SELECTION.
Mr. Darwin's Theory of the Origin of Species by Natural Selection.
Memoir by Mr. Wallace.
Manner in which favoured Races prevail in the Struggle for Existence.
Formation of new Races by breeding.
Hypotheses of definite and indefinite Modifiability equally arbitrary.
Competition and Extinction of Races.
Progression not a necessary Accompaniment of Variation.
Distinct Classes of Phenomena which Natural Selection explains.
Unity of Type, Rudimentary Organs, Geographical Distribution,
Relation of the extinct to the living Fauna and Flora, and
mutual Relations of successive Groups of Fossil Forms.
Light thrown on Embryological Development by Natural Selection.
Why large Genera have more variable Species than small ones.
Dr. Hooker on the Evidence afforded by the Vegetable Kingdom
in favour of Creation by Variation.
Steenstrup on alternation of Generations.
How far the Doctrine of Independent Creation is opposed to the
Laws now governing the Migration of Species.
For many years after the promulgation of Lamarck's doctrine of
progressive development, geologists were much occupied with the question
whether the past changes in the animate and inanimate world were brought
about by sudden and paroxysmal action, or gradually and continuously, by
causes differing neither in kind nor degree from those now in operation.
The anonymous author of "The Vestiges of Creation" published in 1844
a treatise, written in a clear and attractive style, which made
the English public familiar with the leading views of Lamarck on
transmutation and progression, but brought no new facts or original
line of argument to support those views, or to combat the principal
objections which the scientific world entertained against them.
No decided step in this direction was made until the publication in 1858
of two papers, one by Mr. Darwin and another by Mr. Wallace, followed in
1859 by Mr. Darwin's celebrated work on "The Origin of Species by Means
of Natural Selection; or, the Preservation of favoured Races in the
Struggle for Life." The author of this treatise had for twenty previous
years strongly inclined to believe that variation and the ordinary laws
of reproduction were among the secondary causes always employed by the
Author of nature, in the introduction from time to time of new species
into the world, and he had devoted himself patiently to the collecting
of facts and making of experiments in zoology and botany, with a view
of testing the soundness of the theory of transmutation. Part of the
manuscript of his projected work was read to Dr. Hooker as early as 1844
and some of the principal results were communicated to me on several
occasions. [40] Dr. Hooker and I had repeatedly urged him to publish
without delay, but in vain, as he was always unwilling to interrupt the
course of his investigations; until at length Mr. Alfred R. Wallace, who
had been engaged for years in collecting and studying the animals of the
East Indian archipelago, thought out independently for himself one of
the most novel and important of Mr. Darwin's theories. This he embodied
in an essay "On the Tendency of Varieties to depart indefinitely from
the original Type." It was written at Ternate in February 1858, and sent
to Mr. Darwin with a request that it might be shown to me if thought
sufficiently novel and interesting. Dr. Hooker and I were of opinion
that it should be immediately printed, and we succeeded in persuading
Mr. Darwin to allow one of the manuscript chapters of his "Origin of
Species," entitled "On the Tendency of Species to form Varieties, and
on the Perpetuation of Species and Varieties by natural Means of
Selection," to appear at the same time.*
(* See "Proceedings of the Linnaean Society" 1858.)
By reference to these memoirs it will be seen that both writers begin by
applying to the animal and vegetable worlds the Malthusian doctrine of
population, or its tendency to increase in a geometrical ratio, while
food can only be made to augment even locally in an arithmetical one.
There being therefore no room or means of subsistence for a large
proportion of the plants and animals which are born into the world, a
great number must annually perish. Hence there is a constant struggle
for existence among the individuals which represent each species and
the vast majority can never reach the adult state, to say nothing of
the multitudes of ova and seeds which are never hatched or allowed to
germinate. Of birds it is estimated that the number of those which die
every year equals the aggregate number by which the species to which
they respectively belong is on the average permanently represented.
