THEORY OF DEVELOPMENT ACCORDING TO LYELL AND DARWIN.
Charles Lyell’s Principles of Geology.—His Natural
History of the Earth’s Development.—Origin of the
Greatest Effects through the Multiplication of the
Smallest Causes.—Unlimited Extent of Geological
Periods.—Lyell’s Refutation of Cuvier’s History of
Creation.—The Establishment of the Uninterrupted
Connection of Historical Development by Lyell and
Darwin.—Biographical Notice of Charles Darwin.—His
Scientific Works.—His Theory of Coral Reefs.—Development
of the Theory of Selection.—A Letter of Darwin’s.—The
Contemporaneous Appearance of Darwin’s and Alfred
Wallace’s Theory of Selection.—Darwin’s Study of Domestic
Animals and Cultivated Plants.—Andreas Wagner’s notions
as to the Special Creation of Cultivated Organisms for the
good of Man.—The Tree of Knowledge in
Paradise.—Comparison between Wild and Cultivated
Organisms.—Darwin’s Study of Domestic
Pigeons.—Importance of Pigeon Breeding.—Common Descent
of all Races of Pigeons.
During the thirty years, from 1830 until 1859, when Darwin’s work
appeared, the ideas of creation introduced by Cuvier remained
predominant in the sciences of organic nature. People rested satisfied
with the unscientific assumption, that in the course of the earth’s
history, a series of inexplicable revolutions had periodically
annihilated the whole world of animals and plants, and that at the end
of each revolution, and the beginning of a new period, a new enlarged,
and improved edition of the organic population had appeared. Although
the number of these editions of creation was altogether problematical,
and in truth could not be fixed at all, and although the numerous
advances which, during this time, were made in all the departments of
zoology and botany demonstrated more and more that Cuvier’s hypothesis
was unfounded and untenable, and that Lamarck’s natural theory of
development was nearer the truth, yet the former maintained its
authority almost universally among biologists. This must, above all, be
ascribed to the veneration which Cuvier had acquired, and strikingly
illustrates how injurious to the progress of humanity a faith in any
definite authority may become. Authority, as Goethe once admirably said,
perpetuates the individual, which as an individual should pass away,
rejects and allows to pass that which should be held fast, and is the
main obstacle to the advance of humanity.
It is only by having regard to the great weight of Cuvier’s authority,
and to the mighty potency of human indolence, which is with difficulty
induced to depart from the broad and comfortable way of everyday
conceptions, and to enter upon new paths not yet made easy, that we can
comprehend how it is that Lamarck’s Theory of Descent did not gain its
due recognition until 1859, after Darwin had given it a new foundation.
The soil had long been prepared for it by the works of Charles Lyell,
another English naturalist, whose views are of great importance for the
natural history of creation, and must accordingly here be briefly
explained.
In 1830 Charles Lyell published, under the title of “Principles of
Geology,” a work in which he thoroughly reformed the science of Geology
and the history of the earth’s development, and effected this reform in
a manner similar to that in which, thirty years later, Darwin in his
work reformed the science of Biology. Lyell’s great treatise, which
radically destroyed Cuvier’s hypothesis of creation, appeared in the
same year in which Cuvier celebrated his triumph over the
nature-philosophy, and established his supremacy in the domain of
morphology for the following thirty years. Whilst Cuvier, by his
artificial hypothesis of creation and his theory of catastrophes
connected with it, directly obstructed the path of the theory of natural
development, and cut off all chance of a natural explanation, Lyell once
more opened a free road, and brought forward convincing geological
evidence to show that Cuvier’s dualistic conceptions were as unfounded
as they were superfluous. He demonstrated that those changes of the
earth’s surface, which are still taking place before our eyes, are
perfectly sufficient to explain everything we know of the development of
the earth’s crust in general, and that it is superfluous and useless to
seek for mysterious causes in inexplicable revolutions. He showed that
we need only have recourse to the hypothesis of exceedingly long periods
of time in order to explain the formation of the crust of the earth in
the simplest and most natural manner by means of the very same causes
which are still active. Many geologists had previously imagined that the
highest chains of mountains which rise on the surface of the earth could
owe their origin only to enormous revolutions transforming a great part
of the earth’s surface, especially to colossal volcanic eruptions. Such
chains of mountains as those of the Alps or the Cordilleras were
believed to have arisen direct from the fiery fluid of the interior of
the earth, through an enormous chasm in the broken crust. Lyell, on the
other hand, showed that we can explain the formation of such enormous
chains of mountains quite naturally by the same slow and imperceptible
risings and depressions of the earth’s surface which are still
continually taking place, and the causes of which are by no means
miraculous. Although these depressions and risings may perhaps amount
only to a few inches, or at most a few feet, in the course of a century;
still, in the course of some millions of years they are perfectly
sufficient to raise up the highest chains of mountains, without the aid
of mysterious and incomprehensible revolutions. In like manner, the
meteorological action of the atmosphere, the influence of rain and snow,
and, lastly, the breakers on the coasts, which by themselves seem to
produce an insignificant effect, must cause the greatest changes if we
only allow sufficiently long periods for their action. The
multiplication of the smallest causes produces the greatest effects.
