the

genus camel of the ancient continent, have not extended themselves

either to Brazil or Mexico; because, during their journey, they must

necessarily have descended into regions that were too hot for

them."[876] In this passage it will be seen that the doctrine of

"specific centres" is tacitly assumed.

_Quadrupeds in Islands._--Islands remote from continents, especially

those of small size, are either destitute of quadrupeds, except such as

have been conveyed to them by man, or contain species peculiar to them.

In the Galapagos archipelago no indigenous quadrupeds were found except

one mouse, which is supposed to be distinct from any hitherto found

elsewhere. A peculiar species of fox is indigenous in the Falkland

Islands, and a rat in New Zealand, which last country, notwithstanding

its magnitude, is destitute of other mammalia, except bats, and these,

says Dr. Prichard, may have made their way along the chain of islands

which extend from the shores of New Guinea far into the Southern

Pacific. The same author remarks, that among the various groups of

fertile islands in the Pacific, no quadrupeds have been met with except

the rat and a few bats as above mentioned, and the dog and hog, which

appear to have been conveyed thither by the natives from New Guinea.

"Rats are to be found even on some desert islands, whither they may have

been conveyed by canoes which have occasionally approached the shore. It

is known, also, that rats occasionally swim in large numbers to

considerable distances."[877]

_Geographical range of the Cetacea._--It is natural to suppose that the

geographical range of the different species of Cetacea should be less

correctly ascertained than that of the terrestrial mammifers. It is,

however, well known that the whales which are obtained by our fishers in

the South Seas are distinct from those of the North; and the same

dissimilarity has been found in all the other marine animals, of the

same class, so far as they have yet been studied by naturalists.

_Dispersion of quadrupeds._--Let us now inquire what facilities the

various land quadrupeds enjoy of spreading themselves over the surface

of the earth. In the first place, as their numbers multiply, all of

them, whether they feed on plants, or prey on other animals, are

disposed to scatter themselves gradually over as wide an area as is

accessible to them. But before they have extended their migrations over

a large space, they are usually arrested either by the sea, or a zone of

uncongenial climate, or some lofty and unbroken chain of mountains, or a

tract already occupied by a hostile and more powerful species.

_Their powers of swimming._--Rivers and narrow friths can seldom

interfere with their progress; for the greater part of them swim well,

and few are without this power when urged by danger and pressing want.

Thus, amongst beasts of prey, the tiger is seen swimming about among

the islands and creeks in the delta of the Ganges, and the jaguar

traverses with ease the largest streams in South America.[878] The bear,

also, and the bison, cross the current of the Mississippi. The popular

error, that the common swine cannot escape by swimming when thrown into

the water, has been contradicted by several curious and

well-authenticated instances during the floods in Scotland of 1829. One

pig, only six months old, after having been carried down from Garmouth

to the bar at the mouth of the Spey, a distance of a quarter of a mile,

swam four miles eastward to Port Gordon, and landed safe. Three others,

of the same age and litter, swam, at the same time, five miles to the

west, and landed at Blackhill.[879]

In an adult and wild state, these animals would doubtless have been more

strong and active, and might, when hard pressed, have performed a much

longer voyage. Hence islands remote from the continent may obtain

inhabitants by casualties which, like the late storms in Morayshire, may

only occur once in many centuries, or thousands of years, under all the

same circumstances. It is obvious that powerful tides, winds, and

currents may sometimes carry along quadrupeds capable, in like manner,

of preserving themselves for hours in the sea, to very considerable

distances; and in this way, perhaps, the tapir (_Tapir Indicus_) may

have become common to Sumatra and the Malayan peninsula.

To the elephant, in particular, the power of crossing rivers is

essential in a wild state, for the quantity of food which a herd of

these animals consumes renders it necessary that they should be

constantly moving from place to place. The elephant crosses the stream

in two ways. If the bed of the river be hard, and the water not of too

great a depth, he fords it. But when he crosses great rivers, such as

the Ganges and the Niger, the elephant swims deep, so deep, that the end

of his trunk only is out of the water; for it is a matter of

indifference to him whether his body be completely immersed, provided he

can bring the tip of his trunk to the surface, so as to breathe the

external air.

