Produced by Amy E. Zelmer
THE ORIGIN OF SPECIES
[1]
By Thomas H. Huxley
MR. DARWIN'S long-standing and well-earned scientific eminence probably
renders him indifferent to that social notoriety which passes by the
name of success; but if the calm spirit of the philosopher have not yet
wholly superseded the ambition and the vanity of the carnal man within
him, he must be well satisfied with the results of his venture in
publishing the 'Origin of Species'. Overflowing the narrow bounds of
purely scientific circles, the "species question" divides with Italy and
the Volunteers the attention of general society. Everybody has read Mr.
Darwin's book, or, at least, has given an opinion upon its merits or
demerits; pietists, whether lay or ecclesiastic, decry it with the mild
railing which sounds so charitable; bigots denounce it with ignorant
invective; old ladies of both sexes consider it a decidedly dangerous
book, and even savants, who have no better mud to throw, quote
antiquated writers to show that its author is no better than an ape
himself; while every philosophical thinker hails it as a veritable
Whitworth gun in the armoury of liberalism; and all competent
naturalists and physiologists, whatever their opinions as to the
ultimate fate of the doctrines put forth, acknowledge that the work
in which they are embodied is a solid contribution to knowledge and
inaugurates a new epoch in natural history.
Nor has the discussion of the subject been restrained within the limits
of conversation. When the public is eager and interested, reviewers must
minister to its wants; and the genuine 'litterateur' is too much in
the habit of acquiring his knowledge from the book he judges--as the
Abyssinian is said to provide himself with steaks from the ox which
carries him--to be withheld from criticism of a profound scientific work
by the mere want of the requisite preliminary scientific acquirement;
while, on the other hand, the men of science who wish well to the new
views, no less than those who dispute their validity, have naturally
sought opportunities of expressing their opinions. Hence it is not
surprising that almost all the critical journals have noticed Mr.
Darwin's work at greater or less length; and so many disquisitions,
of every degree of excellence, from the poor product of ignorance, too
often stimulated by prejudice, to the fair and thoughtful essay of
the candid student of Nature, have appeared, that it seems an almost
hopeless task to attempt to say anything new upon the question.
But it may be doubted if the knowledge and acumen of prejudged
scientific opponents, or the subtlety of orthodox special pleaders, have
yet exerted their full force in mystifying the real issues of the great
controversy which has been set afoot, and whose end is hardly likely
to be seen by this generation; so that, at this eleventh hour, and even
failing anything new, it may be useful to state afresh that which is
true, and to put the fundamental positions advocated by Mr. Darwin in
such a form that they may be grasped by those whose special studies lie
in other directions. And the adoption of this course may be the more
advisable, because, notwithstanding its great deserts, and indeed partly
on account of them, the 'Origin of Species' is by no means an easy book
to read--if by reading is implied the full comprehension of an author's
meaning.
We do not speak jestingly in saying that it is Mr. Darwin's misfortune
to know more about the question he has taken up than any man living.
Personally and practically exercised in zoology, in minute anatomy,
in geology; a student of geographical distribution, not on maps and
in museums only, but by long voyages and laborious collection; having
largely advanced each of these branches of science, and having spent
many years in gathering and sifting materials for his present work,
the store of accurately registered facts upon which the author of the
'Origin of Species' is able to draw at will is prodigious.
But this very superabundance of matter must have been embarrassing to
a writer who, for the present, can only put forward an abstract of his
views; and thence it arises, perhaps, that notwithstanding the clearness
of the style, those who attempt fairly to digest the book find much of
it a sort of intellectual pemmican--a mass of facts crushed and pounded
into shape, rather than held together by the ordinary medium of an
obvious logical bond; due attention will, without doubt, discover this
bond, but it is often hard to find.
Again, from sheer want of room, much has to be taken for granted which
might readily enough be proved; and hence, while the adept, who can
supply the missing links in the evidence from his own knowledge,
discovers fresh proof of the singular thoroughness with which all
difficulties have been considered and all unjustifiable suppositions
avoided, at every reperusal of Mr. Darwin's pregnant paragraphs, the
novice in biology is apt to complain of the frequency of what he fancies
is gratuitous assumption.
Thus while it may be doubted if, for some years, any one is likely to be
competent to pronounce judgment on all the issues raised by Mr. Darwin,
there is assuredly abundant room for him, who, assuming the humbler,
though perhaps as useful, office of an interpreter between the 'Origin
of Species' and the public, contents himself with endeavouring to
point out the nature of the problems which it discusses; to distinguish
between the ascertained facts and the theoretical views which it
contains; and finally, to show the extent to which the explanation it
offers satisfies the requirements of scientific logic. At any rate, it
is this office which we purpose to undertake in the following pages.