The trial of strength which must decide what individuals are to survive
and what to succumb occurs in the season when the means of subsistence
are fewest, or enemies most numerous, or when the individuals are
enfeebled by climate or other causes; and it is then that those
varieties which have any, even the slightest, advantage over others come
off victorious. They may often owe their safety to what would seem to a
casual observer a trifling difference, such as a darker or lighter shade
of colour rendering them less visible to a species which preys upon
them, or sometimes to attributes more obviously advantageous, such as
greater cunning or superior powers of flight or swiftness of foot. These
peculiar qualities and faculties, bodily and instinctive, may enable
them to outlive their less favoured rivals, and being transmitted by the
force of inheritance to their offspring will constitute new races, or
what Mr. Darwin calls "incipient species." If one variety, being
in other respects just equal to its competitors, happens to be more
prolific, some of its offspring will stand a greater chance of being
among those which will escape destruction, and their descendants, being
in like manner very fertile, will continue to multiply at the expense of
all less prolific varieties.
As breeders of domestic animals, when they choose certain varieties in
preference to others to breed from, speak technically of their method as
that of "selecting," Mr. Darwin calls the combination of natural causes,
which may enable certain varieties of wild animals or plants to prevail
over others of the same species, "natural selection."
A breeder finds that a new race of cattle with short horns or without
horns may be formed in the course of several generations by choosing
varieties having the most stunted horns as his stock from which to
breed; so nature, by altering in the course of ages, the conditions
of life, the geographical features of a country, its climate, the
associated plants and animals, and consequently the food and enemies
of a species and its mode of life, may be said, by this means to select
certain varieties best adapted for the new state of things. Such
new races may often supplant the original type from which they
have diverged, although that type may have been perpetuated without
modification for countless anterior ages in the same region, so long as
it was in harmony with the surrounding conditions then prevailing.
Lamarck, when speculating on the origin of the long neck of the giraffe,
imagined that quadruped to have stretched himself up in order to reach
the boughs of lofty trees, until by continued efforts and longing to
reach higher he obtained an elongated neck. Mr. Darwin and Mr. Wallace
simply suppose that, in a season of scarcity, a longer-necked variety,
having the advantage in this respect over most of the herd, as being
able to browse on foliage out of their reach, survived them and
transmitted its peculiarity of cervical conformation to its successors.
By the multiplying of slight modifications in the course of thousands of
generations and by the handing down of the newly-acquired peculiarities
by inheritance, a greater and greater divergence from the original
standard is supposed to be effected, until what may be called a new
species, or in a greater lapse of time a new genus will be the result.
Every naturalist admits that there is a general tendency in animals and
plants to vary; but it is usually taken for granted, though he have
no means of proving the assumption to be true, that there are certain
limits beyond which each species cannot pass under any circumstances or
in any number of generations. Mr. Darwin and Mr. Wallace say that the
opposite hypothesis, which assumes that every species is capable
of varying indefinitely from its original type, is not a whit more
arbitrary, and has this manifest claim to be preferred, that it will
account for a multitude of phenomena which the ordinary theory is
incapable of explaining.
We have no right, they say, to assume, should we find that a variable
species can no longer be made to vary in a certain direction, that it
has reached the utmost limit to which it might under more favourable
conditions or if more time were allowed be made to diverge from the
parent type.
Hybridisation is not considered by Mr. Darwin as a cause of new species,
but rather as tending to keep variation within bounds. Varieties which
are nearly allied cross readily with each other, and with the parent
stock, and such crossing tends to keep the species true to its type,
while forms which are less nearly related, although they may intermarry,
produce no mule offspring capable of perpetuating their kind.
The competition of races and species, observes Mr. Darwin, is always
most severe between those which are most closely allied and which
fill nearly the same place in the economy of nature. Hence when the
conditions of existence are modified the original stock runs great risk
of being superseded by some one of its modified offshoots. The new race
or species may not be absolutely superior in the sum of its powers and
endowments to the parent stock, and may even be more simple in structure
and of a lower grade of intelligence, as well as of organisation,
provided on the whole it happens to have some slight advantage over
its rivals. Progression, therefore, is not a necessary accompaniment
of variation and natural selection, though when a higher organisation
happens to be coincident with superior fitness to new conditions, the
new species will have greater power and a greater chance of permanently
maintaining and extending its ground. One of the principal claims of Mr.
