CHAPTER XV.

RECAPITULATION AND CONCLUSION.

Recapitulation of the objections to the theory of Natural

Selection—Recapitulation of the general and special circumstances in

its favour—Causes of the general belief in the immutability of

species—How far the theory of Natural Selection may be extended—Effects

of its adoption on the study of Natural History—Concluding remarks.

As this whole volume is one long argument, it may be convenient to the

reader to have the leading facts and inferences briefly recapitulated.

That many and serious objections may be advanced against the theory of

descent with modification through variation and natural selection, I do

not deny. I have endeavoured to give to them their full force. Nothing

at first can appear more difficult to believe than that the more

complex organs and instincts have been perfected, not by means superior

to, though analogous with, human reason, but by the accumulation of

innumerable slight variations, each good for the individual possessor.

Nevertheless, this difficulty, though appearing to our imagination

insuperably great, cannot be considered real if we admit the following

propositions, namely, that all parts of the organisation and instincts

offer, at least individual differences—that there is a struggle for

existence leading to the preservation of profitable deviations of

structure or instinct—and, lastly, that gradations in the state of

perfection of each organ may have existed, each good of its kind. The

truth of these propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what

gradations many structures have been perfected, more especially among

broken and failing groups of organic beings, which have suffered much

extinction; but we see so many strange gradations in nature, that we

ought to be extremely cautious in saying that any organ or instinct, or

any whole structure, could not have arrived at its present state by

many graduated steps. There are, it must be admitted, cases of special

difficulty opposed to the theory of natural selection; and one of the

most curious of these is the existence in the same community of two or

three defined castes of workers or sterile female ants; but I have

attempted to show how these difficulties can be mastered.

With respect to the almost universal sterility of species when first

crossed, which forms so remarkable a contrast with the almost universal

fertility of varieties when crossed, I must refer the reader to the

recapitulation of the facts given at the end of the ninth chapter,

which seem to me conclusively to show that this sterility is no more a

special endowment than is the incapacity of two distinct kinds of trees

to be grafted together; but that it is incidental on differences

confined to the reproductive systems of the intercrossed species. We

see the truth of this conclusion in the vast difference in the results

of crossing the same two species reciprocally—that is, when one species

is first used as the father and then as the mother. Analogy from the

consideration of dimorphic and trimorphic plants clearly leads to the

same conclusion, for when the forms are illegitimately united, they

yield few or no seed, and their offspring are more or less sterile; and

these forms belong to the same undoubted species, and differ from each

other in no respect except in their reproductive organs and functions.

Although the fertility of varieties when intercrossed, and of their

mongrel offspring, has been asserted by so many authors to be

universal, this cannot be considered as quite correct after the facts

given on the high authority of Gärtner and Kölreuter. Most of the

varieties which have been experimented on have been produced under

domestication; and as domestication (I do not mean mere confinement)

almost certainly tends to eliminate that sterility which, judging from

analogy, would have affected the parent-species if intercrossed, we

ought not to expect that domestication would likewise induce sterility

in their modified descendants when crossed. This elimination of

sterility apparently follows from the same cause which allows our

domestic animals to breed freely under diversified circumstances; and

this again apparently follows from their having been gradually

accustomed to frequent changes in their conditions of life.

A double and parallel series of facts seems to throw much light on the

sterility of species, when first crossed, and of their hybrid

offspring. On the one side, there is good reason to believe that slight

changes in the conditions of life give vigour and fertility to all

organic beings. We know also that a cross between the distinct

individuals of the same variety, and between distinct varieties,

increases the number of their offspring, and certainly gives to them

increased size and vigour. This is chiefly owing to the forms which are

crossed having been exposed to somewhat different conditions of life;

for I have ascertained by a labourious series of experiments that if

all the individuals of the same variety be subjected during several

generations to the same conditions, the good derived from crossing is

often much diminished or wholly disappears. This is one side of the

case. On the other side, we know that species which have long been

exposed to nearly uniform conditions, when they are subjected under

confinement to new and greatly changed conditions, either perish, or if

they survive, are rendered sterile, though retaining perfect health.

This does not occur, or only in a very slight degree, with our

domesticated productions, which have long been exposed to fluctuating

conditions. Hence when we find that hybrids produced by a cross between

two distinct species are few in number, owing to their perishing soon

after conception or at a very early age, or if surviving that they are

rendered more or less sterile, it seems highly probable that this

result is due to their having been in fact subjected to a great change

in their conditions of life, from being compounded of two distinct

organisations. He who will explain in a definite manner why, for

instance, an elephant or a fox will not breed under confinement in its

native country, whilst the domestic pig or dog will breed freely under

the most diversified conditions, will at the same time be able to give

a definite answer to the question why two distinct species, when

crossed, as well as their hybrid offspring, are generally rendered more

or less sterile, while two domesticated varieties when crossed and

their mongrel offspring are perfectly fertile.

Turning to geographical distribution, the difficulties encountered on

the theory of descent with modification are serious enough. All the

individuals of the same species, and all the species of the same genus,

or even higher group, are descended from common parents; and therefore,

in however distant and isolated parts of the world they may now be

found, they must in the course of successive generations have travelled

from some one point to all the others. We are often wholly unable even

to conjecture how this could have been effected. Yet, as we have reason

to believe that some species have retained the same specific form for

very long periods of time, immensely long as measured by years, too

much stress ought not to be laid on the occasional wide diffusion of

the same species; for during very long periods there will always have

been a good chance for wide migration by many means. A broken or

interrupted range may often be accounted for by the extinction of the

species in the intermediate regions. It cannot be denied that we are as

yet very ignorant as to the full extent of the various climatical and

geographical changes which have affected the earth during modern

periods; and such changes will often have facilitated migration. As an

example, I have attempted to show how potent has been the influence of

the Glacial period on the distribution of the same and of allied

species throughout the world. We are as yet profoundly ignorant of the

many occasional means of transport. With respect to distinct species of

the same genus, inhabiting distant and isolated regions, as the process

of modification has necessarily been slow, all the means of migration

will have been possible during a very long period; and consequently the

difficulty of the wide diffusion of the species of the same genus is in

some degree lessened.

As according to the theory of natural selection an interminable number

of intermediate forms must have existed, linking together all the

species in each group by gradations as fine as our existing varieties,

it may be asked, Why do we not see these linking forms all around us?

Why are not all organic beings blended together in an inextricable

chaos? With respect to existing forms, we should remember that we have

no right to expect (excepting in rare cases) to discover _directly_

connecting links between them, but only between each and some extinct

and supplanted form. Even on a wide area, which has during a long

period remained continuous, and of which the climatic and other

conditions of life change insensibly in proceeding from a district

occupied by one species into another district occupied by a closely

allied species, we have no just right to expect often to find

intermediate varieties in the intermediate zones. For we have reason to

believe that only a few species of a genus ever undergo change; the

other species becoming utterly extinct and leaving no modified progeny.

Of the species which do change, only a few within the same country

change at the same time; and all modifications are slowly effected. I

have also shown that the intermediate varieties which probably at first

existed in the intermediate zones, would be liable to be supplanted by

the allied forms on either hand; for the latter, from existing in

greater numbers, would generally be modified and improved at a quicker

rate than the intermediate varieties, which existed in lesser numbers;

so that the intermediate varieties would, in the long run, be

supplanted and exterminated.

On this doctrine of the extermination of an infinitude of connecting

links, between the living and extinct inhabitants of the world, and at

each successive period between the extinct and still older species, why

is not every geological formation charged with such links? Why does not

every collection of fossil remains afford plain evidence of the

gradation and mutation of the forms of life? Although geological

research has undoubtedly revealed the former existence of many links,

bringing numerous forms of life much closer together, it does not yield

the infinitely many fine gradations between past and present species

required on the theory, and this is the most obvious of the many

objections which may be urged against it. Why, again, do whole groups

of allied species appear, though this appearance is often false, to

have come in suddenly on the successive geological stages? Although we

now know that organic beings appeared on this globe, at a period

incalculably remote, long before the lowest bed of the Cambrian system

was deposited, why do we not find beneath this system great piles of

strata stored with the remains of the progenitors of the Cambrian

fossils? For on the theory, such strata must somewhere have been

deposited at these ancient and utterly unknown epochs of the world’s

history.

I can answer these questions and objections only on the supposition

that the geological record is far more imperfect than most geologists

believe. The number of specimens in all our museums is absolutely as

nothing compared with the countless generations of countless species

which have certainly existed. The parent form of any two or more

species would not be in all its characters directly intermediate

between its modified offspring, any more than the rock-pigeon is

directly intermediate in crop and tail between its descendants, the

pouter and fantail pigeons. We should not be able to recognise a

species as the parent of another and modified species, if we were to

examine the two ever so closely, unless we possessed most of the

intermediate links; and owing to the imperfection of the geological

record, we have no just right to expect to find so many links. If two

or three, or even more linking forms were discovered, they would simply

be ranked by many naturalists as so many new species, more especially

if found in different geological substages, let their differences be

ever so slight. Numerous existing doubtful forms could be named which

are probably varieties; but who will pretend that in future ages so

many fossil links will be discovered, that naturalists will be able to

decide whether or not these doubtful forms ought to be called

varieties? Only a small portion of the world has been geologically

explored. Only organic beings of certain classes can be preserved in a

fossil condition, at least in any great number. Many species when once

formed never undergo any further change but become extinct without

leaving modified descendants; and the periods during which species have

undergone modification, though long as measured by years, have probably

been short in comparison with the periods during which they retained

the same form. It is the dominant and widely ranging species which vary

most frequently and vary most, and varieties are often at first

local—both causes rendering the discovery of intermediate links in any

one formation less likely. Local varieties will not spread into other

and distant regions until they are considerably modified and improved;

and when they have spread, and are discovered in a geological

formation, they appear as if suddenly created there, and will be simply

classed as new species. Most formations have been intermittent in their

accumulation; and their duration has probably been shorter than the

average duration of specific forms. Successive formations are in most

cases separated from each other by blank intervals of time of great

length, for fossiliferous formations thick enough to resist future

degradation can, as a general rule, be accumulated only where much

sediment is deposited on the subsiding bed of the sea. During the

alternate periods of elevation and of stationary level the record will

generally be blank. During these latter periods there will probably be

more variability in the forms of life; during periods of subsidence,

more extinction.