It may be safely assumed that our readers have a general conception of
the nature of the objects to which the word "species" is applied; but it
has, perhaps, occurred to a few, even to those who are naturalists 'ex
professo', to reflect, that, as commonly employed, the term has a double
sense and denotes two very different orders of relations. When we call a
group of animals, or of plants, a species, we may imply thereby, either
that all these animals or plants have some common peculiarity of form
or structure; or, we may mean that they possess some common functional
character. That part of biological science which deals with form
and structure is called Morphology--that which concerns itself with
function, Physiology--so that we may conveniently speak of these two
senses, or aspects, of "species"--the one as morphological, the other
as physiological. Regarded from the former point of view, a species
is nothing more than a kind of animal or plant, which is distinctly
definable from all others, by certain constant, and not merely sexual,
morphological peculiarities. Thus horses form a species, because the
group of animals to which that name is applied is distinguished from all
others in the world by the following constantly associated characters.
They have--1, A vertebral column; 2, Mammae; 3, A placental embryo; 4,
Four legs; 5, A single well-developed toe in each foot provided with a
hoof; 6, A bushy tail; and 7, Callosities on the inner sides of both
the fore and the hind legs. The asses, again, form a distinct species,
because, with the same characters, as far as the fifth in the above
list, all asses have tufted tails, and have callosities only on the
inner side of the fore-legs. If animals were discovered having the
general characters of the horse, but sometimes with callosities only
on the fore-legs, and more or less tufted tails; or animals having the
general characters of the ass, but with more or less bushy tails,
and sometimes with callosities on both pairs of legs, besides being
intermediate in other respects--the two species would have to be merged
into one. They could no longer be regarded as morphologically distinct
species, for they would not be distinctly definable one from the other.
However bare and simple this definition of species may appear to be,
we confidently appeal to all practical naturalists, whether zoologists,
botanists, or palaeontologists, to say if, in the vast majority of
cases, they know, or mean to affirm anything more of the group of
animals or plants they so denominate than what has just been stated.
Even the most decided advocates of the received doctrines respecting
species admit this.
"I apprehend," says Professor Owen [2], "that few naturalists nowadays,
in describing and proposing a name for what they call 'a new species,'
use that term to signify what was meant by it twenty or thirty years
ago; that is, an originally distinct creation, maintaining its primitive
distinction by obstructive generative peculiarities. The proposer of the
new species now intends to state no more than he actually knows; as, for
example, that the differences on which he founds the specific character
are constant in individuals of both sexes, so far as observation has
reached; and that they are not due to domestication or to artificially
superinduced external circumstances, or to any outward influence within
his cognizance; that the species is wild, or is such as it appears by
Nature."
If we consider, in fact, that by far the largest proportion of recorded
existing species are known only by the study of their skins, or bones,
or other lifeless exuvia; that we are acquainted with none, or next to
none, of their physiological peculiarities, beyond those which can be
deduced from their structure, or are open to cursory observation; and
that we cannot hope to learn more of any of those extinct forms of life
which now constitute no inconsiderable proportion of the known Flora and
Fauna of the world: it is obvious that the definitions of these species
can be only of a purely structural, or morphological, character. It is
probable that naturalists would have avoided much confusion of ideas
if they had more frequently borne the necessary limitations of our
knowledge in mind. But while it may safely be admitted that we are
acquainted with only the morphological characters of the vast majority
of species--the functional or physiological, peculiarities of a few have
been carefully investigated, and the result of that study forms a large
and most interesting portion of the physiology of reproduction.
The student of Nature wonders the more and is astonished the less, the
more conversant he becomes with her operations; but of all the perennial
miracles she offers to his inspection, perhaps the most worthy of
admiration is the development of a plant or of an animal from its
embryo. Examine the recently laid egg of some common animal, such as a
salamander or newt. It is a minute spheroid in which the best microscope
will reveal nothing but a structureless sac, enclosing a glairy fluid,
holding granules in suspension. But strange possibilities lie dormant
in that semi-fluid globule. Let a moderate supply of warmth reach its
watery cradle, and the plastic matter undergoes changes so rapid, yet
so steady and purposelike in their succession, that one can only compare
them to those operated by a skilled modeller upon a formless lump of
clay. As with an invisible trowel, the mass is divided and subdivided
into smaller and smaller portions, until it is reduced to an aggregation
of granules not too large to build withal the finest fabrics of the
nascent organism. And, then, it is as if a delicate finger traced out
the line to be occupied by the spinal column, and moulded the contour
of the body; pinching up the head at one end, the tail at the other,
and fashioning flank and limb into due salamandrine proportions, in so
artistic a way, that, after watching the process hour by hour, one is
almost involuntarily possessed by the notion, that some more subtle aid
to vision than an achromatic, would show the hidden artist, with his
plan before him, striving with skilful manipulation to perfect his work.
