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, André, 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 André 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.[62] 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 direction; 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 form, 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-Séquard,
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 ones _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 Mr. 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 vertebræ 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 vertebræ, 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 a 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
phænomena 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 different 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 ensuring 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 follow, 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
phænomena 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
phænomena 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 phænomena 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?