With respect to the absence of strata rich in fossils beneath the

Cambrian formation, I can recur only to the hypothesis given in the

tenth chapter; namely, that though our continents and oceans have

endured for an enormous period in nearly their present relative

positions, we have no reason to assume that this has always been the

case; consequently formations much older than any now known may lie

buried beneath the great oceans. With respect to the lapse of time not

having been sufficient since our planet was consolidated for the

assumed amount of organic change, and this objection, as urged by Sir

William Thompson, is probably one of the gravest as yet advanced, I can

only say, firstly, that we do not know at what rate species change, as

measured by years, and secondly, that many philosophers are not as yet

willing to admit that we know enough of the constitution of the

universe and of the interior of our globe to speculate with safety on

its past duration.

That the geological record is imperfect all will admit; but that it is

imperfect to the degree required by our theory, few will be inclined to

admit. If we look to long enough intervals of time, geology plainly

declares that species have all changed; and they have changed in the

manner required by the theory, for they have changed slowly and in a

graduated manner. We clearly see this in the fossil remains from

consecutive formations invariably being much more closely related to

each other than are the fossils from widely separated formations.

Such is the sum of the several chief objections and difficulties which

may justly be urged against the theory; and I have now briefly

recapitulated the answers and explanations which, as far as I can see,

may be given. I have felt these difficulties far too heavily during

many years to doubt their weight. But it deserves especial notice that

the more important objections relate to questions on which we are

confessedly ignorant; nor do we know how ignorant we are. We do not

know all the possible transitional gradations between the simplest and

the most perfect organs; it cannot be pretended that we know all the

varied means of Distribution during the long lapse of years, or that we

know how imperfect is the Geological Record. Serious as these several

objections are, in my judgment they are by no means sufficient to

overthrow the theory of descent with subsequent modification.

Now let us turn to the other side of the argument. Under domestication

we see much variability, caused, or at least excited, by changed

conditions of life; but often in so obscure a manner, that we are

tempted to consider the variations as spontaneous. Variability is

governed by many complex laws, by correlated growth, compensation, the

increased use and disuse of parts, and the definite action of the

surrounding conditions. There is much difficulty in ascertaining how

largely our domestic productions have been modified; but we may safely

infer that the amount has been large, and that modifications can be

inherited for long periods. As long as the conditions of life remain

the same, we have reason to believe that a modification, which has

already been inherited for many generations, may continue to be

inherited for an almost infinite number of generations. On the other

hand we have evidence that variability, when it has once come into

play, does not cease under domestication for a very long period; nor do

we know that it ever ceases, for new varieties are still occasionally

produced by our oldest domesticated productions.

Variability is not actually caused by man; he only unintentionally

exposes organic beings to new conditions of life and then nature acts

on the organisation and causes it to vary. But man can and does select

the variations given to him by nature, and thus accumulates them in any

desired manner. He thus adapts animals and plants for his own benefit

or pleasure. He may do this methodically, or he may do it unconsciously

by preserving the individuals most useful or pleasing to him without

any intention of altering the breed. It is certain that he can largely

influence the character of a breed by selecting, in each successive

generation, individual differences so slight as to be inappreciable

except by an educated eye. This unconscious process of selection has

been the great agency in the formation of the most distinct and useful

domestic breeds. That many breeds produced by man have to a large

extent the character of natural species, is shown by the inextricable

doubts whether many of them are varieties or aboriginally distinct

species.

There is no reason why the principles which have acted so efficiently

under domestication should not have acted under nature. In the survival

of favoured individuals and races, during the constantly recurrent

Struggle for Existence, we see a powerful and ever-acting form of

Selection. The struggle for existence inevitably follows from the high

geometrical ratio of increase which is common to all organic beings.

This high rate of increase is proved by calculation—by the rapid

increase of many animals and plants during a succession of peculiar

seasons, and when naturalised in new countries. More individuals are

born than can possibly survive. A grain in the balance may determine

which individuals shall live and which shall die—which variety or

species shall increase in number, and which shall decrease, or finally

become extinct. As the individuals of the same species come in all

respects into the closest competition with each other, the struggle

will generally be most severe between them; it will be almost equally

severe between the varieties of the same species, and next in severity

between the species of the same genus. On the other hand the struggle

will often be severe between beings remote in the scale of nature. The

slightest advantage in certain individuals, at any age or during any

season, over those with which they come into competition, or better

adaptation in however slight a degree to the surrounding physical

conditions, will, in the long run, turn the balance.

With animals having separated sexes, there will be in most cases a

struggle between the males for the possession of the females. The most

vigorous males, or those which have most successfully struggled with

their conditions of life, will generally leave most progeny. But

success will often depend on the males having special weapons or means

of defence or charms; and a slight advantage will lead to victory.

As geology plainly proclaims that each land has undergone great

physical changes, we might have expected to find that organic beings

have varied under nature, in the same way as they have varied under

domestication. And if there has been any variability under nature, it

would be an unaccountable fact if natural selection had not come into

play. It has often been asserted, but the assertion is incapable of

proof, that the amount of variation under nature is a strictly limited

quantity. Man, though acting on external characters alone and often

capriciously, can produce within a short period a great result by

adding up mere individual differences in his domestic productions; and

every one admits that species present individual differences. But,

besides such differences, all naturalists admit that natural varieties

exist, which are considered sufficiently distinct to be worthy of

record in systematic works. No one has drawn any clear distinction

between individual differences and slight varieties; or between more

plainly marked varieties and subspecies and species. On separate

continents, and on different parts of the same continent, when divided

by barriers of any kind, and on outlying islands, what a multitude of

forms exist, which some experienced naturalists rank as varieties,

others as geographical races or sub species, and others as distinct,

though closely allied species!

If, then, animals and plants do vary, let it be ever so slightly or

slowly, why should not variations or individual differences, which are

in any way beneficial, be preserved and accumulated through natural

selection, or the survival of the fittest? If man can by patience

select variations useful to him, why, under changing and complex

conditions of life, should not variations useful to nature’s living

products often arise, and be preserved or selected? What limit can be

put to this power, acting during long ages and rigidly scrutinising the

whole constitution, structure, and habits of each creature, favouring

the good and rejecting the bad? I can see no limit to this power, in

slowly and beautifully adapting each form to the most complex relations

of life. The theory of natural selection, even if we look no further

than this, seems to be in the highest degree probable. I have already

recapitulated, as fairly as I could, the opposed difficulties and

objections: now let us turn to the special facts and arguments in

favour of the theory.

On the view that species are only strongly marked and permanent

varieties, and that each species first existed as a variety, we can see

why it is that no line of demarcation can be drawn between species,

commonly supposed to have been produced by special acts of creation,

and varieties which are acknowledged to have been produced by secondary

laws. On this same view we can understand how it is that in a region

where many species of a genus have been produced, and where they now

flourish, these same species should present many varieties; for where

the manufactory of species has been active, we might expect, as a

general rule, to find it still in action; and this is the case if

varieties be incipient species. Moreover, the species of the larger

genera, which afford the greater number of varieties or incipient

species, retain to a certain degree the character of varieties; for

they differ from each other by a less amount of difference than do the

species of smaller genera. The closely allied species also of a larger

genera apparently have restricted ranges, and in their affinities they

are clustered in little groups round other species—in both respects

resembling varieties. These are strange relations on the view that each

species was independently created, but are intelligible if each existed

first as a variety.

As each species tends by its geometrical rate of reproduction to

increase inordinately in number; and as the modified descendants of

each species will be enabled to increase by as much as they become more

diversified in habits and structure, so as to be able to seize on many

and widely different places in the economy of nature, there will be a

constant tendency in natural selection to preserve the most divergent

offspring of any one species. Hence during a long-continued course of

modification, the slight differences characteristic of varieties of the

same species, tend to be augmented into the greater differences

characteristic of the species of the same genus. New and improved

varieties will inevitably supplant and exterminate the older, less

improved and intermediate varieties; and thus species are rendered to a

large extent defined and distinct objects. Dominant species belonging

to the larger groups within each class tend to give birth to new and

dominant forms; so that each large group tends to become still larger,

and at the same time more divergent in character. But as all groups

cannot thus go on increasing in size, for the world would not hold

them, the more dominant groups beat the less dominant. This tendency in

the large groups to go on increasing in size and diverging in

character, together with the inevitable contingency of much extinction,

explains the arrangement of all the forms of life in groups subordinate

to groups, all within a few great classes, which has prevailed

throughout all time. This grand fact of the grouping of all organic

beings under what is called the Natural System, is utterly inexplicable

on the theory of creation.

As natural selection acts solely by accumulating slight, successive,

favourable variations, it can produce no great or sudden modifications;

it can act only by short and slow steps. Hence, the canon of “Natura

non facit saltum,” which every fresh addition to our knowledge tends to

confirm, is on this theory intelligible. We can see why throughout

nature the same general end is gained by an almost infinite diversity

of means, for every peculiarity when once acquired is long inherited,

and structures already modified in many different ways have to be

adapted for the same general purpose. We can, in short, see why nature

is prodigal in variety, though niggard in innovation. But why this

should be a law of nature if each species has been independently

created no man can explain.

Many other facts are, as it seems to me, explicable on this theory. How

strange it is that a bird, under the form of a woodpecker, should prey

on insects on the ground; that upland geese, which rarely or never

swim, would possess webbed feet; that a thrush-like bird should dive

and feed on sub-aquatic insects; and that a petrel should have the

habits and structure fitting it for the life of an auk! and so in

endless other cases. But on the view of each species constantly trying

to increase in number, with natural selection always ready to adapt the

slowly varying descendants of each to any unoccupied or ill-occupied

place in nature, these facts cease to be strange, or might even have

been anticipated.