Animals of the deer kind frequently take to the water, especially in the

rutting season, when the stags are seen, swimming for several leagues at

a time, from island to island, in search of the does, especially in the

Canadian lakes; and in some countries where there are islands near the

sea-shore, they fearlessly enter the sea and swim to them. In hunting

excursions, in North America, the elk of that country is frequently

pursued for great distances through the water.

The large herbivorous animals, which are gregarious, can never remain

long in a confined region, as they consume so much vegetable food. The

immense herds of bisons (_Bos Americanus_) which often, in the great

valleys of the Mississippi and its tributaries blacken the surface of

the prairie lands, are continually shifting their quarters, followed by

wolves, which prowl about in their rear. "It is no exaggeration," says

Mr. James, "to assert, that in one place, on the banks of the Platte, at

least ten thousand bisons burst on our sight in an instant. In the

morning, we again sought the living picture; but upon all the plain,

which last evening was so teeming with noble animals, not one

remained."[880]

_Migratory instincts._--Besides the disposition common to the

individuals of every species slowly to extend their range in search of

food, in proportion as their numbers augment, a migratory instinct often

developes itself in an extraordinary manner, when, after an unusually

prolific season, or upon a sudden scarcity of provisions, great

multitudes are threatened by famine. It may be useful to enumerate some

examples of these migrations, because they may put us upon our guard

against attributing a high antiquity to a particular species merely

because it is diffused over a great space; they show clearly how soon,

in a state of nature, a newly created species might spread itself, in

every direction, from a single point.

In very severe winters, great numbers of the black bears of America

migrate from Canada into the United States; but in milder seasons, when

they have been well fed, they remain and hybernate in the north.[881]

The rein-deer, which, in Scandinavia, can scarcely exist to the south of

the sixty-fifth parallel, descends, in consequence of the greater

coldness of the climate, to the fiftieth degree in Chinese Tartary, and

often roves into a country of more southern latitude than any part of

England.

In Lapland, and other high latitudes, the common squirrels, whenever

they are compelled, by want of provisions, to quit their usual abodes,

migrate in amazing numbers, and travel directly forwards, allowing

neither rocks nor forests, nor the broadest waters, to turn them from

their course. Great numbers are often drowned in attempting to pass

friths and rivers. In like manner the small Norway rat sometimes pursues

its migrations in a straight line across rivers and lakes; and Pennant

informs us, that when the rats, in Kamtschatka, become too numerous,

they gather together in the spring, and proceed in great bodies

westward, swimming over rivers, lakes, and arms of the sea. Many are

drowned or destroyed by water-fowl or fish. As soon as they have crossed

the river Penginsk, at the head of the gulf of the same name, they turn

southward, and reach the rivers Judoma and Okotsk by the middle of July;

a district more than 800 miles distant from their point of departure.

The lemings, also, a small kind of rat, are described as natives of the

mountains of Kolen, in Lapland; and once or twice in a quarter of a

century they appear in vast numbers, advancing along the ground, and

"devouring every green thing." Innumerable bands march from the Kolen,

through Nordland and Finmark, to the Western Ocean, which they

immediately enter; and after swimming about for some time, perish. Other

bands take their route through Swedish Lapland, to the Bothnian Gulf,

where they are drowned in the same manner. They are followed in their

journeys by bears, wolves, and foxes, which prey upon them incessantly.

They generally move in lines, which are about three feet from each

other, and exactly parallel, going directly forward through rivers and

lakes; and when they meet with stacks of hay or corn, gnawing their way

through them instead of passing round.[882] These excursions usually

precede a rigorous winter, of which the lemings seem in some way

forewarned.

[Illustration: Fig. 97.

The Leming, or Lapland Marmot (Mus Lemmus, Linn.)]