As life advances, and the young amphibian ranges the waters, the terror
of his insect contemporaries, not only are the nutritious particles
supplied by its prey, by the addition of which to its frame, growth
takes place, laid down, each in its proper spot, and in such due
proportion to the rest, as to reproduce the form, the colour, and the
size, characteristic of the parental stock; but even the wonderful
powers of reproducing lost parts possessed by these animals are
controlled by the same governing tendency. Cut off the legs, the tail,
the jaws, separately or all together, and, as Spallanzani showed long
ago, these parts not only grow again, but the redintegrated limb is
formed on the same type as those which were lost. The new jaw, or leg,
is a newt's, and never by any accident more like that of a frog. What is
true of the newt is true of every animal and of every plant; the acorn
tends to build itself up again into a woodland giant such as that from
whose twig it fell; the spore of the humblest lichen reproduces the
green or brown incrustation which gave it birth; and at the other end of
the scale of life, the child that resembled neither the paternal nor the
maternal side of the house would be regarded as a kind of monster.
So that the one end to which, in all living beings, the formative
impulse is tending--the one scheme which the Archaeus of the old
speculators strives to carry out, seems to be to mould the offspring
into the likeness of the parent. It is the first great law of
reproduction, that the offspring tends to resemble its parent or
parents, more closely than anything else.
Science will some day show us how this law is a necessary consequence
of the more general laws which govern matter; but, for the present, more
can hardly be said than that it appears to be in harmony with them. We
know that the phenomena of vitality are not something apart from other
physical phenomena, but one with them; and matter and force are the two
names of the one artist who fashions the living as well as the
lifeless. Hence living bodies should obey the same great laws as other
matter--nor, throughout Nature, is there a law of wider application than
this, that a body impelled by two forces takes the direction of their
resultant. But living bodies may be regarded as nothing but extremely
complex bundles of forces held in a mass of matter, as the complex
forces of a magnet are held in the steel by its coercive force; and,
since the differences of sex are comparatively slight, or, in other
words, the sum of the forces in each has a very similar tendency, their
resultant, the offspring, may reasonably be expected to deviate but
little from a course parallel to either, or to both.
Represent the reason of the law to ourselves by what physical metaphor
or analogy we will, however, the great matter is to apprehend its
existence and the importance of the consequences deducible from it. For
things which are like to the same are like to one another; and if; in
a great series of generations, every offspring is like its parent, it
follows that all the offspring and all the parents must be like
one another; and that, given an original parental stock, with the
opportunity of undisturbed multiplication, the law in question
necessitates the production, in course of time, of an indefinitely large
group, the whole of whose members are at once very similar and are blood
relations, having descended from the same parent, or pair of parents.
The proof that all the members of any given group of animals, or plants,
had thus descended, would be ordinarily considered sufficient to entitle
them to the rank of physiological species, for most physiologists
consider species to be definable as "the offspring of a single primitive
stock."
But though it is quite true that all those groups we call species 'may',
according to the known laws of reproduction, have descended from a
single stock, and though it is very likely they really have done so,
yet this conclusion rests on deduction and can hardly hope to establish
itself upon a basis of observation. And the primitiveness of the
supposed single stock, which, after all, is the essential part of the
matter, is not only a hypothesis, but one which has not a shadow of
foundation, if by "primitive" he meant "independent of any other living
being." A scientific definition, of which an unwarrantable hypothesis
forms an essential part, carries its condemnation within itself;
but, even supposing such a definition were, in form, tenable, the
physiologist who should attempt to apply it in Nature would soon find
himself involved in great, if not inextricable, difficulties. As we have
said, it is indubitable that offspring 'tend' to resemble the parental
organism, but it is equally true that the similarity attained never
amounts to identity, either in form or in structure. There is always a
certain amount of deviation, not only from the precise characters of a
single parent, but when, as in most animals and many plants, the sexes
are lodged in distinct individuals, from an exact mean between the two
parents. And indeed, on general principles, this slight deviation seems
as intelligible as the general similarity, if we reflect how complex the
co-operating "bundles of forces" are, and how improbable it is that, in
any case, their true resultant shall coincide with any mean between
the more obvious characters of the two parents. Whatever be its cause,
however, the co-existence of this tendency to minor variation with the
tendency to general similarity, is of vast importance in its bearing on
the question of the origin of species.