We can to a certain extent understand how it is that there is so much

beauty throughout nature; for this may be largely attributed to the

agency of selection. That beauty, according to our sense of it, is not

universal, must be admitted by every one who will look at some venomous

snakes, at some fishes, and at certain hideous bats with a distorted

resemblance to the human face. Sexual selection has given the most

brilliant colours, elegant patterns, and other ornaments to the males,

and sometimes to both sexes of many birds, butterflies and other

animals. With birds it has often rendered the voice of the male musical

to the female, as well as to our ears. Flowers and fruit have been

rendered conspicuous by brilliant colours in contrast with the green

foliage, in order that the flowers may be easily seen, visited and

fertilised by insects, and the seeds disseminated by birds. How it

comes that certain colours, sounds and forms should give pleasure to

man and the lower animals, that is, how the sense of beauty in its

simplest form was first acquired, we do not know any more than how

certain odours and flavours were first rendered agreeable.

As natural selection acts by competition, it adapts and improves the

inhabitants of each country only in relation to their co-inhabitants;

so that we need feel no surprise at the species of any one country,

although on the ordinary view supposed to have been created and

specially adapted for that country, being beaten and supplanted by the

naturalised productions from another land. Nor ought we to marvel if

all the contrivances in nature be not, as far as we can judge,

absolutely perfect; as in the case even of the human eye; or if some of

them be abhorrent to our ideas of fitness. We need not marvel at the

sting of the bee, when used against the enemy, causing the bee’s own

death; at drones being produced in such great numbers for one single

act, and being then slaughtered by their sterile sisters; at the

astonishing waste of pollen by our fir-trees; at the instinctive hatred

of the queen-bee for her own fertile daughters; at ichneumonidæ feeding

within the living bodies of caterpillars; and at other such cases. The

wonder, indeed, is, on the theory of natural selection, that more cases

of the want of absolute perfection have not been detected.

The complex and little known laws governing the production of varieties

are the same, as far as we can judge, with the laws which have governed

the production of distinct species. In both cases physical conditions

seem to have produced some direct and definite effect, but how much we

cannot say. Thus, when varieties enter any new station, they

occasionally assume some of the characters proper to the species of

that station. With both varieties and species, use and disuse seem to

have produced a considerable effect; for it is impossible to resist

this conclusion when we look, for instance, at the logger-headed duck,

which has wings incapable of flight, in nearly the same condition as in

the domestic duck; or when we look at the burrowing tucu-tucu, which is

occasionally blind, and then at certain moles, which are habitually

blind and have their eyes covered with skin; or when we look at the

blind animals inhabiting the dark caves of America and Europe. With

varieties and species, correlated variation seems to have played an

important part, so that when one part has been modified other parts

have been necessarily modified. With both varieties and species,

reversions to long-lost characters occasionally occur. How inexplicable

on the theory of creation is the occasional appearance of stripes on

the shoulders and legs of the several species of the horse-genus and of

their hybrids! How simply is this fact explained if we believe that

these species are all descended from a striped progenitor, in the same

manner as the several domestic breeds of the pigeon are descended from

the blue and barred rock-pigeon!

On the ordinary view of each species having been independently created,

why should specific characters, or those by which the species of the

same genus differ from each other, be more variable than the generic

characters in which they all agree? Why, for instance, should the

colour of a flower be more likely to vary in any one species of a

genus, if the other species possess differently coloured flowers, than

if all possessed the same coloured flowers? If species are only

well-marked varieties, of which the characters have become in a high

degree permanent, we can understand this fact; for they have already

varied since they branched off from a common progenitor in certain

characters, by which they have come to be specifically distinct from

each other; therefore these same characters would be more likely again

to vary than the generic characters which have been inherited without

change for an immense period. It is inexplicable on the theory of

creation why a part developed in a very unusual manner in one species

alone of a genus, and therefore, as we may naturally infer, of great

importance to that species, should be eminently liable to variation;

but, on our view, this part has undergone, since the several species

branched off from a common progenitor, an unusual amount of variability

and modification, and therefore we might expect the part generally to

be still variable. But a part may be developed in the most unusual

manner, like the wing of a bat, and yet not be more variable than any

other structure, if the part be common to many subordinate forms, that

is, if it has been inherited for a very long period; for in this case

it will have been rendered constant by long-continued natural

selection.

Glancing at instincts, marvellous as some are, they offer no greater

difficulty than do corporeal structures on the theory of the natural

selection of successive, slight, but profitable modifications. We can

thus understand why nature moves by graduated steps in endowing

different animals of the same class with their several instincts. I

have attempted to show how much light the principle of gradation throws

on the admirable architectural powers of the hive-bee. Habit no doubt

often comes into play in modifying instincts; but it certainly is not

indispensable, as we see in the case of neuter insects, which leave no

progeny to inherit the effects of long-continued habit. On the view of

all the species of the same genus having descended from a common

parent, and having inherited much in common, we can understand how it

is that allied species, when placed under widely different conditions

of life, yet follow nearly the same instincts; why the thrushes of

tropical and temperate South America, for instance, line their nests

with mud like our British species. On the view of instincts having been

slowly acquired through natural selection, we need not marvel at some

instincts being not perfect and liable to mistakes, and at many

instincts causing other animals to suffer.

If species be only well-marked and permanent varieties, we can at once

see why their crossed offspring should follow the same complex laws in

their degrees and kinds of resemblance to their parents—in being

absorbed into each other by successive crosses, and in other such

points—as do the crossed offspring of acknowledged varieties. This

similarity would be a strange fact, if species had been independently

created and varieties had been produced through secondary laws.

If we admit that the geological record is imperfect to an extreme

degree, then the facts, which the record does give, strongly support

the theory of descent with modification. New species have come on the

stage slowly and at successive intervals; and the amount of change

after equal intervals of time, is widely different in different groups.

The extinction of species and of whole groups of species, which has

played so conspicuous a part in the history of the organic world,

almost inevitably follows from the principle of natural selection; for

old forms are supplanted by new and improved forms. Neither single

species nor groups of species reappear when the chain of ordinary

generation is once broken. The gradual diffusion of dominant forms,

with the slow modification of their descendants, causes the forms of

life, after long intervals of time, to appear as if they had changed

simultaneously throughout the world. The fact of the fossil remains of

each formation being in some degree intermediate in character between

the fossils in the formations above and below, is simply explained by

their intermediate position in the chain of descent. The grand fact

that all extinct beings can be classed with all recent beings,

naturally follows from the living and the extinct being the offspring

of common parents. As species have generally diverged in character

during their long course of descent and modification, we can understand

why it is that the more ancient forms, or early progenitors of each

group, so often occupy a position in some degree intermediate between

existing groups. Recent forms are generally looked upon as being, on

the whole, higher in the scale of organisation than ancient forms; and

they must be higher, in so far as the later and more improved forms

have conquered the older and less improved forms in the struggle for

life; they have also generally had their organs more specialised for

different functions. This fact is perfectly compatible with numerous

beings still retaining simple and but little improved structures,

fitted for simple conditions of life; it is likewise compatible with

some forms having retrograded in organisation, by having become at each

stage of descent better fitted for new and degraded habits of life.

Lastly, the wonderful law of the long endurance of allied forms on the

same continent—of marsupials in Australia, of edentata in America, and

other such cases—is intelligible, for within the same country the

existing and the extinct will be closely allied by descent.

Looking to geographical distribution, if we admit that there has been

during the long course of ages much migration from one part of the

world to another, owing to former climatical and geographical changes

and to the many occasional and unknown means of dispersal, then we can

understand, on the theory of descent with modification, most of the

great leading facts in Distribution. We can see why there should be so

striking a parallelism in the distribution of organic beings throughout

space, and in their geological succession throughout time; for in both

cases the beings have been connected by the bond of ordinary

generation, and the means of modification have been the same. We see

the full meaning of the wonderful fact, which has struck every

traveller, namely, that on the same continent, under the most diverse

conditions, under heat and cold, on mountain and lowland, on deserts

and marshes, most of the inhabitants within each great class are

plainly related; for they are the descendants of the same progenitors

and early colonists. On this same principle of former migration,

combined in most cases with modification, we can understand, by the aid

of the Glacial period, the identity of some few plants, and the close

alliance of many others, on the most distant mountains, and in the

northern and southern temperate zones; and likewise the close alliance

of some of the inhabitants of the sea in the northern and southern

temperate latitudes, though separated by the whole intertropical ocean.

Although two countries may present physical conditions as closely

similar as the same species ever require, we need feel no surprise at

their inhabitants being widely different, if they have been for a long

period completely sundered from each other; for as the relation of

organism to organism is the most important of all relations, and as the

two countries will have received colonists at various periods and in

different proportions, from some other country or from each other, the

course of modification in the two areas will inevitably have been

different.

On this view of migration, with subsequent modification, we see why

oceanic islands are inhabited by only few species, but of these, why

many are peculiar or endemic forms. We clearly see why species

belonging to those groups of animals which cannot cross wide spaces of

the ocean, as frogs and terrestrial mammals, do not inhabit oceanic

islands; and why, on the other hand, new and peculiar species of bats,

animals which can traverse the ocean, are often found on islands far

distant from any continent. Such cases as the presence of peculiar

species of bats on oceanic islands and the absence of all other

terrestrial mammals, are facts utterly inexplicable on the theory of

independent acts of creation.

The existence of closely allied representative species in any two

areas, implies, on the theory of descent with modification, that the

same parent-forms formerly inhabited both areas; and we almost

invariably find that wherever many closely allied species inhabit two

areas, some identical species are still common to both. Wherever many

closely allied yet distinct species occur, doubtful forms and varieties

belonging to the same groups likewise occur. It is a rule of high

generality that the inhabitants of each area are related to the

inhabitants of the nearest source whence immigrants might have been

derived. We see this in the striking relation of nearly all the plants

and animals of the Galapagos Archipelago, of Juan Fernandez, and of the

other American islands, to the plants and animals of the neighbouring

American mainland; and of those of the Cape de Verde Archipelago, and

of the other African islands to the African mainland. It must be

admitted that these facts receive no explanation on the theory of

creation.