Drops of water produce a cavity in a rock.
I shall afterwards be obliged again to recur to the immeasurable length
of geological periods which are necessary for this purpose, for, as we
shall see, Darwin’s theory, as well as that of Lyell, renders the
assumption of immense periods absolutely necessary. If the earth and its
organisms have actually developed in a natural way, this slow and
gradual development must certainly have taken a length of time which
surpasses our powers of comprehension. But as many men see in this very
circumstance one of the principal difficulties in the way of those
theories of development, I beg leave here to remark that we have not a
single rational ground for conceiving the time requisite to be limited
in any way. Not only many ordinary persons, but even eminent
naturalists, make it their chief objection to these theories, that they
arbitrarily claim too great a length of time: yet the ground of
objection is scarcely intelligible. For it is absolutely impossible to
see what can, in any way, limit us in assuming long periods of time. We
have long known, even from the structure of the stratified crust of the
earth alone, that its origin and the formation of neptunic rocks from
water must have taken, at least, several millions of years. From a
strictly philosophical point of view, it makes no difference whether we
hypothetically assume for this process ten millions or ten thousand
billions of years. Before us and behind us lies eternity. If the
assumption of such enormous periods is opposed to the feelings of many,
I regard this simply as the consequence of false notions which are
impressed upon us from our earliest youth concerning the short history
of the earth, which is said to embrace only a few thousands of years.
Albert Lange, in his “History of Materialism,”(12) has convincingly
shown that from a strictly philosophical point of view it is far less
objectionable in a scientific hypothesis to assume periods which are too
long than periods which are too short. Every process of development is
the more intelligible the longer it is assumed to last. A short and
limited period is the most improbable.
I have no space here to enter minutely into Lyell’s great work, and will
therefore mention only its most important result, which is, that he
completely refuted Cuvier’s history of creation with its mythical
revolutions, and established in its place the constant and slow
transformation of the earth’s crust by the continued action of forces,
which are still working on the earth’s surface, viz., the movement of
water and the volcanic fluid of the interior of earth. Lyell thus
demonstrated a continuous and uninterrupted connection of the whole
history of the earth, and he proved it so irrefutably, and established
so convincingly the supremacy of the “existing causes,” that is, of the
causes which are still active in the transformation of the earth’s
crust, that Geology in a short time completely renounced Cuvier’s
hypothesis.
Now, it is remarkable that Palæontology, the science of petrifactions,
so far as it was pursued by botanists and zoologists, remained
apparently unaffected by this great progress in geology. Biology still
continued to assume repeated new creations of the whole animal and
vegetable kingdoms, at the beginning of every new period of the earth’s
history, although this hypothesis of individual creations, shoved into
the world one after the other, without the assumption of Cuvier’s
cataclysms, became pure nonsense, and lost its foundation. It is
evidently perfectly absurd to assume a distinct new creation of the
whole world of animals and plants at definite epochs, without the crust
of the earth itself experiencing any considerable general revolution.
And although this conception is most closely connected with Cuvier’s
theory of catastrophes, still it prevailed when the latter had been
completely destroyed and abandoned.