As a general rule, the extent to which an offspring differs from its
parent is slight enough; but, occasionally, the amount of difference is
much more strongly marked, and then the divergent offspring receives the
name of a Variety. Multitudes, of what there is every reason to believe
are such varieties, are known, but the origin of very few has been
accurately recorded, and of these we will select two as more especially
illustrative of the main features of variation. The first of them is
that of the "Ancon," or "Otter" sheep, of which a careful account is
given by Colonel David Humphreys, F.R.S., in a letter to Sir Joseph
Banks, published in the Philosophical Transactions for 1813. It appears
that one Seth Wright, the proprietor of a farm on the banks of the
Charles River, in Massachusetts, possessed a flock of fifteen ewes and
a ram of the ordinary kind. In the year 1791, one of the ewes presented
her owner with a male lamb, differing, for no assignable reason, from
its parents by a proportionally long body and short bandy legs, whence
it was unable to emulate its relatives in those sportive leaps over the
neighbours' fences, in which they were in the habit of indulging, much
to the good farmer's vexation.
The second case is that detailed by a no less unexceptionable authority
than Reaumur, in his 'Art de faire eclore les Poulets'. A Maltese
couple, named Kelleia, whose hands and feet were constructed upon the
ordinary human model, had born to them a son, Gratio, who possessed six
perfectly movable fingers on each hand, and six toes, not quite so well
formed, on each foot. No cause could be assigned for the appearance of
this unusual variety of the human species.
Two circumstances are well worthy of remark in both these cases. In
each, the variety appears to have arisen in full force, and, as it were,
'per saltum'; a wide and definite difference appearing, at once, between
the Ancon ram and the ordinary sheep; between the six-fingered and
six-toed Gratio Kelleia and ordinary men. In neither case is it possible
to point out any obvious reason for the appearance of the variety.
Doubtless there were determining causes for these as for all other
phenomena; but they do not appear, and we can be tolerably certain that
what are ordinarily understood as changes in physical conditions, as in
climate, in food, or the like, did not take place and had nothing to do
with the matter. It was no case of what is commonly called adaptation
to circumstances; but, to use a conveniently erroneous phrase, the
variations arose spontaneously. The fruitless search after final causes
leads their pursuers a long way; but even those hardy teleologists, who
are ready to break through all the laws of physics in chase of their
favourite will-o'-the-wisp, may be puzzled to discover what purpose
could be attained by the stunted legs of Seth Wright's ram or the
hexadactyle members of Gratio Kelleia.
Varieties then arise we know not why; and it is more than probable that
the majority of varieties have arisen in this "spontaneous" manner,
though we are, of course, far from denying that they may be traced,
in some cases, to distinct external influences; which are assuredly
competent to alter the character of the tegumentary covering, to
change colour, to increase or diminish the size of muscles, to modify
constitution, and, among plants, to give rise to the metamorphosis of
stamens into petals, and so forth. But however they may have arisen,
what especially interests us at present is, to remark that, once in
existence, varieties obey the fundamental law of reproduction that like
tends to produce like; and their offspring exemplify it by tending
to exhibit the same deviation from the parental stock as themselves.
Indeed, there seems to be, in many instances, a pre-potent influence
about a newly-arisen variety which gives it what one may call an unfair
advantage over the normal descendants from the same stock. This is
strikingly exemplified by the case of Gratio Kelleia, who married a
woman with the ordinary pentadactyle extremities, and had by her
four children, Salvator, George, Andre, and Marie. Of these children
Salvator, the eldest boy, had six fingers and six toes, like his father;
the second and third, also boys, had five fingers and five toes,
like their mother, though the hands and feet of George were slightly
deformed. The last, a girl, had five fingers and five toes, but the
thumbs were slightly deformed. The variety thus reproduced itself purely
in the eldest, while the normal type reproduced itself purely in the
third, and almost purely in the second and last: so that it would seem,
at first, as if the normal type were more powerful than the variety.
But all these children grew up and intermarried with normal wives and
husband, and then, note what took place: Salvator had four children,
three of whom exhibited the hexadactyle members of their grandfather and
father, while the youngest had the pentadactyle limbs of the mother
and grandmother; so that here, notwithstanding a double pentadactyle
dilution of the blood, the hexadactyle variety had the best of it. The
same pre-potency of the variety was still more markedly exemplified in
the progeny of two of the other children, Marie and George. Marie (whose
thumbs only were deformed) gave birth to a boy with six toes, and three
other normally formed children; but George, who was not quite so pure a
pentadactyle, begot, first, two girls, each of whom had six fingers
and toes; then a girl with six fingers on each hand and six toes on the
right foot, but only five toes on the left; and lastly, a boy with only
five fingers and toes. In these instances, therefore, the variety, as
it were, leaped over one generation to reproduce itself in full force in
the next. Finally, the purely pentadactyle Andre was the father of many
children, not one of whom departed from the normal parental type.