The fact, as we have seen, that all past and present organic beings can

be arranged within a few great classes, in groups subordinate to

groups, and with the extinct groups often falling in between the recent

groups, is intelligible on the theory of natural selection with its

contingencies of extinction and divergence of character. On these same

principles we see how it is that the mutual affinities of the forms

within each class are so complex and circuitous. We see why certain

characters are far more serviceable than others for classification; why

adaptive characters, though of paramount importance to the beings, are

of hardly any importance in classification; why characters derived from

rudimentary parts, though of no service to the beings, are often of

high classificatory value; and why embryological characters are often

the most valuable of all. The real affinities of all organic beings, in

contradistinction to their adaptive resemblances, are due to

inheritance or community of descent. The Natural System is a

genealogical arrangement, with the acquired grades of difference,

marked by the terms, varieties, species, genera, families, &c.; and we

have to discover the lines of descent by the most permanent characters,

whatever they may be, and of however slight vital importance.

The similar framework of bones in the hand of a man, wing of a bat, fin

of the porpoise, and leg of the horse—the same number of vertebræ

forming the neck of the giraffe and of the elephant—and innumerable

other such facts, at once explain themselves on the theory of descent

with slow and slight successive modifications. The similarity of

pattern in the wing and in the leg of a bat, though used for such

different purpose—in the jaws and legs of a crab—in the petals,

stamens, and pistils of a flower, is likewise, to a large extent,

intelligible on the view of the gradual modification of parts or

organs, which were aboriginally alike in an early progenitor in each of

these classes. On the principle of successive variations not always

supervening at an early age, and being inherited at a corresponding not

early period of life, we clearly see why the embryos of mammals, birds,

reptiles, and fishes should be so closely similar, and so unlike the

adult forms. We may cease marvelling at the embryo of an air-breathing

mammal or bird having branchial slits and arteries running in loops,

like those of a fish which has to breathe the air dissolved in water by

the aid of well-developed branchiæ.

Disuse, aided sometimes by natural selection, will often have reduced

organs when rendered useless under changed habits or conditions of

life; and we can understand on this view the meaning of rudimentary

organs. But disuse and selection will generally act on each creature,

when it has come to maturity and has to play its full part in the

struggle for existence, and will thus have little power on an organ

during early life; hence the organ will not be reduced or rendered

rudimentary at this early age. The calf, for instance, has inherited

teeth, which never cut through the gums of the upper jaw, from an early

progenitor having well-developed teeth; and we may believe, that the

teeth in the mature animal were formerly reduced by disuse owing to the

tongue and palate, or lips, having become excellently fitted through

natural selection to browse without their aid; whereas in the calf, the

teeth have been left unaffected, and on the principle of inheritance at

corresponding ages have been inherited from a remote period to the

present day. On the view of each organism with all its separate parts

having been specially created, how utterly inexplicable is it that

organs bearing the plain stamp of inutility, such as the teeth in the

embryonic calf or the shrivelled wings under the soldered wing-covers

of many beetles, should so frequently occur. Nature may be said to have

taken pains to reveal her scheme of modification, by means of

rudimentary organs, of embryological and homologous structures, but we

are too blind to understand her meaning.

I have now recapitulated the facts and considerations which have

thoroughly convinced me that species have been modified, during a long

course of descent. This has been effected chiefly through the natural

selection of numerous successive, slight, favourable variations; aided

in an important manner by the inherited effects of the use and disuse

of parts; and in an unimportant manner, that is, in relation to

adaptive structures, whether past or present, by the direct action of

external conditions, and by variations which seem to us in our

ignorance to arise spontaneously. It appears that I formerly underrated

the frequency and value of these latter forms of variation, as leading

to permanent modifications of structure independently of natural

selection. But as my conclusions have lately been much misrepresented,

and it has been stated that I attribute the modification of species

exclusively to natural selection, I may be permitted to remark that in

the first edition of this work, and subsequently, I placed in a most

conspicuous position—namely, at the close of the Introduction—the

following words: “I am convinced that natural selection has been the

main but not the exclusive means of modification.” This has been of no

avail. Great is the power of steady misrepresentation; but the history

of science shows that fortunately this power does not long endure.

It can hardly be supposed that a false theory would explain, in so

satisfactory a manner as does the theory of natural selection, the

several large classes of facts above specified. It has recently been

objected that this is an unsafe method of arguing; but it is a method

used in judging of the common events of life, and has often been used

by the greatest natural philosophers. The undulatory theory of light

has thus been arrived at; and the belief in the revolution of the earth

on its own axis was until lately supported by hardly any direct

evidence. It is no valid objection that science as yet throws no light

on the far higher problem of the essence or origin of life. Who can

explain what is the essence of the attraction of gravity? No one now

objects to following out the results consequent on this unknown element

of attraction; notwithstanding that Leibnitz formerly accused Newton of

introducing “occult qualities and miracles into philosophy.”

I see no good reasons why the views given in this volume should shock

the religious feelings of any one. It is satisfactory, as showing how

transient such impressions are, to remember that the greatest discovery

ever made by man, namely, the law of the attraction of gravity, was

also attacked by Leibnitz, “as subversive of natural, and inferentially

of revealed, religion.” A celebrated author and divine has written to

me that “he has gradually learned to see that it is just as noble a

conception of the Deity to believe that He created a few original forms

capable of self-development into other and needful forms, as to believe

that He required a fresh act of creation to supply the voids caused by

the action of His laws.”

Why, it may be asked, until recently did nearly all the most eminent

living naturalists and geologists disbelieve in the mutability of

species? It cannot be asserted that organic beings in a state of nature

are subject to no variation; it cannot be proved that the amount of

variation in the course of long ages is a limited quantity; no clear

distinction has been, or can be, drawn between species and well-marked

varieties. It cannot be maintained that species when intercrossed are

invariably sterile and varieties invariably fertile; or that sterility

is a special endowment and sign of creation. The belief that species

were immutable productions was almost unavoidable as long as the

history of the world was thought to be of short duration; and now that

we have acquired some idea of the lapse of time, we are too apt to

assume, without proof, that the geological record is so perfect that it

would have afforded us plain evidence of the mutation of species, if

they had undergone mutation.

But the chief cause of our natural unwillingness to admit that one

species has given birth to other and distinct species, is that we are

always slow in admitting any great changes of which we do not see the

steps. The difficulty is the same as that felt by so many geologists,

when Lyell first insisted that long lines of inland cliffs had been

formed, and great valleys excavated, by the agencies which we still see

at work. The mind cannot possibly grasp the full meaning of the term of

even a million years; it cannot add up and perceive the full effects of

many slight variations, accumulated during an almost infinite number of

generations.

Although I am fully convinced of the truth of the views given in this

volume under the form of an abstract, I by no means expect to convince

experienced naturalists whose minds are stocked with a multitude of

facts all viewed, during a long course of years, from a point of view

directly opposite to mine. It is so easy to hide our ignorance under

such expressions as the “plan of creation,” “unity of design,” &c., and

to think that we give an explanation when we only restate a fact. Any

one whose disposition leads him to attach more weight to unexplained

difficulties than to the explanation of a certain number of facts will

certainly reject the theory. A few naturalists, endowed with much

flexibility of mind, and who have already begun to doubt the

immutability of species, may be influenced by this volume; but I look

with confidence to the future, to young and rising naturalists, who

will be able to view both sides of the question with impartiality.

Whoever is led to believe that species are mutable will do good service

by conscientiously expressing his conviction; for thus only can the

load of prejudice by which this subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that a

multitude of reputed species in each genus are not real species; but

that other species are real, that is, have been independently created.

This seems to me a strange conclusion to arrive at. They admit that a

multitude of forms, which till lately they themselves thought were

special creations, and which are still thus looked at by the majority

of naturalists, and which consequently have all the external

characteristic features of true species—they admit that these have been

produced by variation, but they refuse to extend the same view to other

and slightly different forms. Nevertheless, they do not pretend that

they can define, or even conjecture, which are the created forms of

life, and which are those produced by secondary laws. They admit

variation as a vera causa in one case, they arbitrarily reject it in

another, without assigning any distinction in the two cases. The day

will come when this will be given as a curious illustration of the

blindness of preconceived opinion. These authors seem no more startled

at a miraculous act of creation than at an ordinary birth. But do they

really believe that at innumerable periods in the earth’s history

certain elemental atoms have been commanded suddenly to flash into

living tissues? Do they believe that at each supposed act of creation

one individual or many were produced? Were all the infinitely numerous

kinds of animals and plants created as eggs or seed, or as full grown?

and in the case of mammals, were they created bearing the false marks

of nourishment from the mother’s womb? Undoubtedly some of these same

questions cannot be answered by those who believe in the appearance or

creation of only a few forms of life or of some one form alone. It has

been maintained by several authors that it is as easy to believe in the

creation of a million beings as of one; but Maupertuis’ philosophical

axiom “of least action” leads the mind more willingly to admit the

smaller number; and certainly we ought not to believe that innumerable

beings within each great class have been created with plain, but

deceptive, marks of descent from a single parent.

As a record of a former state of things, I have retained in the

foregoing paragraphs, and elsewhere, several sentences which imply that

naturalists believe in the separate creation of each species; and I

have been much censured for having thus expressed myself. But

undoubtedly this was the general belief when the first edition of the

present work appeared. I formerly spoke to very many naturalists on the

subject of evolution, and never once met with any sympathetic

agreement. It is probable that some did then believe in evolution, but

they were either silent or expressed themselves so ambiguously that it

was not easy to understand their meaning. Now, things are wholly

changed, and almost every naturalist admits the great principle of

evolution. There are, however, some who still think that species have

suddenly given birth, through quite unexplained means, to new and

totally different forms. But, as I have attempted to show, weighty

evidence can be opposed to the admission of great and abrupt

modifications. Under a scientific point of view, and as leading to

further investigation, but little advantage is gained by believing that

new forms are suddenly developed in an inexplicable manner from old and

widely different forms, over the old belief in the creation of species

from the dust of the earth.