Vast troops of the wild ass, or _onager_ of the ancients, which inhabit

the mountainous deserts of Great Tartary, feed, during the summer, in

the tracts east and north of Lake Aral. In the autumn they collect in

herds of hundreds, and even thousands, and direct their course towards

Persia, to enjoy a warm retreat during winter.[883] Bands of two or

three hundred quaggas, a species of wild ass, are sometimes seen to

migrate from the tropical plains of southern Africa to the vicinity of

the Malaleveen River. During their migrations they are followed by

lions, who slaughter them night by night.[884]

The migratory swarms of the springbok, or Cape antelope, afford another

illustration of the rapidity with which a species under certain

circumstances may be diffused over a continent. When the stagnant pools

of the immense deserts south of the Orange River dry up, which often

happens after intervals of three or four years, myriads of these animals

desert the parched soil, and pour down like a deluge on the cultivated

regions near the Cape. The havoc committed by them resembles that of the

African locusts; and so crowded are the herds, that "the lion has been

seen to walk in the midst of the compressed phalanx with only as much

room between him and his victims as the fears of those immediately

around could procure by pressing outwards."[885]

[Illustration: Fig. 98.

Mydaus meliceps, or badger-headed Mydaus. Length, including the tail, 16

inches.]

Dr. Horsfield mentions a singular fact in regard to the geographical

distribution of the _Mydaus meliceps_, an animal intermediate between

the polecat and badger. It inhabits Java, and is "confined exclusively

to those mountains which have an elevation of more than seven thousand

feet above the level of the ocean; on these it occurs with the same

regularity as many plants. The long extended surface of Java, abounding

with conical points which exceed this elevation, affords many places

favorable for its resort. On ascending these mountains, the traveller

scarcely fails to meet with this animal, which, from its peculiarities,

is universally known to the inhabitants of these elevated tracts, while

to those of the plains it is as strange to an animal from a foreign

county. In my visits to the mountainous districts, I uniformly met with

it; and, as far as the information of the natives can be relied on, it

is found on all the mountains."[886]

Now, if asked to conjecture how the Mydaus arrived at the elevated

regions of each of these isolated mountains, we might say that, before

the island was peopled by man, by whom their numbers are now thinned,

they may occasionally have multiplied so as to be forced to collect

together and migrate: in which case notwithstanding the slowness of

their motions, some few would succeed in reaching another mountain, some

twenty, or even, perhaps, fifty miles distant; for although the climate

of the hot intervening plains would be unfavourable to them, they might

support it for a time, and would find there abundance of insects on

which they feed. Volcanic eruptions, which, at different times have

covered the summits of some of those lofty cones with sterile sand and

ashes, may have occasionally contributed to force on these migrations.

_Drifting of animals on ice-floes._--The power of the terrestrial

mammalia to cross the sea is very limited, and it was before stated that

the same species is scarcely ever common to districts widely separated

by the ocean. If there be some exceptions to this rule, they generally

admit of explanation; for there are natural means whereby some animals

may be floated across the water, and the sea may in the course of ages

wear a wide passage through a neck of land, leaving individuals of a

species on each side of the new channel. Polar bears are known to have

been frequently drifted on the ice from Greenland to Iceland; they can

also swim to considerable distances, for Captain Parry, on the return of

his ships through Barrow's Straits, met with a bear swimming in the

water about midway between the shores, which were about forty miles

apart, and where no ice was in sight.[887] "Near the east coast of

Greenland," observes Scoresby, "they have been seen on the ice in such

quantities, that they were compared to flocks of sheep on a common; and

they are often found on field-ice, above two hundred miles from the

shore."[888] Wolves, in the arctic regions, often venture upon the ice

near the shore, for the purpose of preying upon young seals which they

surprise when asleep. When these ice-floes get detached, the wolves are

often carried out to sea; and though some may be drifted to islands or

continents, the greater part of them perish, and have been often heard

in this situation howling dreadfully, as they die by famine.[889]

During the short summer which visits Melville Island, various plants

push forth their leaves and flowers the moment the snow is off the

ground, and form a carpet spangled with the most lively colours. These

secluded spots are reached annually by herds of musk-oxen and reindeer,

which travel immense distances over dreary and desolate regions, to

graze undisturbed on these luxuriant pastures.[890] The rein-deer often

pass along in the same manner, by the chain of the Aleutian Islands,

from Behring's Straits to Kamtschatka, subsisting on the moss found in

these islands during their passage.[891] But the musk-ox,

notwithstanding its migratory habits, and its long journeys over the

ice, does not exist, either in Asia or Greenland.[892]

_On floating islands of drift-wood._--Within the tropics there are no

ice-floes; but, as if to compensate for that mode of transportation,

there are floating islets of matted trees, which are often borne along

through considerable spaces. These are sometimes seen sailing at the

distance of fifty or one hundred miles from the mouth of the Ganges,

with living trees standing erect upon them. The Amazon, the Congo, and

the Orinoco, also produce these verdant rafts, which are formed in the

manner already described when speaking of the great raft of the

Atchafalaya, an arm of the Mississippi, where a natural bridge of

timber, ten miles long, and more than two hundred yards wide, existed

for more than forty years, supporting a luxuriant vegetation, and rising

and sinking with the water which flowed beneath it.