It was reserved for the great English naturalist, Charles Darwin, to
remove this contradiction, and to show that the organic beings of the
earth have a history as continuous and connected as the inorganic crust
of the earth; that animals and plants have arisen from one another by as
gradual a transmutation as that by which the varying forms of the
earth’s crust, the forms of the continents, and of the seas surrounding
and separating them, have arisen out of earlier and quite different
forms. In this respect we may truly say that in the domain of Zoology
and Botany Darwin made the same progress as Lyell, his great countryman,
in the domain of Geology. Both proved the _uninterrupted connection of
the historical development_, and demonstrated a gradual transmutation of
the different conditions succeeding one another.
The special merit of Darwin, as I have already remarked in a preceding
chapter, is twofold. In the first place, he has treated the Theory of
Descent, put forth by Lamarck and Goethe, in a much more comprehensive
manner, as a whole, and carried it out in a much more connected manner,
than had been done by any one of his predecessors. Secondly, he has
established the causal foundation of this Theory of Descent by the
Theory of Selection, which is peculiarly his own; that is, he has
demonstrated the acting _causes of the changes_ which the Theory of
Descent simply stated, as _facts_. The Theory of Descent, introduced
into Biology in 1809, by Lamarck, asserts that all the different species
of animals and plants are descended from a single or some few most
simple prototypes, produced by spontaneous generation. The Theory of
Selection, established in 1859 by Darwin, shows us _why_ this must be
so; it points out the acting causes in a manner with which Kant would
have been delighted, and indeed, in the domain of organic nature, Darwin
has become the Newton whose advent Kant thought himself entitled
prophetically to deny.
Now, before we approach Darwin’s theory, it will perhaps be of interest
to notice a few details as to the personal character of this great
naturalist, as to his life, and the way in which he was led to form his
doctrine. Charles Robert Darwin was born at Shrewsbury, on the Severn,
on the 12th of February, 1809; therefore, at present he is sixty-three
years old. In his seventeenth year (1825) he entered the University of
Edinburgh, and two years later Christ’s College, Cambridge. When
scarcely twenty-two years old, in 1831, he was invited to take part in a
scientific expedition which was sent out by England, in order to survey
accurately the southernmost point of South America, and to examine
several parts of the South Seas. This expedition, like many other
voyages of inquiry fitted out in a praiseworthy manner by England, had
scientific objects, and at the same time was intended to solve practical
problems relating to navigation. The vessel, commanded by Captain
Fitzroy, appropriately bore the symbolic name of the _Beagle_. The
voyage of the _Beagle_, which lasted five years, was of the highest
importance to the full development of Darwin’s genius; for in the very
first year, when he set his foot on the soil of South America, the
outline of the doctrine of descent dawned upon him. Darwin himself has
described this voyage in a work which is written in a very attractive
style, and the perusal of which I strongly recommend to the reader. This
book of travel, which lies far above the usual average in interest, not
only shows in a very charming manner Darwin’s amiable character, but we
can in many ways recognize the various steps by which he arrived at his
conceptions. The result of the voyage was, first, a large scientific
work, the zoological and geological portion of which belong in a great
measure to Darwin; and secondly, a celebrated work by him alone on Coral
Reefs, which in itself would have sufficed to secure to him a lasting
reputation. It is well known that the islands in the South Seas consist
for the most part of coral reefs, and are surrounded by them. Formerly
no satisfactory explanation could be given of their different and
remarkable forms, and of their relation to those islands which are not
formed of corals. It was reserved for Darwin to solve this difficult
problem, for together with the constructive action of the coral
zoophytes, he assumed geological risings and depressions of the bottom
of the sea to account for the origin of the different forms of reefs.
Darwin’s Theory of the Origin of Coral Reefs, like his later one as to
the Origin of Organic Species, is a theory which fully explains the
phenomenon, and for this purpose assumes only the simplest natural
causes, without hypothetically supporting it with any unknown processes.
Among the remaining works of Darwin, I must not pass over his excellent
monograph on Cirrhipedia, a curious class of marine animals, which in
their outward appearance resemble mussels, and were actually considered
by Cuvier as Molluscs possessing two shells, while in truth they
belonged to the Crustacea (crabs).