It may be asked how far I extend the doctrine of the modification of

species. The question is difficult to answer, because the more distinct

the forms are which we consider, by so much the arguments in favour of

community of descent become fewer in number and less in force. But some

arguments of the greatest weight extend very far. All the members of

whole classes are connected together by a chain of affinities, and all

can be classed on the same principle, in groups subordinate to groups.

Fossil remains sometimes tend to fill up very wide intervals between

existing orders.

Organs in a rudimentary condition plainly show that an early progenitor

had the organ in a fully developed condition, and this in some cases

implies an enormous amount of modification in the descendants.

Throughout whole classes various structures are formed on the same

pattern, and at a very early age the embryos closely resemble each

other. Therefore I cannot doubt that the theory of descent with

modification embraces all the members of the same great class or

kingdom. I believe that animals are descended from at most only four or

five progenitors, and plants from an equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all

animals and plants are descended from some one prototype. But analogy

may be a deceitful guide. Nevertheless all living things have much in

common, in their chemical composition, their cellular structure, their

laws of growth, and their liability to injurious influences. We see

this even in so trifling a fact as that the same poison often similarly

affects plants and animals; or that the poison secreted by the gall-fly

produces monstrous growths on the wild rose or oak-tree. With all

organic beings, excepting perhaps some of the very lowest, sexual

reproduction seems to be essentially similar. With all, as far as is at

present known, the germinal vesicle is the same; so that all organisms

start from a common origin. If we look even to the two main

divisions—namely, to the animal and vegetable kingdoms—certain low

forms are so far intermediate in character that naturalists have

disputed to which kingdom they should be referred. As Professor Asa

Gray has remarked, “the spores and other reproductive bodies of many of

the lower algæ may claim to have first a characteristically animal, and

then an unequivocally vegetable existence.” Therefore, on the principle

of natural selection with divergence of character, it does not seem

incredible that, from some such low and intermediate form, both animals

and plants may have been developed; and, if we admit this, we must

likewise admit that all the organic beings which have ever lived on

this earth may be descended from some one primordial form. But this

inference is chiefly grounded on analogy, and it is immaterial whether

or not it be accepted. No doubt it is possible, as Mr. G.H. Lewes has

urged, that at the first commencement of life many different forms were

evolved; but if so, we may conclude that only a very few have left

modified descendants. For, as I have recently remarked in regard to the

members of each great kingdom, such as the Vertebrata, Articulata, &c.,

we have distinct evidence in their embryological, homologous, and

rudimentary structures, that within each kingdom all the members are

descended from a single progenitor.

When the views advanced by me in this volume, and by Mr. Wallace or

when analogous views on the origin of species are generally admitted,

we can dimly foresee that there will be a considerable revolution in

natural history. Systematists will be able to pursue their labours as

at present; but they will not be incessantly haunted by the shadowy

doubt whether this or that form be a true species. This, I feel sure

and I speak after experience, will be no slight relief. The endless

disputes whether or not some fifty species of British brambles are good

species will cease. Systematists will have only to decide (not that

this will be easy) whether any form be sufficiently constant and

distinct from other forms, to be capable of definition; and if

definable, whether the differences be sufficiently important to deserve

a specific name. This latter point will become a far more essential

consideration than it is at present; for differences, however slight,

between any two forms, if not blended by intermediate gradations, are

looked at by most naturalists as sufficient to raise both forms to the

rank of species.

Hereafter we shall be compelled to acknowledge that the only

distinction between species and well-marked varieties is, that the

latter are known, or believed to be connected at the present day by

intermediate gradations, whereas species were formerly thus connected.

Hence, without rejecting the consideration of the present existence of

intermediate gradations between any two forms, we shall be led to weigh

more carefully and to value higher the actual amount of difference

between them. It is quite possible that forms now generally

acknowledged to be merely varieties may hereafter be thought worthy of

specific names; and in this case scientific and common language will

come into accordance. In short, we shall have to treat species in the

same manner as those naturalists treat genera, who admit that genera

are merely artificial combinations made for convenience. This may not

be a cheering prospect; but we shall at least be freed from the vain

search for the undiscovered and undiscoverable essence of the term

species.

The other and more general departments of natural history will rise

greatly in interest. The terms used by naturalists, of affinity,

relationship, community of type, paternity, morphology, adaptive

characters, rudimentary and aborted organs, &c., will cease to be

metaphorical and will have a plain signification. When we no longer

look at an organic being as a savage looks at a ship, as something

wholly beyond his comprehension; when we regard every production of

nature as one which has had a long history; when we contemplate every

complex structure and instinct as the summing up of many contrivances,

each useful to the possessor, in the same way as any great mechanical

invention is the summing up of the labour, the experience, the reason,

and even the blunders of numerous workmen; when we thus view each

organic being, how far more interesting—I speak from experience—does

the study of natural history become!

A grand and almost untrodden field of inquiry will be opened, on the

causes and laws of variation, on correlation, on the effects of use and

disuse, on the direct action of external conditions, and so forth. The

study of domestic productions will rise immensely in value. A new

variety raised by man will be a far more important and interesting

subject for study than one more species added to the infinitude of

already recorded species. Our classifications will come to be, as far

as they can be so made, genealogies; and will then truly give what may

be called the plan of creation. The rules for classifying will no doubt

become simpler when we have a definite object in view. We possess no

pedigree or armorial bearings; and we have to discover and trace the

many diverging lines of descent in our natural genealogies, by

characters of any kind which have long been inherited. Rudimentary

organs will speak infallibly with respect to the nature of long-lost

structures. Species and groups of species which are called aberrant,

and which may fancifully be called living fossils, will aid us in

forming a picture of the ancient forms of life. Embryology will often

reveal to us the structure, in some degree obscured, of the prototypes

of each great class.

When we can feel assured that all the individuals of the same species,

and all the closely allied species of most genera, have, within a not

very remote period descended from one parent, and have migrated from

some one birth-place; and when we better know the many means of

migration, then, by the light which geology now throws, and will

continue to throw, on former changes of climate and of the level of the

land, we shall surely be enabled to trace in an admirable manner the

former migrations of the inhabitants of the whole world. Even at

present, by comparing the differences between the inhabitants of the

sea on the opposite sides of a continent, and the nature of the various

inhabitants of that continent in relation to their apparent means of

immigration, some light can be thrown on ancient geography.

The noble science of geology loses glory from the extreme imperfection

of the record. The crust of the earth, with its embedded remains, must

not be looked at as a well-filled museum, but as a poor collection made

at hazard and at rare intervals. The accumulation of each great

fossiliferous formation will be recognised as having depended on an

unusual occurrence of favourable circumstances, and the blank intervals

between the successive stages as having been of vast duration. But we

shall be able to gauge with some security the duration of these

intervals by a comparison of the preceding and succeeding organic

forms. We must be cautious in attempting to correlate as strictly

contemporaneous two formations, which do not include many identical

species, by the general succession of the forms of life. As species are

produced and exterminated by slowly acting and still existing causes,

and not by miraculous acts of creation; and as the most important of

all causes of organic change is one which is almost independent of

altered and perhaps suddenly altered physical conditions, namely, the

mutual relation of organism to organism—the improvement of one organism

entailing the improvement or the extermination of others; it follows,

that the amount of organic change in the fossils of consecutive

formations probably serves as a fair measure of the relative, though

not actual lapse of time. A number of species, however, keeping in a

body might remain for a long period unchanged, whilst within the same

period, several of these species, by migrating into new countries and

coming into competition with foreign associates, might become modified;

so that we must not overrate the accuracy of organic change as a

measure of time.

In the future I see open fields for far more important researches.

Psychology will be securely based on the foundation already well laid

by Mr. Herbert Spencer, that of the necessary acquirement of each

mental power and capacity by gradation. Much light will be thrown on

the origin of man and his history.

Authors of the highest eminence seem to be fully satisfied with the

view that each species has been independently created. To my mind it

accords better with what we know of the laws impressed on matter by the

Creator, that the production and extinction of the past and present

inhabitants of the world should have been due to secondary causes, like

those determining the birth and death of the individual. When I view

all beings not as special creations, but as the lineal descendants of

some few beings which lived long before the first bed of the Cambrian

system was deposited, they seem to me to become ennobled. Judging from

the past, we may safely infer that not one living species will transmit

its unaltered likeness to a distinct futurity. And of the species now

living very few will transmit progeny of any kind to a far distant

futurity; for the manner in which all organic beings are grouped, shows

that the greater number of species in each genus, and all the species

in many genera, have left no descendants, but have become utterly

extinct. We can so far take a prophetic glance into futurity as to

foretell that it will be the common and widely spread species,

belonging to the larger and dominant groups within each class, which

will ultimately prevail and procreate new and dominant species. As all

the living forms of life are the lineal descendants of those which

lived long before the Cambrian epoch, we may feel certain that the

ordinary succession by generation has never once been broken, and that

no cataclysm has desolated the whole world. Hence, we may look with

some confidence to a secure future of great length. And as natural

selection works solely by and for the good of each being, all corporeal

and mental endowments will tend to progress towards perfection.

It is interesting to contemplate a tangled bank, clothed with many

plants of many kinds, with birds singing on the bushes, with various

insects flitting about, and with worms crawling through the damp earth,

and to reflect that these elaborately constructed forms, so different

from each other, and dependent upon each other in so complex a manner,

have all been produced by laws acting around us. These laws, taken in

the largest sense, being Growth with reproduction; Inheritance which is

almost implied by reproduction; Variability from the indirect and

direct action of the conditions of life, and from use and disuse; a

Ratio of Increase so high as to lead to a Struggle for Life, and as a

consequence to Natural Selection, entailing Divergence of Character and

the Extinction of less improved forms. Thus, from the war of nature,

from famine and death, the most exalted object which we are capable of

conceiving, namely, the production of the higher animals, directly

follows. There is grandeur in this view of life, with its several

powers, having been originally breathed by the Creator into a few forms

or into one; and that, whilst this planet has gone circling on

according to the fixed law of gravity, from so simple a beginning

endless forms most beautiful and most wonderful have been, and are

being evolved.

GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.*

* I am indebted to the kindness of Mr. W.S. Dallas for this

Glossary, which has been given because several readers have

complained to me that some of the terms used were unintelligible to

them. Mr. Dallas has endeavoured to give the explanations of the

terms in as popular a form as possible.

ABERRANT.—Forms or groups of animals or plants which deviate in

important characters from their nearest allies, so as not to be easily

included in the same group with them, are said to be aberrant.

ABERRATION (in Optics).—In the refraction of light by a convex lens the

rays passing through different parts of the lens are brought to a focus

at slightly different distances—this is called _spherical aberration;_

at the same time the coloured rays are separated by the prismatic

action of the lens and likewise brought to a focus at different

distances—this is _chromatic aberration_.

ABNORMAL.—Contrary to the general rule.

ABORTED.—An organ is said to be aborted, when its development has been

arrested at a very early stage.

ALBINISM.—Albinos are animals in which the usual colouring matters

characteristic of the species have not been produced in the skin and

its appendages. Albinism is the state of being an albino.

ALGÆ.—A class of plants including the ordinary sea-weeds and the

filamentous fresh-water weeds.

ALTERNATION OF GENERATIONS.—This term is applied to a peculiar mode of

reproduction which prevails among many of the lower animals, in which

the egg produces a living form quite different from its parent, but

from which the parent-form is reproduced by a process of budding, or by

the division of the substance of the first product of the egg.

AMMONITES.—A group of fossil, spiral, chambered shells, allied to the

existing pearly Nautilus, but having the partitions between the

chambers waved in complicated patterns at their junction with the outer

wall of the shell.

ANALOGY.—That resemblance of structures which depends upon similarity

of function, as in the wings of insects and birds. Such structures are

said to be _analogous_, and to be _analogues_ of each other.

ANIMALCULE.—A minute animal: generally applied to those visible only by

the microscope.

ANNELIDS.—A class of worms in which the surface of the body exhibits a

more or less distinct division into rings or segments, generally

provided with appendages for locomotion and with gills. It includes the

ordinary marine worms, the earth-worms, and the leeches.

ANTENNÆ.—Jointed organs appended to the head in Insects, Crustacea and

Centipedes, and not belonging to the mouth.

ANTHERS.—The summits of the stamens of flowers, in which the pollen or

fertilising dust is produced.

APLACENTALIA, APLACENTATA or Aplacental Mammals.—See _mammalia_.

ARCHETYPAL.—Of or belonging to the Archetype, or ideal primitive form

upon which all the beings of a group seem to be organised.

ARTICULATA.—A great division of the Animal Kingdom characterised

generally by having the surface of the body divided into rings called

segments, a greater or less number of which are furnished with jointed

legs (such as Insects, Crustaceans and Centipedes).

ASYMMETRICAL.—Having the two sides unlike.

ATROPHIED.—Arrested in development at a very early stage.

BALANUS.—The genus including the common Acorn-shells which live in

abundance on the rocks of the sea-coast.

BATRACHIANS.—A class of animals allied to the Reptiles, but undergoing

a peculiar metamorphosis, in which the young animal is generally

aquatic and breathes by gills. (_Examples_, Frogs, Toads, and Newts.)

BOULDERS.—Large transported blocks of stone generally embedded in clays

or gravels.

BRACHIOPODA.—A class of marine Mollusca, or soft-bodied animals,

furnished with a bivalve shell, attached to submarine objects by a

stalk which passes through an aperture in one of the valves, and

furnished with fringed arms, by the action of which food is carried to

the mouth.

BRANCHIÆ.—Gills or organs for respiration in water.

BRANCHIAL.—Pertaining to gills or branchiæ.

CAMBRIAN SYSTEM.—A series of very ancient Palæozoic rocks, between the

Laurentian and the Silurian. Until recently these were regarded as the

oldest fossiliferous rocks.

CANIDÆ.—The Dog-family, including the Dog, Wolf, Fox, Jackal, &c.

CARAPACE.—The shell enveloping the anterior part of the body in

Crustaceans generally; applied also to the hard shelly pieces of the

Cirripedes.

CARBONIFEROUS.—This term is applied to the great formation which

includes, among other rocks, the coal-measures. It belongs to the

oldest, or Palæozoic, system of formations.

CAUDAL.—Of or belonging to the tail.

CEPHALOPODS.—The highest class of the Mollusca, or soft-bodied animals,

characterised by having the mouth surrounded by a greater or less

number of fleshy arms or tentacles, which, in most living species, are

furnished with sucking-cups. (_Examples_, Cuttle-fish, Nautilus.)

CETACEA.—An order of Mammalia, including the Whales, Dolphins, &c.,

having the form of the body fish-like, the skin naked, and only the

fore limbs developed.

CHELONIA.—An order of Reptiles including the Turtles, Tortoises, &c.

CIRRIPEDES.—An order of Crustaceans including the Barnacles and

Acorn-shells. Their young resemble those of many other Crustaceans in

form; but when mature they are always attached to other objects, either

directly or by means of a stalk, and their bodies are enclosed by a

calcareous shell composed of several pieces, two of which can open to

give issue to a bunch of curled, jointed tentacles, which represent the

limbs.

COCCUS.—The genus of Insects including the Cochineal. In these the male

is a minute, winged fly, and the female generally a motionless,

berry-like mass.

COCOON.—A case usually of silky material, in which insects are

frequently enveloped during the second or resting-stage (pupa) of their

existence. The term “cocoon-stage” is here used as equivalent to

“pupa-stage.”

CŒLOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which

have the seed hollowed on the inner face.

COLEOPTERA.—Beetles, an order of Insects, having a biting mouth and the

first pair of wings more or less horny, forming sheaths for the second

pair, and usually meeting in a straight line down the middle of the

back.

COLUMN.—A peculiar organ in the flowers of Orchids, in which the

stamens, style and stigma (or the reproductive parts) are united.

COMPOSITÆ or COMPOSITOUS PLANTS.—Plants in which the inflorescence

consists of numerous small flowers (florets) brought together into a

dense head, the base of which is enclosed by a common envelope.

(_Examples_, the Daisy, Dandelion, &c.)

CONFERVÆ.—The filamentous weeds of fresh water.

CONGLOMERATE.—A rock made up of fragments of rock or pebbles, cemented

together by some other material.

COROLLA.—The second envelope of a flower usually composed of coloured,

leaf-like organs (petals), which may be united by their edges either in

the basal part or throughout.

CORRELATION.—The normal coincidence of one phenomenon, character, &c.,

with another.

CORYMB.—A bunch of flowers in which those springing from the lower part

of the flower stalks are supported on long stalks so as to be nearly on

a level with the upper ones.

COTYLEDONS.—The first or seed-leaves of plants.

CRUSTACEANS.—A class of articulated animals, having the skin of the

body generally more or less hardened by the deposition of calcareous

matter, breathing by means of gills. (_Examples_, Crab, Lobster,

Shrimp, &c.)

CURCULIO.—The old generic term for the Beetles known as Weevils,

characterised by their four-jointed feet, and by the head being

produced into a sort of beak, upon the sides of which the antennæ are

inserted.

CUTANEOUS.—Of or belonging to the skin.

DEGRADATION.—The wearing down of land by the action of the sea or of

meteoric agencies.

DENUDATION.—The wearing away of the surface of the land by water.

DEVONIAN SYSTEM or FORMATION.—A series of Palæozoic rocks, including

the Old Red Sandstone.

DICOTYLEDONS, or DICOTYLEDONOUS PLANTS.—A class of plants characterised

by having two seed-leaves, by the formation of new wood between the

bark and the old wood (exogenous growth) and by the reticulation of the

veins of the leaves. The parts of the flowers are generally in

multiples of five.

DIFFERENTATION.—The separation or discrimination of parts or organs

which in simpler forms of life are more or less united.

DIMORPHIC.—Having two distinct forms.—DIMORPHISM is the condition of

the appearance of the same species under two dissimilar forms.

DIOECIOUS.—Having the organs of the sexes upon distinct individuals.

DIORITE.—A peculiar form of Greenstone.

DORSAL.—Of or belonging to the back.

EDENTATA.—A peculiar order of Quadrupeds, characterised by the absence

of at least the middle incisor (front) teeth in both jaws. (_Examples_,

the Sloths and Armadillos.)

ELYTRA.—The hardened fore-wings of Beetles, serving as sheaths for the

membranous hind-wings, which constitute the true organs of flight.

EMBRYO.—The young animal undergoing development within the egg or womb.

EMBRYOLOGY.—The study of the development of the embryo.

ENDEMIC.—Peculiar to a given locality.

ENTOMOSTRACA.—A division of the class Crustacea, having all the

segments of the body usually distinct, gills attached to the feet or

organs of the mouth, and the feet fringed with fine hairs. They are

generally of small size.

EOCENE.—The earliest of the three divisions of the Tertiary epoch of

geologists. Rocks of this age contain a small proportion of shells

identical with species now living.

EPHEMEROUS INSECTS.—Insects allied to the May-fly.

FAUNA.—The totality of the animals naturally inhabiting a certain

country or region, or which have lived during a given geological

period.

FELIDÆ.—The Cat-family.

FERAL.—Having become wild from a state of cultivation or domestication.

FLORA.—The totality of the plants growing naturally in a country, or

during a given geological period.

FLORETS.—Flowers imperfectly developed in some respects, and collected

into a dense spike or head, as in the Grasses, the Dandelion, &c.

FOETAL.—Of or belonging to the foetus, or embryo in course of

development.