Darwin's theory to acceptance is that it enables us to dispense with a
law of progression as a necessary accompaniment of variation. It will
account equally well for what is called degradation, or a retrograde
movement towards a simpler structure, and does not require Lamarck's
continual creation of monads; for this was a necessary part of his
system, in order to explain how, after the progressive power had been
at work for myriads of ages, there were as many beings of the simplest
structure in existence as ever.
Mr. Darwin argues, and with no small success, that all true
classification in zoology and botany is in fact genealogical, and that
community of descent is the hidden bond which naturalists have been
unconsciously seeking, while they often imagined that they were looking
for some unknown plan of creation.
As the "Origin of Species"* is in itself a condensed abstract of a much
larger work not yet published [41] I could not easily give an analysis
of its contents within narrower limits than those of the original, but
it may be useful to enumerate briefly some of the principal classes of
phenomena on which the theory of "natural selection" would throw light.
(* "Origin of Species" page 121.)
In the first place it would explain, says Mr. Darwin, the unity of type
which runs through the whole organic world, and why there is sometimes a
fundamental agreement in structure in the same class of beings which is
quite independent of their habits of life, for such structure, derived
by inheritance from a remote progenitor, has been modified in the course
of ages in different ways according to the conditions of existence.
It would also explain why all living and extinct beings are united, by
complex radiating and circuitous lines of affinity with one another into
one grand system;* also, there having been a continued extinction of old
races and species in progress and a formation of new ones by variation,
why in some genera which are largely represented, or to which a great
many species belong, many of these are closely but unequally related;
also, why there are distinct geographical provinces of species of
animals and plants, for after long isolation by physical barriers each
fauna and flora by varying continually must become distinct from its
ancestral type, and from the new forms assumed by other descendants
which have diverged from the same stock.
(* "Origin" page 498.)
The theory of indefinite modification would also explain why rudimentary
organs are so useful in classification, being the remnants preserved
by inheritance of organs which the present species once used--as in the
case of the rudiments of eyes in insects and reptiles inhabiting dark
caverns, or of the wings of birds and beetles which have lost all
power of flight. In such cases the affinities of species are often more
readily discerned by reference to these imperfect structures than
by others of much more physiological importance to the individuals
themselves.
The same hypothesis would explain why there are no mammalia in islands
far from continents, except bats, which can reach them by flying; and
also why the birds, insects, plants, and other inhabitants of islands,
even when specifically unlike, usually agree generically with those of
the nearest continent, it being assumed that the original stock of such
species came by migration from the nearest land.
Variation and natural selection would also afford a key to a multitude
of geological facts otherwise wholly unaccounted for, as for example why
there is generally an intimate connection between the living animals
and plants of each great division of the globe and the extinct fauna and
flora of the Post-Tertiary or Tertiary formations of the same region;
as, for example, in North America, where we not only find among the
living mollusca peculiar forms foreign to Europe, such as Gnathodon
and Fulgur (a subgenus of Fusus), but meet also with extinct species of
those same genera in the Tertiary fauna of the same part of the world.
In like manner, among the mammalia we find in Australia not only living
kangaroos and wombats, but fossil individuals of extinct species of the
same genera. So also there are recent and fossil sloths, armadilloes
and other Edentata in South America, and living and extinct species
of elephant, rhinoceros, tiger, and bear in the great Europeo-Asiatic
continent. The theory of the origin of new species by variation will
also explain why a species which has once died out never reappears
and why the fossil fauna and flora recede farther and farther from the
living type in proportion as we trace them back to remoter ages. It
would also account for the fact that when we have to intercalate a new
set of fossiliferous strata between two groups previously known, the
newly discovered fossils serve to fill up gaps between specific or
generic types previously familiar to us, supplying often the missing
links of the chain, which, if transmutation is accepted, must once have
been continuous.