On these green islets of the Mississippi, observes Malte-Brun, young

trees take root, and the pistia and nenuphar display their yellow

flowers: serpents, birds, and the cayman alligator, come to repose

there, and all are sometimes carried to the sea and engulphed in its

waters.[893]

Spix and Martius relate that, during their travels in Brazil, they were

exposed to great danger while ascending the Amazon in a canoe, from the

vast quantity of drift-wood constantly propelled against them by the

current; so much so, that their safety depended on the crew being always

on the alert to turn aside the trunks of trees with long poles. The tops

alone of some trees appeared above water, others had their roots

attached to them with so much soil that they might be compared to

floating islets. On these, say the travellers, we saw some very singular

assemblages of animals, pursuing peacefully their uncertain way in

strange companionship. On one raft were several grave-looking storks,

perched by the side of a party of monkeys, who made comical gestures,

and burst into loud cries, on seeing the canoe. On another was seen a

number of ducks and divers, sitting by a group of squirrels. Next came

down upon the stem of a large rotten cedar tree, an enormous crocodile,

by the side of a tiger-cat, both animals regarding each other with

hostility and mistrust, but the saurian being evidently most at his

ease, as conscious of his superior strength.[894]

Similar green rafts, principally composed of canes and brushwood, are

called "camelotes" on the Parana in South America; and they are

occasionally carried down by inundations, bearing on them the tiger,

cayman, squirrels, and other quadrupeds, which are said to be always

terror-stricken on their floating habitation. No less than four tigers

(pumas) were landed in this manner in one night at Monte Video, lat. 35

degrees S., to the great alarm of the inhabitants, who found them

prowling about the streets in the morning.[895]

In a memoir lately published, a naval officer relates that, as he

returned from China by the eastern passage, he fell in, among the

Moluccas, with several small floating islands of this kind, covered with

mangrove trees interwoven with underwood. The trees and shrubs retained

their verdure, receiving nourishment from a stratum of soil which formed

a white beach round the margin of each raft, where it was exposed to the

washing of the waves and the rays of the sun.[896] The occurrence of

soil in such situations may easily be explained; for all the natural

bridges of timber which occasionally connect the islands of the Ganges,

Mississippi, and other rivers, with their banks, are exposed to floods

of water, densely charged with sediment.

Captain W. H. Smyth informs me, that, when cruising in the Cornwallis

amidst the Philippine Islands, he has more than once seen, after those

dreadful hurricanes called typhoons, floating masses of wood, with

trees growing upon them, and ships have sometimes been in imminent

peril, as often as these islands were mistaken for terra firma, when, in

fact, they were in rapid motion.

It is highly interesting to trace, in imagination, the effects of the

passage of these rafts from the mouth of a large river to some

archipelago, such as those in the South Pacific, raised from the deep,

in comparatively modern times, by the operations of the volcano and the

earthquake, and the joint labours of coral animals and testacea. If a

storm arise, and the frail vessel be wrecked, still many a bird and

insect may succeed in gaining, by flight, some island of the newly

formed group, while the seeds and berries of herbs and shrubs, which

fall into the waves, may be thrown upon the strand. But if the surface

of the deep be calm, and the rafts are carried along by a current, or

wafted by some slight breath of air fanning the foliage of the green

trees, it may arrive, after a passage of several weeks, at the bay of an

island, into which its plants and animals may be poured out as from an

ark, and thus a colony of several hundred new species may at once be

naturalized.

The reader should be reminded, that I merely advert to the

transportation of these rafts as of extremely rare and accidental

occurrence; but it may account, in tropical countries, for some of the

rare exceptions to the general law of the confined range of mammiferous

species.