FORAMINIFERA.—A class of animals of very low organisation and generally

of small size, having a jelly-like body, from the surface of which

delicate filaments can be given off and retracted for the prehension of

external objects, and having a calcareous or sandy shell, usually

divided into chambers and perforated with small apertures.

FOSSILIFEROUS.—Containing fossils.

FOSSORIAL.—Having a faculty of digging. The Fossorial Hymenoptera are a

group of Wasp-like Insects, which burrow in sandy soil to make nests

for their young.

FRENUM (pl. FRENA).—A small band or fold of skin.

FUNGI (sing. FUNGUS).—A class of cellular plants, of which Mushrooms,

Toadstools, and Moulds, are familiar examples.

FURCULA.—The forked bone formed by the union of the collar-bones in

many birds, such as the common Fowl.

GALLINACEOUS BIRDS.—An order of birds of which the common Fowl, Turkey,

and Pheasant, are well-known examples.

GALLUS.—The genus of birds which includes the common Fowl.

GANGLION.—A swelling or knot from which nerves are given off as from a

centre.

GANOID FISHES.—Fishes covered with peculiar enamelled bony scales. Most

of them are extinct.

GERMINAL VESICLE.—A minute vesicle in the eggs of animals, from which

the development of the embryo proceeds.

GLACIAL PERIOD.—A period of great cold and of enormous extension of ice

upon the surface of the earth. It is believed that glacial periods have

occurred repeatedly during the geological history of the earth, but the

term is generally applied to the close of the Tertiary epoch, when

nearly the whole of Europe was subjected to an arctic climate.

GLAND.—An organ which secretes or separates some peculiar product from

the blood or sap of animals or plants.

GLOTTIS.—The opening of the windpipe into the œsophagus or gullet.

GNEISS.—A rock approaching granite in composition, but more or less

laminated, and really produced by the alteration of a sedimentary

deposit after its consolidation.

GRALLATORES.—The so-called wading-birds (storks, cranes, snipes, &c.),

which are generally furnished with long legs, bare of feathers above

the heel, and have no membranes between the toes.

GRANITE.—A rock consisting essentially of crystals of felspar and mica

in a mass of quartz.

HABITAT.—The locality in which a plant or animal naturally lives.

HEMIPTERA.—An order or sub-order of insects, characterised by the

possession of a jointed beak or rostrum, and by having the fore-wings

horny in the basal portion and membranous at the extremity, where they

cross each other. This group includes the various species of bugs.

HERMAPHRODITE.—Possessing the organs of both sexes.

HOMOLOGY.—That relation between parts which results from their

development from corresponding embryonic parts, either in different

animals, as in the case of the arm of man, the fore-leg of a quadruped,

and the wing of a bird; or in the same individual, as in the case of

the fore and hind legs in quadrupeds, and the segments or rings and

their appendages of which the body of a worm, a centipede, &c., is

composed. The latter is called _serial homology_. The parts which stand

in such a relation to each other are said to be _homologous_, and one

such part or organ is called the _homologue_ of the other. In different

plants the parts of the flower are homologous, and in general these

parts are regarded as homologous with leaves.

HOMOPTERA.—An order or sub-order of insects having (like the Hemiptera)

a jointed beak, but in which the fore-wings are either wholly

membranous or wholly leathery, The _Cicadæ_, frog-hoppers, and

_Aphides_, are well-known examples.

HYBRID.—The offspring of the union of two distinct species.

HYMENOPTERA.—An order of insects possessing biting jaws and usually

four membranous wings in which there are a few veins. Bees and wasps

are familiar examples of this group.

HYPERTROPHIED.—Excessively developed.

ICHNEUMONIDÆ.—A family of hymenopterous insects, the members of which

lay their eggs in the bodies or eggs of other insects.

IMAGO.—The perfect (generally winged) reproductive state of an insect.

INDIGENES.—The aboriginal animal or vegetable inhabitants of a country

or region.

INFLORESCENCE.—The mode of arrangement of the flowers of plants.

INFUSORIA.—A class of microscopic animalcules, so called from their

having originally been observed in infusions of vegetable matters. They

consist of a gelatinous material enclosed in a delicate membrane, the

whole or part of which is furnished with short vibrating hairs (called

cilia), by means of which the animalcules swim through the water or

convey the minute particles of their food to the orifice of the mouth.

INSECTIVOROUS.—Feeding on insects.

INVERTEBRATA, or INVERTEBRATE ANIMALS.—Those animals which do not

possess a backbone or spinal column.

LACUNÆ.—Spaces left among the tissues in some of the lower animals and

serving in place of vessels for the circulation of the fluids of the

body.

LAMELLATED.—Furnished with lamellæ or little plates.

LARVA (pl. LARVÆ).—The first condition of an insect at its issuing from

the egg, when it is usually in the form of a grub, caterpillar, or

maggot.

LARYNX.—The upper part of the windpipe opening into the gullet.

LAURENTIAN.—A group of greatly altered and very ancient rocks, which is

greatly developed along the course of the St. Laurence, whence the

name. It is in these that the earliest known traces of organic bodies

have been found.

LEGUMINOSÆ.—An order of plants represented by the common peas and

beans, having an irregular flower in which one petal stands up like a

wing, and the stamens and pistil are enclosed in a sheath formed by two

other petals. The fruit is a pod (or legume).

LEMURIDÆ.—A group of four-handed animals, distinct from the monkeys and

approaching the insectivorous quadrupeds in some of their characters

and habits. Its members have the nostrils curved or twisted, and a claw

instead of a nail upon the first finger of the hind hands.

LEPIDOPTERA.—An order of insects, characterised by the possession of a

spiral proboscis, and of four large more or less scaly wings. It

includes the well-known butterflies and moths.

LITTORAL.—Inhabiting the seashore.

LOESS.—A marly deposit of recent (Post-Tertiary) date, which occupies a

great part of the valley of the Rhine.

MALACOSTRACA.—The higher division of the Crustacea, including the

ordinary crabs, lobsters, shrimps, &c., together with the woodlice and

sand-hoppers.

MAMMALIA.—The highest class of animals, including the ordinary hairy

quadrupeds, the whales and man, and characterised by the production of

living young which are nourished after birth by milk from the teats

(_Mammæ_, _Mammary glands_) of the mother. A striking difference in

embryonic development has led to the division of this class into two

great groups; in one of these, when the embryo has attained a certain

stage, a vascular connection, called the _placenta_, is formed between

the embryo and the mother; in the other this is wanting, and the young

are produced in a very incomplete state. The former, including the

greater part of the class, are called _Placental Mammals;_ the latter,

or _Aplacental Mammals_, include the Marsupials and Monotremes

(_Ornithorhynchus_).

MAMMIFEROUS.—Having mammæ or teats (see MAMMALIA).

MANDIBLES.—in insects, the first or uppermost pair of jaws, which are

generally solid, horny, biting organs. In birds the term is applied to

both jaws with their horny coverings. In quadrupeds the mandible is

properly the lower jaw.

MARSUPIALS.—An order of Mammalia in which the young are born in a very

incomplete state of development, and carried by the mother, while

sucking, in a ventral pouch (marsupium), such as the kangaroos,

opossums, &c. (see MAMMALIA).

MAXILLÆ.—in insects, the second or lower pair of jaws, which are

composed of several joints and furnished with peculiar jointed

appendages called palpi, or feelers.

MELANISM.—The opposite of albinism; an undue development of colouring

material in the skin and its appendages.

METAMORPHIC ROCKS.—Sedimentary rocks which have undergone alteration,

generally by the action of heat, subsequently to their deposition and

consolidation.

MOLLUSCA.—One of the great divisions of the animal kingdom, including

those animals which have a soft body, usually furnished with a shell,

and in which the nervous ganglia, or centres, present no definite

general arrangement. They are generally known under the denomination of

“shellfish”; the cuttle-fish, and the common snails, whelks, oysters,

mussels, and cockles, may serve as examples of them.

MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS.—Plants in which the seed

sends up only a single seed-leaf (or cotyledon); characterised by the

absence of consecutive layers of wood in the stem (endogenous growth),

by the veins of the leaves being generally straight, and by the parts

of the flowers being generally in multiples of three. (_Examples_,

Grasses, Lilies, Orchids, Palms, &c.)

MORAINES.—The accumulations of fragments of rock brought down by

glaciers.

MORPHOLOGY.—The law of form or structure independent of function.

MYSIS-STAGE.—A stage in the development of certain crustaceans

(prawns), in which they closely resemble the adults of a genus

(_Mysis_) belonging to a slightly lower group.

NASCENT.—Commencing development.

NATATORY.—Adapted for the purpose of swimming.

NAUPLIUS-FORM.—The earliest stage in the development of many Crustacea,

especially belonging to the lower groups. In this stage the animal has

a short body, with indistinct indications of a division into segments,

and three pairs of fringed limbs. This form of the common fresh-water

_Cyclops_ was described as a distinct genus under the name of

_Nauplius_.

NEURATION.—The arrangement of the veins or nervures in the wings of

insects.

NEUTERS.—Imperfectly developed females of certain social insects (such

as ants and bees), which perform all the labours of the community.

Hence, they are also called _workers_.

NICTITATING MEMBRANE.—A semi-transparent membrane, which can be drawn

across the eye in birds and reptiles, either to moderate the effects of

a strong light or to sweep particles of dust, &c., from the surface of

the eye.

OCELLI.—The simple eyes or stemmata of insects, usually situated on the

crown of the head between the great compound eyes.

ŒSOPHAGUS.—The gullet.

OOLITIC.—A great series of secondary rocks, so called from the texture

of some of its members, which appear to be made up of a mass of small

EGG-LIKE calcareous bodies.

OPERCULUM.—A calcareous plate employed by many Molluscæ to close the

aperture of their shell. The OPERCULAR VALVES of Cirripedes are those

which close the aperture of the shell.

ORBIT.—The bony cavity for the reception of the eye.

ORGANISM.—An organised being, whether plant or animal.

ORTHOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which

have the seed straight.