If a variation which approaches the nature of a monstrosity can strive
thus forcibly to reproduce itself, it is not wonderful that less
aberrant modifications should tend to be preserved even more strongly;
and the history of the Ancon sheep is, in this respect, particularly
instructive. With the "'cuteness" characteristic of their nation, the
neighbours of the Massachusetts farmer imagined it would be an excellent
thing if all his sheep were imbued with the stay-at-home tendencies
enforced by Nature upon the newly-arrived ram; and they advised Wright
to kill the old patriarch of his fold, and install the Ancon ram in his
place. The result justified their sagacious anticipations, and coincided
very nearly with what occurred to the progeny of Gratio Kelleia. The
young lambs were almost always either pure Ancons, or pure ordinary
sheep. [3] But when sufficient Ancon sheep were obtained to interbreed
with one another, it was found that the offspring was always pure Ancon.
Colonel Humphreys, in fact, states that he was acquainted with only "one
questionable case of a contrary nature." Here, then, is a remarkable
and well-established instance, not only of a very distinct race being
established 'per saltum', but of that race breeding "true" at once, and
showing no mixed forms, even when crossed with another breed.
By taking care to select Ancons of both sexes, for breeding from, it
thus became easy to establish an extremely well-marked race; so peculiar
that, even when herded with other sheep, it was noted that the Ancons
kept together. And there is every reason to believe that the existence
of this breed might have been indefinitely protracted; but the
introduction of the Merino sheep, which were not only very superior to
the Ancons in wool and meat, but quite as quiet and orderly, led to the
complete neglect of the new breed, so that, in 1813, Colonel Humphreys
found it difficult to obtain the specimen, whose skeleton was presented
to Sir Joseph Banks. We believe that, for many years, no remnant of it
has existed in the United States.
Gratio Kelleia was not the progenitor of a race of six-fingered men, as
Seth Wright's ram became a nation of Ancon sheep, though the tendency of
the variety to perpetuate itself appears to have been fully as strong
in the one case as in the other. And the reason of the difference is
not far to seek. Seth Wright took care not to weaken the Ancon blood by
matching his Ancon ewes with any but males of the same variety, while
Gratio Kelleia's sons were too far removed from the patriarchal times
to intermarry with their sisters; and his grandchildren seem not to have
been attracted by their six-fingered cousins. In other words, in the one
example a race was produced, because, for several generations, care
was taken to 'select' both parents of the breeding stock from animals
exhibiting a tendency to vary in the same condition; while, in the
other, no race was evolved, because no such selection was exercised.
A race is a propagated variety; and as, by the laws of reproduction,
offspring tend to assume the parental forms, they will be more likely to
propagate a variation exhibited by both parents than that possessed by
only one.
There is no organ of the body of an animal which may not, and does not,
occasionally, vary more or less from the normal type; and there is
no variation which may not be transmitted and which, if selectively
transmitted, may not become the foundation of a race. This great truth,
sometimes forgotten by philosophers, has long been familiar to practical
agriculturists and breeders; and upon it rest all the methods of
improving the breeds of domestic animals, which, for the last century,
have been followed with so much success in England. Colour, form, size,
texture of hair or wool, proportions of various parts, strength or
weakness of constitution, tendency to fatten or to remain lean, to give
much or little milk, speed, strength, temper, intelligence, special
instincts; there is not one of these characters whose transmission is
not an every-day occurrence within the experience of cattle-breeders,
stock-farmers, horse-dealers, and dog and poultry fanciers. Nay, it
is only the other day that an eminent physiologist, Dr. Brown-Sequard,
communicated to the Royal Society his discovery that epilepsy,
artificially produced in guinea-pigs, by a means which he has
discovered, is transmitted to their offspring.
But a race, once produced, is no more a fixed and immutable entity than
the stock whence it sprang; variations arise among its members, and
as these variations are transmitted like any others, new races may be
developed out of the pre-existing one 'ad infinitum', or, at least,
within any limit at present determined. Given sufficient time and
sufficiently careful selection, and the multitude of races which
may arise from a common stock is as astonishing as are the extreme
structural differences which they may present. A remarkable example of
this is to be found in the rock-pigeon, which Dr. Darwin has, in our
opinion, satisfactorily demonstrated to be the progenitor of all our
domestic pigeons, of which there are certainly more than a hundred
well-marked races. The most noteworthy of these races are, the four
great stocks known to the "fancy" as tumblers, pouters, carriers, and
fantails; birds which not only differ most singularly in size, colour,
and habits, but in the form of the beak and of the skull: in the
proportions of the beak to the skull; in the number of tail-feathers; in
the absolute and relative size of the feet; in the presence or absence
of the uropygial gland; in the number of vertebrae in the back; in
short, in precisely those characters in which the genera and species of
birds differ from one another.