_Migrations of the Cetacea._--Many of the Cetacea, the whales of the

northern seas for example, are found to desert one tract of the sea, and

to visit another very distant, when they are urged by want of food, or

danger. The seals also retire from the coast of Greenland in July,

return again in September, and depart again in March, to return in June.

They proceed in great droves northwards, directing their course where

the sea is most free from ice, and are observed to be extremely fat when

they set out on this expedition, and very lean when they come home

again.[897]

_Species of the Mediterranean, Black Sea, and Caspian identical._--Some

naturalists have wondered that the sea-calves, dolphins, and other

marine mammalia of the Mediterranean and Black Sea, should be identical

with those found in the Caspian: and among other fanciful theories, they

have suggested that they may dive through subterranean conduits, and

thus pass from one sea into the other. But as the occurrence of wolves

and other noxious animals, on both sides of the British Channel, was

adduced, by Verstegan and Desmarest, as one of many arguments to prove

that England and France were once united; so the correspondence of the

aquatic species of the inland seas of Asia with those of the Black Sea

tend to confirm the hypothesis, for which there are abundance of

independent geological data, that those seas were connected together by

straits at no remote period of the earth's history.

_Geographical Distribution and Migrations of Birds._

I shall now offer a few observations on some of the other divisions of

the animal kingdom. Birds, notwithstanding their great locomotive

powers, form no exception to the general rules already laid down; but,

in this class, as in plants and terrestrial quadrupeds, different groups

of species are circumscribed within definite limits. We find, for

example, one assemblage in the Brazils, another in the same latitudes in

Central Africa, another in India, and a fourth in New Holland. Of

twenty-six different species of land birds found in the Galapagos

archipelago, all, with the exception of one, are distinct from those

inhabiting other parts of the globe;[898] and in other archipelagos a

single island sometimes contains a species found in no other spot on the

whole earth; as is exemplified in some of the parrot tribes. In this

extensive family, which are, with few exceptions, inhabitants of

tropical regions, the American group has not one in common with the

African, nor either of these with the parrots of India.[899]

Another illustration is afforded by that minute and beautiful tribe, the

humming-birds. The whole of them are, in the first place, peculiar to

the new world; but some species are confined to Mexico, while others

exist only in some of the West India Islands, and have not been found

elsewhere in the western hemisphere. Yet there are species of this

family which have a vast range, as the _Trochilus flammifrons_ (or

_Mellisuga Kingii_), which is found over a space of 2500 miles on the

west coast of South America, from the hot dry country of Lima to the

humid forests of Tierra del Fuego. Captain King, during his survey in

the years 1826-30, found this bird at the Straits of Magellan, in the

month of May--the depth of winter--sucking the flowers of a large

species of fuchsia, then in bloom, in the midst of a shower of snow.

The ornithology of our own country affords one well-known and striking

exemplification of the law of a limited specific range; for the common

grouse (_Tetra scoticus_) occurs nowhere in the known world except in

the British isles.

Some species of the vulture tribe are said to be cosmopolites; and the

common wild goose (_Anas anser_, Linn.), if we may believe some

ornithologists, is a general inhabitant of the globe, being met with

from Lapland to the Cape of Good Hope, frequent in Arabia, Persia,

China, and Japan, and in the American continent from Hudson's Bay to

South Carolina.[900] An extraordinary range has also been attributed to

the nightingale, which extends from western Europe to Persia, and still

farther. In a work entitled Specchio Comparativo,[901] by Charles

Bonaparte, many species of birds are enumerated as common to Rome and

Philadelphia: the greater part of these are migratory, but some of them,

such as the long-eared owl (_Strix otus_), are permanent in both

countries. The correspondence of the ornithological fauna of the eastern

and western hemispheres increases considerably, as might have been

anticipated, in high northern latitudes.[902]

_Their facilities of diffusion._--In parallel zones of the northern and

southern hemispheres, a great general correspondence of form is

observable, both in the aquatic and terrestrial birds; but there is

rarely any specific identity; and this phenomenon is truly remarkable,

when we recollect the readiness with which some birds, not gifted with

great powers of flight, shift their quarters to different regions, and

the facility with which others, possessing great strength of wing,

perform their aarial voyage. Some migrate periodically from high

latitudes, to avoid the cold of winter, and the accompaniments of

cold,--scarcity of insects and vegetable food; others, it is said, for

some particular kinds of nutriment required for rearing their young: for

this purpose they often traverse the ocean for thousands of miles, and

recross it at other periods, with equal security.