OSCULANT.—Forms or groups apparently intermediate between and

connecting other groups are said to be osculant.

OVA.—Eggs.

OVARIUM or OVARY (in plants).—The lower part of the pistil or female

organ of the flower, containing the ovules or incipient seeds; by

growth after the other organs of the flower have fallen, it usually

becomes converted into the fruit.

OVIGEROUS.—Egg-bearing.

OVULES (of plants).—The seeds in the earliest condition.

PACHYDERMS.—A group of Mammalia, so called from their thick skins, and

including the elephant, rhinoceros, hippopotamus, &c.

PALÆOZOIC.—The oldest system of fossiliferous rocks.

PALPI.—Jointed appendages to some of the organs of the mouth in insects

and Crustacea.

PAPILIONACEÆ.—An order of plants (see LEGUMINOSÆ), The flowers of these

plants are called _papilionaceous_, or butterfly-like, from the fancied

resemblance of the expanded superior petals to the wings of a

butterfly.

PARASITE.—An animal or plant living upon or in, and at the expense of,

another organism.

PARTHENOGENESIS.—The production of living organisms from unimpregnated

eggs or seeds.

PEDUNCULATED.—Supported upon a stem or stalk. The pedunculated oak has

its acorns borne upon a footstool.

PELORIA or PELORISM.—The appearance of regularity of structure in the

flowers of plants which normally bear irregular flowers.

PELVIS.—The bony arch to which the hind limbs of vertebrate animals are

articulated.

PETALS.—The leaves of the corolla, or second circle of organs in a

flower. They are usually of delicate texture and brightly coloured.

PHYLLODINEOUS.—Having flattened, leaf-like twigs or leafstalks instead

of true leaves.

PIGMENT.—The colouring material produced generally in the superficial

parts of animals. The cells secreting it are called _pigment-cells_.

PINNATE.—Bearing leaflets on each side of a central stalk.

PISTILS.—The female organs of a flower, which occupy a position in the

centre of the other floral organs. The pistil is generally divisible

into the ovary or germen, the style and the stigma.

PLACENTALIA, PLACENTATA.—or PLACENTAL MAMMALS, See MAMMALIA.

PLANTIGRADES.—Quadrupeds which walk upon the whole sole of the foot,

like the bears.

PLASTIC.—Readily capable of change.

PLEISTOCENE PERIOD.—The latest portion of the Tertiary epoch.

PLUMULE (in plants).—The minute bud between the seed-leaves of

newly-germinated plants.

PLUTONIC ROCKS.—Rocks supposed to have been produced by igneous action

in the depths of the earth.

POLLEN.—The male element in flowering plants; usually a fine dust

produced by the anthers, which, by contact with the stigma effects the

fecundation of the seeds. This impregnation is brought about by means

of tubes (_pollen-tubes_) which issue from the pollen-grains adhering

to the stigma, and penetrate through the tissues until they reach the

ovary.

POLYANDROUS (flowers).—Flowers having many stamens.

POLYGAMOUS PLANTS.—Plants in which some flowers are unisexual and

others hermaphrodite. The unisexual (male and female) flowers, may be

on the same or on different plants.

POLYMORPHIC.—Presenting many forms.

POLYZOARY.—The common structure formed by the cells of the Polyzoa,

such as the well-known seamats.

PREHENSILE.—Capable of grasping.

PREPOTENT.—Having a superiority of power.

PRIMARIES.—The feathers forming the tip of the wing of a bird, and

inserted upon that part which represents the hand of man.

PROCESSES.—Projecting portions of bones, usually for the attachment of

muscles, ligaments, &c.

PROPOLIS.—A resinous material collected by the hivebees from the

opening buds of various trees.

PROTEAN.—Exceedingly variable.

PROTOZOA.—The lowest great division of the animal kingdom. These

animals are composed of a gelatinous material, and show scarcely any

trace of distinct organs. The Infusoria, Foraminifera, and sponges,

with some other forms, belong to this division.

PUPA (pl. PUPÆ).—The second stage in the development of an insect, from

which it emerges in the perfect (winged) reproductive form. In most

insects the _pupal stage_ is passed in perfect repose. The _chrysalis_

is the pupal state of butterflies.

RADICLE.—The minute root of an embryo plant.

RAMUS.—One half of the lower jaw in the Mammalia. The portion which

rises to articulate with the skull is called the _ascending ramus_.

RANGE.—The extent of country over which a plant or animal is naturally

spread. _Range in time_ expresses the distribution of a species or

group through the fossiliferous beds of the earth’s crust.

RETINA.—The delicate inner coat of the eye, formed by nervous filaments

spreading from the optic nerve, and serving for the perception of the

impressions produced by light.

RETROGRESSION.—Backward development. When an animal, as it approaches

maturity, becomes less perfectly organised than might be expected from

its early stages and known relationships, it is said to undergo a

_retrogade development_ or _metamorphosis_.

RHIZOPODS.—A class of lowly organised animals (Protozoa), having a

gelatinous body, the surface of which can be protruded in the form of

root-like processes or filaments, which serve for locomotion and the

prehension of food. The most important order is that of the

Foraminifera.

RODENTS.—The gnawing Mammalia, such as the rats, rabbits, and

squirrels. They are especially characterised by the possession of a

single pair of chisel-like cutting teeth in each jaw, between which and

the grinding teeth there is a great gap.

RUBUS.—The bramble genus.

RUDIMENTARY.—Very imperfectly developed.

RUMINANTS.—The group of quadrupeds which ruminate or chew the cud, such

as oxen, sheep, and deer. They have divided hoofs, and are destitute of

front teeth in the upper jaw.

SACRAL.—Belonging to the sacrum, or the bone composed usually of two or

more united vertebræ to which the sides of the pelvis in vertebrate

animals are attached.

SARCODE.—The gelatinous material of which the bodies of the lowest

animals (Protozoa) are composed.

SCUTELLÆ.—The horny plates with which the feet of birds are generally

more or less covered, especially in front.

SEDIMENTARY FORMATIONS.—Rocks deposited as sediments from water.

SEGMENTS.—The transverse rings of which the body of an articulate

animal or annelid is composed.

SEPALS.—The leaves or segments of the calyx, or outermost envelope of

an ordinary flower. They are usually green, but sometimes brightly

coloured.

SERRATURES.—Teeth like those of a saw.

SESSILE.—Not supported on a stem or footstalk.

SILURIAN SYSTEM.—A very ancient system of fossiliferous rocks belonging

to the earlier part of the Palæozoic series.

SPECIALISATION.—The setting apart of a particular organ for the

performance of a particular function.

SPINAL CORD.—The central portion of the nervous system in the

Vertebrata, which descends from the brain through the arches of the

vertebræ, and gives off nearly all the nerves to the various organs of

the body.

STAMENS.—The male organs of flowering plants, standing in a circle

within the petals. They usually consist of a filament and an anther,

the anther being the essential part in which the pollen, or fecundating

dust, is formed.

STERNUM.—The breast-bone.

STIGMA.—The apical portion of the pistil in flowering plants.

STIPULES.—Small leafy organs placed at the base of the footstalks of

the leaves in many plants.

STYLE.—The middle portion of the perfect pistil, which rises like a

column from the ovary and supports the stigma at its summit.

SUBCUTANEOUS.—Situated beneath the skin.

SUCTORIAL.—Adapted for sucking.

SUTURES (in the skull).—The lines of junction of the bones of which the

skull is composed.

TARSUS (pl. TARSI).—The jointed feet of articulate animals, such as

insects.

TELEOSTEAN FISHES.—Fishes of the kind familiar to us in the present

day, having the skeleton usually completely ossified and the scales

horny.

TENTACULA or TENTACLES.—Delicate fleshy organs of prehension or touch

possessed by many of the lower animals.

TERTIARY.—The latest geological epoch, immediately preceding the

establishment of the present order of things.

TRACHEA.—The windpipe or passage for the admission of air to the lungs.

TRIDACTYLE.—Three-fingered, or composed of three movable parts attached

to a common base.

TRILOBITES.—A peculiar group of extinct crustaceans, somewhat

resembling the woodlice in external form, and, like some of them,

capable of rolling themselves up into a ball. Their remains are found

only in the Palæozoic rocks, and most abundantly in those of Silurian

age.

TRIMORPHIC.—Presenting three distinct forms.

UMBELLIFERÆ.—An order of plants in which the flowers, which contain

five stamens and a pistil with two styles, are supported upon

footstalks which spring from the top of the flower stem and spread out

like the wires of an umbrella, so as to bring all the flowers in the

same head (_umbel_) nearly to the same level. (_Examples_, Parsley and

Carrot.)

UNGULATA.—Hoofed quadrupeds.

UNICELLULAR.—Consisting of a single cell.

VASCULAR.—Containing blood-vessels.

VERMIFORM.—Like a worm.

VERTEBRATA or VERTEBRATE ANIMALS.—The highest division of the animal

kingdom, so called from the presence in most cases of a backbone

composed of numerous joints or _vertebræ_, which constitutes the centre

of the skeleton and at the same time supports and protects the central

parts of the nervous system.

WHORLS.—The circles or spiral lines in which the parts of plants are

arranged upon the axis of growth.

WORKERS.—See neuters.

ZOËA-STAGE.—The earliest stage in the development of many of the higher

Crustacea, so called from the name of _Zoëa_ applied to these young

animals when they were supposed to constitute a peculiar genus.

ZOOIDS.—In many of the lower animals (such as the Corals, Medusæ, &c.)

reproduction takes place in two ways, namely, by means of eggs and by a

process of budding with or without separation from the parent of the

product of the latter, which is often very different from that of the

egg. The individuality of the species is represented by the whole of

the form produced between two sexual reproductions; and these forms,

which are apparently individual animals, have been called _zooids_.

INDEX.

Aberrant groups, 379.

Abyssinia, plants of, 340.

Acclimatisation, 112.

Adoxa, 173.

Affinities of extinct species, 301.

—, of organic beings, 378.