And it is most remarkable and instructive to observe, that none of these
races can be shown to have been originated by the action of changes
in what are commonly called external circumstances, upon the wild
rock-pigeon. On the contrary, from time immemorial, pigeon-fanciers have
had essentially similar methods of treating their pets, which have
been housed, fed, protected and cared for in much the same way in all
pigeonries. In fact, there is no case better adapted than that of
the pigeons to refute the doctrine which one sees put forth on
high authority, that "no other characters than those founded on the
development of bone for the attachment of muscles" are capable of
variation. In precise contradiction of this hasty assertion, Mr.
Darwin's researches prove that the skeleton of the wings in domestic
pigeons has hardly varied at all from that of the wild type; while, on
the other hand, it is in exactly those respects, such as the relative
length of the beak and skull, the number of the vertebrae, and the
number of the tail-feathers, in which muscular exertion can have no
important influence, that the utmost amount of variation has taken
place.
We have said that the following out of the properties exhibited by
physiological species would lead us into difficulties, and at this point
they begin to be obvious; for if, as the result of spontaneous variation
and of selective breeding, the progeny of a common stock may become
separated into groups distinguished from one another by constant, not
sexual, morphological characters, it is clear that the physiological
definition of species is likely to clash with the morphological
definition. No one would hesitate to describe the pouter and the tumbler
as distinct species, if they were found fossil, or if their skins and
skeletons were imported, as those of exotic wild birds commonly
are--and without doubt, if considered alone, they are good and distinct
morphological species. On the other hand, they are not physiological
species, for they are descended from a common stock, the rock-pigeon.
Under these circumstances, as it is admitted on all sides that races
occur in Nature, how are we to know whether any apparently distinct
animals are really of different physiological species, or not, seeing
that the amount of morphological difference is no safe guide? Is there
any test of a physiological species? The usual answer of physiologists
is in the affirmative. It is said that such a test is to be found in
the phenomena of hybridization--in the results of crossing races, as
compared with the results of crossing species.
So far as the evidence goes at present, individuals, of what are
certainly known to be mere races produced by selection, however distinct
they may appear to be, not only breed freely together, but the offspring
of such crossed races are only perfectly fertile with one another. Thus,
the spaniel and the greyhound, the dray-horse and the Arab, the
pouter and the tumbler, breed together with perfect freedom, and their
mongrels, if matched with other mongrels of the same kind, are equally
fertile.
On the other hand, there can be no doubt that the individuals of
many natural species are either absolutely infertile if crossed with
individuals of other species, or, if they give rise to hybrid offspring,
the hybrids so produced are infertile when paired together. The horse
and the ass, for instance, if so crossed, give rise to the mule, and
there is no certain evidence of offspring ever having been produced by a
male and female mule. The unions of the rock-pigeon and the
ring-pigeon appear to be equally barren of result. Here, then, says the
physiologist, we have a means of distinguishing any two true species
from any two varieties. If a male and a female, selected from each
group, produce offspring, and that offspring is fertile with others
produced in the same way, the groups are races and not species. If, on
the other hand, no result ensues, or if the offspring are infertile with
others produced in the same way, they are true physiological species.
The test would be an admirable one, if, in the first place, it were
always practicable to apply it, and if, in the second, it always yielded
results susceptible of a definite interpretation. Unfortunately, in
the great majority of cases, this touchstone for species is wholly
inapplicable.
The constitution of many wild animals is so altered by confinement that
they will not breed even with their own females, so that the negative
results obtained from crosses are of no value; and the antipathy of wild
animals of the same species for one another, or even of wild and tame
members of the same species, is ordinarily so great, that it is hopeless
to look for such unions in Nature. The hermaphrodism of most plants,
the difficulty in the way of insuring the absence of their own, or the
proper working of other pollen, are obstacles of no less magnitude
in applying the test to them. And, in both animals and plants, is
superadded the further difficulty, that experiments must be continued
over a long time for the purpose of ascertaining the fertility of the
mongrel or hybrid progeny, as well as of the first crosses from which
they spring.
Not only do these great practical difficulties lie in the way of
applying the hybridization test, but even when this oracle can be
questioned, its replies are sometimes as doubtful as those of Delphi.