Periodical migrations, no less regular, are mentioned by Humboldt, of

many American water-fowl, from one part of the tropics to another, in a

zone where there is the same temperature throughout the year. Immense

flights of ducks leave the valley of the Orinoco, when the increasing

depth of its waters and the flooding of its shores prevent them from

catching fish, insects, and aquatic worms. They then betake themselves

to the Rio Negro and Amazon, having passed from the eighth and third

degrees of north latitude to the first and fourth of south latitude,

directing their course south-south-east. In September, when the Orinoco

decreases and re-enters its channel, these birds return

northwards.[903]

The insectivorous swallows which visit our island would perish during

winter, if they did not annually repair to warmer climes. It is supposed

that in these aerial excursions the average rapidity of their flight is

not less than fifty miles an hour; so that, when aided by the wind, they

soon reach warmer latitudes. Spallanzani calculated that the swallow can

fly at the rate of ninety-two miles an hour, and conceived that the

rapidity of the swift might be three times greater.[904] The rate of

flight of the eider duck (_Anas mollissima_) is said to be ninety miles

an hour; and Bachman says that the hawk, wild pigeon (_Columba

migratoria_), and several species of wild ducks, in North America, fly

at the rate of forty miles an hour, or nearly a thousand miles in

twenty-four hours.[905]

When we reflect how easily different species, in a great lapse of ages,

may be each overtaken by gales and hurricanes, and, abandoning

themselves to the tempest, be scattered at random through various

regions of the earth's surface, where the temperature of the atmosphere,

the vegetation, and the animal productions, might be suited to their

wants, we shall be prepared to find some species capriciously

distributed, and to be sometimes unable to determine the native

countries of each. Captain Smyth informs me, that, when engaged in his

survey of the Mediterranean, he encountered a gale in the Gulf of Lyons,

at the distance of between twenty and thirty leagues from the coast of

France, which bore along many land birds of various species, some of

which alighted on the ship, while others were thrown with violence

against the sails. In this manner islands become tenanted by species of

birds inhabiting the nearest mainland.

_Geographical Distribution and Dissemination of Reptiles._

A few facts respecting the third great class of vertebrated animals will

suffice to show that the plan of nature in regard to their location on

the globe is perfectly analogous to that already exemplified in other

parts of the organic creation, and has probably been determined by

similar causes.

_Habitations of reptiles._--Of the great saurians, the gavials which

inhabit the Ganges differ from the cayman of America, or the crocodile

of the Nile. The monitor of New Holland is specifically distinct from

the Indian species; these latter, again, from the African, and all from

their congeners in the new world. So in regard to snakes; we find the

boa of America represented by the python, a different though nearly

allied genus in India. America is the country of the rattlesnake;

Africa, of the cerastes; and Asia, of the hooded snake, or cobra di

capello. The amphibious genera Siren and Menopoma belong to North

America, possessing both lungs and gills, and respiring at pleasure

either air or water. The only analogous animal of the old world is the

_Proteus anguinus_ of the lakes of Lower Carniola, and the grotto of

Adelsberg between Trieste and Vienna.[906]

There is a legend that St. Patrick expelled all reptiles from Ireland;

and certain it is that none of the three species of snakes common in

England, nor the toad, have been observed there by naturalists. They

have our common frog, and our water-newt, and according to Ray (Quad.

264.), the green lizard (_Lacerta viridis_).

_Migrations of the larger reptiles._--The range of the large reptiles

is, in general, quite as limited as that of some orders of the

terrestrial mammalia. The great saurians sometimes cross a considerable

tract in order to pass from one river to another; but their motions by

land are generally slower than those of quadrupeds. By water, however,

they may transport themselves to distant situations more easily. The

larger alligator of the Ganges sometimes descends beyond the brackish

water of the delta into the sea; and in such cases it might chance to be

drifted away by a current, and survive till it reached a shore at some

distance; but such casualties are probably very rare.