One of the most original speculations in Mr. Darwin's work is derived
from the fact that, in the breeding of animals, it is often observed
that at whatever age any variation first appears in the parent, it tends
to reappear at a corresponding age in the offspring. Hence the young
individuals of two races which have sprung from the same parent stock
are usually more like each other than the adults. Thus the puppies
of the greyhound and bull-dog are much more nearly alike in their
proportions than the grown-up dogs, and in like manner the foals of the
carthorse and racehorse than the adult individuals. For the same reason
we may understand why the species of the same genus, or genera of the
same family, resemble each other more nearly in their embryonic than in
their more fully developed state, or how it is that in the eyes of
most naturalists the structure of the embryo is even more important in
classification than that of the adult, "for the embryo is the animal in
its less modified state, and in so far it reveals the structure of its
progenitor. In two groups of animals, however much they may at present
differ from each other in structure and habits, if they pass through
the same or similar embryonic stages, we may feel assured that they
have both descended from the same or nearly similar parents, and are
therefore in that degree closely related. Thus community in embryonic
structure reveals community of descent, however much the structure of
the adult may have been modified."*
(* Darwin, "Origin" etc. page 448.)
If then there had been a system of progressive development, the
successive changes through which the embryo of a species of a high
class, a mammifer for example, now passes, may be expected to present us
with a picture of the stages through which, in the course of ages, that
class of animals has successively passed in advancing from a lower to a
higher grade. Hence the embryonic states exhibited one after the other
by the human individual bear a certain amount of resemblance to those
of the fish, reptile, and bird before assuming those of the highest
division of the vertebrata.
Mr. Darwin, after making a laborious analysis of many floras, found that
those genera which are represented by a large number of species contain
a greater number of variable species, relatively speaking, than the
smaller genera or those less numerously represented. This fact he
adduces in support of his opinion that varieties are incipient species,
for he observes that the existence of the larger genera implies that
the manufacturing of species has been active in the period immediately
preceding our own, in which case we ought generally to find the same
forces still in full activity, more especially as we have every reason
to believe the process by which new species are produced is a slow one.*
(* "Origin of Species" chapter 2 page 56.)
Dr. Hooker tells us that he was long disposed to doubt this result,
as he was acquainted with so many variable small genera, but after
examining Mr. Darwin's data, he was compelled to acquiesce in his
generalisation.*
(* "Introductory Essay to the Flora of Australia" page 6.)
It is one of those conclusions, to verify which requires the
investigation of many thousands of species, and to which exceptions may
easily be adduced both in the animal and vegetable kingdoms, so that
it will be long before we can expect it to be thoroughly tested, and
if true, fairly appreciated. Among the most striking exceptions will
be some genera still large, but which are beginning to decrease, the
conditions favourable to their former predominance having already
begun to change. To many, this doctrine of "natural selection," or
"the preservation of favoured races in the struggle for life," seems so
simple, when once clearly stated, and so consonant with known facts and
received principles, that they have difficulty in conceiving how it can
constitute a great step in the progress of science. Such is often the
case with important discoveries, but in order to assure ourselves that
the doctrine was by no means obvious, we have only to refer back to the
writings of skilful naturalists who attempted in the earlier part of the
nineteenth century to theorise on this subject, before the invention of
this new method of explaining how certain forms are supplanted by new
ones and in what manner these last are selected out of innumerable
varieties and rendered permanent.
DR. HOOKER ON THE THEORY OF "CREATION BY VARIATION" AS APPLIED TO THE
VEGETABLE KINGDOM.
Of Dr. Hooker, whom I have often cited in this chapter, Mr. Darwin has
spoken in the Introduction to his "Origin of Species," as one "who had,
for fifteen years, aided him in every possible way, by his large stores
of knowledge, and his excellent judgment." This distinguished botanist
published his "Introductory Essay to the Flora of Australia" in December
1859, the year after the memoir on "Natural Selection" was communicated
to the Linnaean Society, and a month after the appearance of the "Origin
of Species."