For example, cases are cited by Mr. Darwin, of plants which are more
fertile with the pollen of another species than with their own; and
there are others, such as certain 'fuci', whose male element will
fertilize the ovule of a plant of distinct species, while the males of
the latter species are ineffective with the females of the first. So
that, in the last-named instance, a physiologist, who should cross the
two species in one way, would decide that they were true species; while
another, who should cross them in the reverse way, would, with equal
justice, according to the rule, pronounce them to be mere races. Several
plants, which there is great reason to believe are mere varieties, are
almost sterile when crossed; while both animals and plants, which have
always been regarded by naturalists as of distinct species, turn out,
when the test is applied, to be perfectly fertile. Again, the sterility
or fertility of crosses seems to bear no relation to the structural
resemblances or differences of the members of any two groups.
Mr. Darwin has discussed this question with singular ability and
circumspection, and his conclusions are summed up as follows, at page
276 of his work:--
"First crosses between forms sufficiently distinct to be ranked as
species, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that
the two most careful experimentalists who have ever lived have come to
diametrically opposite conclusions in ranking forms by this test. The
sterility is innately variable in individuals of the same species, and
is eminently susceptible of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity, but is
governed by several curious and complex laws. It is generally different
and sometimes widely different, in reciprocal crosses between the same
two species. It is not always equal in degree in a first cross, and in
the hybrid produced from this cross.
"In the same manner as in grafting trees, the capacity of one species
or variety to take on another is incidental on generally unknown
differences in their vegetative systems; so in crossing, the greater
or less facility of one species to unite with another is incidental
on unknown differences in their reproductive systems. There is no more
reason to think that species have been specially endowed with various
degrees of sterility to prevent them crossing and breeding in Nature,
than to think that trees have been specially endowed with various and
somewhat analogous degrees of difficulty in being grafted together, in
order to prevent them becoming inarched in our forests.
"The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances;
in some cases largely on the early death of the embryo. The sterility of
hybrids which have their reproductive systems imperfect, and which
have had this system and their whole organization disturbed by being
compounded of two distinct species, seems closely allied to that
sterility which so frequently affects pure species when their natural
conditions of life have been disturbed. This view is supported by a
parallelism of another kind: namely, that the crossing of forms, only
slightly different, is favourable to the vigour and fertility of
the offspring; and that slight changes in the conditions of life are
apparently favourable to the vigour and fertility of all organic beings.
It is not surprising that the degree of difficulty in uniting two
species, and the degree of sterility of their hybrid offspring, should
generally correspond, though due to distinct causes; for both depend
on the amount of difference of some kind between the species which are
crossed. Nor is it surprising that the facility of effecting a first
cross, the fertility of hybrids produced from it, and the capacity of
being grafted together--though this latter capacity evidently depends
on widely different circumstances--should all run to a certain extent
parallel with the systematic affinity of the forms which are subjected
to experiment; for systematic affinity attempts to express all kinds of
resemblance between all species.
"First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable we
are to argue in a circle with respect to varieties in a state of Nature;
and when we remember that the greater number of varieties have
been produced under domestication by the selection of mere external
differences, and not of differences in the reproductive system. In
all other respects, excluding fertility, there is a close general
resemblance between hybrids and mongrels."--Pp. 276-8.
We fully agree with the general tenor of this weighty passage; but
forcible as are these arguments, and little as the value of fertility or
infertility as a test of species may be, it must not be forgotten that
the really important fact, so far as the inquiry into the origin of
species goes, is, that there are such things in Nature as groups of
animals and of plants, whose members are incapable of fertile union with
those of other groups; and that there are such things as hybrids, which
are absolutely sterile when crossed with other hybrids. For, if such
phenomena as these were exhibited by only two of those assemblages of
living objects, to which the name of species (whether it be used in its
physiological or in its morphological sense) is given, it would have
to be accounted for by any theory of the origin of species, and every
theory which could not account for it would be, so far, imperfect.
Up to this point, we have been dealing with matters of fact, and the
statements which we have laid before the reader would, to the best of
our knowledge, be admitted to contain a fair exposition of what is at
present known respecting the essential properties of species, by all who
have studied the question. And whatever may be his theoretical views, no
naturalist will probably be disposed to demur to the following summary
of that exposition:--
Living beings, whether animals or plants, are divisible into multitudes
of distinctly definable kinds, which are morphological species. They are
also divisible into groups of individuals, which breed freely together,
tending to reproduce their like, and are physiological species. Normally
resembling their parents, the offspring of members of these species are
still liable to vary; and the variation may be perpetuated by selection,
as a race, which race, in many cases, presents all the characteristics
of a morphological species. But it is not as yet proved that a race
ever exhibits, when crossed with another race of the same species, those
phenomena of hybridization which are exhibited by many species when
crossed with other species. On the other hand, not only is it not proved
that all species give rise to hybrids infertile 'inter se', but there
is much reason to believe that, in crossing, species exhibit every
gradation from perfect sterility to perfect fertility.