Having, in the course of his extensive travels, studied the botany of
arctic, temperate, and tropical regions, and written on the flora of
India, which he had examined at all heights above the sea from the
plains of Bengal to the limits of perpetual snow in the Himalaya, and
having specially devoted his attention to "geographical varieties," or
those changes of character which plants exhibit when traced over wide
areas and seen under new conditions; being also practically versed in
the description and classification of new plants, from various parts of
the world, and having been called upon carefully to consider the
claims of thousands of varieties to rank as species, no one was better
qualified by observation and reflection to give an authoritative opinion
on the question, whether the present vegetation of the globe is or is
not in accordance with the theory which Mr. Darwin has proposed. We
cannot but feel, therefore, deeply interested when we find him making
the following declaration:
"The mutual relations of the plants of each great botanical province,
and, in fact, of the world generally, is just such as would have
resulted if variation had gone on operating throughout indefinite
periods, in the same manner as we see it act in a limited number of
centuries, so as gradually to give rise in the course of time, to the
most widely divergent forms."
In the same essay, this author remarks, "The element of mutability
pervades the whole Vegetable Kingdom; no class, nor order, nor genus of
more than a few species claims absolute exemption from it, whilst the
grand total of unstable forms, generally assumed to be species, probably
exceeds that of the stable." Yet he contends that species are neither
visionary, nor even arbitrary creations of the naturalist, but
realities, though they may not remain true for ever. The majority
of them, he remarks, are so far constant, "within the range of our
experience," and their forms and characters so faithfully handed down
through thousands of generations, that they admit of being treated as if
they were permanent and immutable. But the range of "our experience" is
so limited, that it will "not account for a single fact in the present
geographical distribution, or origin of any one species of plant, nor
for the amount of variation it has undergone, nor will it indicate the
time when it first appeared, nor the form it had when created."*
(* Hooker, "Introductory Essay to the Flora of Australia.")
To what an extent the limits of species are indefinable, is evinced, he
says, by the singular fact that, among those botanists who believe them
to be immutable, the number of flowering plants is by some assumed to be
80,000, and by others over 150,000. The general limitation of species
to certain areas suggests the idea that each of them, with all their
varieties, have sprung from a common parent and have spread in various
directions from a common centre. The frequency also of the grouping of
genera within certain geographical limits is in favour of the same
law, although the migration of species may sometimes cause apparent
exceptions to the rule and make the same types appear to have originated
independently at different spots.*
(* Ibid. page 13.)
Certain genera of plants, which, like the brambles, roses, and willows
in Europe, consist of a continuous series of varieties between the terms
of which no intermediate forms can be intercalated, may be supposed
to be newer types and on the increase, and therefore undergoing much
variation; whereas genera which present no such perplexing gradations
may be of older date and may have been losing species and varieties by
extinction. In this case, the annihilation of intermediate forms which
once existed makes it an easy task to distinguish those which remain.
It had usually been supposed by the advocates of the immutability of
species that domesticated races, if allowed to run wild, always revert
to their parent type. Mr. Wallace had said in reply that a domesticated
species, if it loses the protection of Man, can only stand its ground
in a wild state by resuming those habits and recovering those attributes
which it may have lost when under domestication. If these faculties are
so much enfeebled as to be irrecoverable it will perish; if not and if
it can adapt itself to the surrounding conditions, it will revert to the
state in which Man first found it: for in one, two, or three thousand
years, which may have elapsed since it was originally tamed, there will
not have been time for such geographical, climatal, and organic changes
as would only be suited to a new race or a new and allied species.
But in regard to plants Dr. Hooker questions the fact of reversion.
According to him, species in general do not readily vary, but when they
once begin to do so the new varieties, as every horticulturist knows,
show a great inclination to go on departing more and more from the
old stock. As the best marked varieties of a wild species occur on the
confines of the area which it inhabits, so the best marked varieties of
a cultivated plant are those last produced by the gardener. Cabbages,
for example, wall fruits, and cereal, show no disposition, when
neglected, to assume the characters of the wild states of these plants.
Hence the difficulty of determining what are the true parent species of
most of our cultivated plants. Thus the finer kinds of apples, if grown
from seed, degenerate and become crabs, but in so doing they do not
revert to the original wild crab-apple, but become crab states of the
varieties to which they belong.*
(* "Introductory Essay to the Flora of Australia" page 9.)