Such are the most essential characteristics of species. Even were man
not one of them--a member of the same system and subject to the same
laws--the question of their origin, their causal connexion, that is,
with the other phenomena of the universe, must have attracted his
attention, as soon as his intelligence had raised itself above the level
of his daily wants.
Indeed history relates that such was the case, and has embalmed for us
the speculations upon the origin of living beings, which were among the
earliest products of the dawning intellectual activity of man. In those
early days positive knowledge was not to be had, but the craving
after it needed, at all hazards, to be satisfied, and according to the
country, or the turn of thought, of the speculator, the suggestion that
all living things arose from the mud of the Nile, from a primeval
egg, or from some more anthropomorphic agency, afforded a sufficient
resting-place for his curiosity. The myths of Paganism are as dead as
Osiris or Zeus, and the man who should revive them, in opposition to the
knowledge of our time, would be justly laughed to scorn; but the coeval
imaginations current among the rude inhabitants of Palestine, recorded
by writers whose very name and age are admitted by every scholar to be
unknown, have unfortunately not yet shared their fate, but, even at
this day, are regarded by nine-tenths of the civilized world as the
authoritative standard of fact and the criterion of the justice of
scientific conclusions, in all that relates to the origin of things,
and, among them, of species. In this nineteenth century, as at the dawn
of modern physical science, the cosmogony of the semi-barbarous Hebrew
is the incubus of the philosopher and the opprobrium of the orthodox.
Who shall number the patient and earnest seekers after truth, from the
days of Galileo until now, whose lives have been embittered and their
good name blasted by the mistaken zeal of Bibliolaters? Who shall count
the host of weaker men whose sense of truth has been destroyed in the
effort to harmonize impossibilities--whose life has been wasted in the
attempt to force the generous new wine of Science into the old bottles
of Judaism, compelled by the outcry of the same strong party?
It is true that if philosophers have suffered, their cause has been
amply avenged. Extinguished theologians lie about the cradle of every
science as the strangled snakes beside that of Hercules; and history
records that whenever science and orthodoxy have been fairly opposed,
the latter has been forced to retire from the lists, bleeding and
crushed if not annihilated; scotched, if not slain. But orthodoxy is the
Bourbon of the world of thought. It learns not, neither can it forget;
and though, at present, bewildered and afraid to move, it is as willing
as ever to insist that the first chapter of Genesis contains the
beginning and the end of sound science; and to visit, with such petty
thunderbolts as its half-paralysed hands can hurl, those who refuse to
degrade Nature to the level of primitive Judaism.
Philosophers, on the other hand, have no such aggressive tendencies.
With eyes fixed on the noble goal to which "per aspera et ardua" they
tend, they may, now and then, be stirred to momentary wrath by the
unnecessary obstacles with which the ignorant, or the malicious,
encumber, if they cannot bar, the difficult path; but why should their
souls be deeply vexed? The majesty of Fact is on their side, and the
elemental forces of Nature are working for them. Not a star comes to the
meridian at its calculated time but testifies to the justice of their
methods--their beliefs are "one with falling rain and with the growing
corn." By doubt they are established, and open inquiry is their bosom
friend. Such men have no fear of traditions however venerable, and no
respect for them when they become mischievous and obstructive; but they
have better than mere antiquarian business in hand, and if dogmas, which
ought to be fossil but are not, are not forced upon their notice, they
are too happy to treat them as non-existent.
The hypotheses respecting the origin of species which profess to stand
upon a scientific basis, and, as such, alone demand serious attention,
are of two kinds. The one, the "special creation" hypothesis, presumes
every species to have originated from one or more stocks, these not
being the result of the modification of any other form of living
matter--or arising by natural agencies--but being produced, as such, by
a supernatural creative act.
The other, the so-called "transmutation" hypothesis, considers that
all existing species are the result of the modification of pre-existing
species, and those of their predecessors, by agencies similar to those
which at the present day produce varieties and races, and therefore in
an altogether natural way; and it is a probable, though not a necessary
consequence of this hypothesis, that all living beings have arisen from
a single stock. With respect to the origin of this primitive stock,
or stocks, the doctrine of the origin of species is obviously not
necessarily concerned. The transmutation hypothesis, for example, is
perfectly consistent either with the conception of a special creation of
the primitive germ, or with the supposition of its having arisen, as a
modification of inorganic matter, by natural causes.
The doctrine of special creation owes its existence very largely to the
supposed necessity of making science accord with the Hebrew cosmogony;
but it is curious to observe that, as the doctrine is at present
maintained by men of science, it is as hopelessly inconsistent with the
Hebrew view as any other hypothesis.