It would lead me into too long a digression were I to attempt to give a
fuller analysis of this admirable essay; but I may add that none of
the observations are more in point, as bearing on the doctrine of what
Hooker terms "creation by variation," than the great extent to which
the internal characters and properties of plants, or their physiological
constitution, are capable of being modified, while they exhibit
externally no visible departure from the normal form. Thus, in one
region a species may possess peculiar medicinal qualities which it wants
in another, or it may be hardier and better able to resist cold. The
average range in altitude, says Hooker, of each species of flowering
plant in the Himalayan Mountains, whether in the tropical, temperate,
or Alpine region, is 4000 feet, which is equivalent to twelve degrees
of isothermals of latitude. If an individual of any of these species be
taken from the upper limits of its range and carried to England, it is
found to be better able to stand our climate than those from the lower
or warmer stations. When several of these internal or physiological
modifications are accompanied by variation in size, habits of growth,
colour of the flowers, and other external characters, and these are
found to be constant in successive generations, botanists may well begin
to differ in opinion as to whether they ought to regard them as distinct
species or not.
ALTERNATION OF GENERATIONS.
Hitherto, no rival hypothesis has been proposed as a substitute for the
doctrine of transmutation; for what we term "independent creation," or
the direct intervention of the Supreme Cause, must simply be considered
as an avowal that we deem the question to lie beyond the domain of
science.
The discovery by Steenstrup of alternate generation enlarges our views
of the range of metamorphosis through which a species may pass, so
that some of its stages (as when a Sertularia and a Medusa interchange)
deviate so far from others as to have been referred by able zoologists
to distinct genera, or even families. But in all these cases the
organism, after running through a certain cycle of change, returns to
the exact point from which it set out, and no new form or species is
thereby introduced into the world. The only secondary cause therefore
which has as yet been even conjecturally brought forward, to explain how
in the ordinary course of nature a new specific form may be generated
is, as Lamarck declared, "variation," and this has been rendered a far
more probable hypothesis by the way in which "natural selection" is
shown to give intensity and permanency to certain varieties.
INDEPENDENT CREATION.
When I formerly advocated the doctrine that species were primordial
creations and not derivative, I endeavoured to explain the manner of
their geographical distribution, and the affinity of living forms to
the fossil types nearest akin to them in the Tertiary strata of the
same part of the globe, by supposing that the creative power, which
originally adapts certain types to aquatic and others to terrestrial
conditions, has at successive geological epochs introduced new forms
best suited to each area and climate, so as to fill the places of those
which may have died out.
In that case, although the new species would differ from the old (for
these would not be revived, having been already proved by the fact of
their extinction to be incapable of holding their ground), still they
would resemble their predecessors generically. For, as Mr. Darwin states
in regard to new races, those of a dominant type inherit the advantages
which made their parent species flourish in the same country, and they
likewise partake in those general advantages which made the genus to
which the parent species belonged a large genus in its own country.
We might therefore, by parity of reasoning, have anticipated that the
creative power, adapting the new types to the new combination of organic
and inorganic conditions of a given region, such as its soil,
climate, and inhabitants, would introduce new modifications of the old
types--marsupials, for example, in Australia, new sloths and armadilloes
in South America, new heaths at the Cape, new roses in the northern
and new calceolarias in the southern hemisphere. But to this line of
argument Mr. Darwin and Dr. Hooker reply that when animals or plants
migrate into new countries, whether assisted by man or without his aid,
the most successful colonisers appertain by no means to those types
which are most allied to the old indigenous species. On the contrary it
more frequently happens that members of genera, orders, or even classes,
distinct and foreign to the invaded country, make their way most rapidly
and become dominant at the expense of the endemic species. Such is the
case with the placental quadrupeds in Australia, and with horses and
many foreign plants in the pampas of South America, and numberless
instances in the United States and elsewhere which might easily be
enumerated. Hence the transmutationists infer that the reason why these
foreign types, so peculiarly fitted for these regions, have never
before been developed there is simply that they were excluded by natural
barriers. But these barriers of sea or desert or mountain could never
have been of the least avail had the creative force acted independently
of material laws or had it not pleased the Author of Nature that the
origin of new species should be governed by some secondary causes
analogous to those which we see preside over the appearance of new
varieties, which never appear except as the offspring of a parent stock
very closely resembling them.