The extraordinary hardships to which Darwin had been exposed during his
voyage in the _Beagle_ had injured his health to such a degree, that
after his return home he was obliged to withdraw from the restless
turmoil of London life, and since then has lived in quiet retirement on
his estate at Down, near Bromley, in Kent. This seclusion from the
restless activity of the great city certainly exercised a beneficial
influence upon Darwin, and it is probable that we owe to it, at least
partially, the formation of the Theory of Selection. Undisturbed by the
various engagements which in London would have wasted his strength, he
was enabled to concentrate his attention upon the great problem to
which his mind had been turned during his voyage in the _Beagle_. In
order to show what kind of observations during the voyage principally
gave rise to the fundamental idea of the Theory of Selection, and in
what manner he afterwards worked it out, I shall insert here a passage
from a letter which he addressed to me on the 8th of October, 1864.
_Letter from Charles Darwin to Haeckel, 8th October, 1864._
“In South America three classes of facts were brought strongly before my
mind. _Firstly_, the manner in which closely allied species replace
species in going southward. _Secondly_, the close affinity of the
species inhabiting the islands near South America to those proper to the
continent. This struck me profoundly, especially the difference of the
species in the adjoining islets in the Galopagos Archipelago. _Thirdly_,
the relation of the living Edentata and Rodentia to the extinct species.
I shall never forget my astonishment when I dug out a gigantic piece of
armour like that of the living armadillo.
“Reflecting on these facts, and collecting analogous ones, it seemed to
me probable that allied species were descended from a common parent. But
for some years I could not conceive how each form became so excellently
adapted to its habits of life. I then began systematically to study
domestic productions, and after a time saw clearly that man’s selective
power was the most important agent. I was prepared, from having studied
the habits of animals, to appreciate the struggle for existence, and my
work in geology gave me some idea of the lapse of past time. Therefore,
when I happened to read “Malthus on Population,” the idea of natural
selection flashed on me. Of all the minor points, the last which I
appreciated was the importance and cause of the principle of
divergence.”
During the leisure and retirement in which Darwin lived after his
return, he occupied himself, as we see from this letter, first and
specially with the study of organisms in their cultivated state; that
is, domestic animals and garden plants. This was undoubtedly the most
likely way to arrive at the Theory of Selection. In this, as in all his
labours, Darwin proceeded with extreme care and accuracy. With wonderful
caution and self-denial, he published nothing on this subject during a
period of twenty-one years, from 1837 to 1858, not even a preliminary
sketch of his theory, which he had written as early as 1844. He was
always anxious to collect still more certain experimental proofs, in
order to be able to establish his theory in a complete form, and on the
broadest possible foundation of experience. While he was thus aiming at
the greatest possible perfection, which might perhaps have led him never
to publish his theory at all, he was fortunately disturbed by a
countryman of his, who, independently of Darwin, had discovered the
Theory of Selection, and in 1858 sent its outlines to Darwin himself,
with the request to hand them to Lyell for publication in some English
journal. This was Alfred Wallace, one of the boldest and most
distinguished scientific travellers of modern times. For many years
Wallace had wandered alone in the wilds of the Sunda Islands, in the
dense primitive forests of the Indian Archipelago; and during this close
and comprehensive study of one of the richest and most interesting parts
of the earth, with its great variety of animals and plants, he had
arrived at exactly the same general views regarding the origin of
organic species as Darwin. Lyell and Hooker, both of whom had long known
Darwin’s work, now induced him to publish a short extract from his
manuscripts simultaneously with the manuscript sent him by Wallace. They
appeared in the _Journal of the Linnean Society_, August, 1858.
Darwin’s great work “On the Origin of Species,” in which the Theory of
Selection is carried out in detail, appeared in November, 1859. Darwin
himself, however, characterizes this book (of which a fifth edition
appeared in 1869, and the German translation by Bronn as early as
1860)(1) as only a preliminary extract from a larger and more detailed
work, which is to contain a mass of facts in favour of his theory, and
comprehensive and experimental proofs. The first part of the larger work
promised by Darwin appeared in 1868, under the title, “The Variations of
Animals and Plants in the State of Domestication,” and has been
translated into German by Victor Carus.(14) It contains a rich abundance
of the most valuable evidence as to the extraordinary changes of organic
forms which man can produce by cultivation and artificial selection.
However much we are indebted to Darwin for this abundance of convincing
facts, still we do not by any means share the opinion of those
naturalists who hold that the Theory of Selection requires for its
actual proof these further details. It is our opinion that Darwin’s
first work, which appeared in 1859, already contains sufficient proof.
The unassailable strength of his theory does not lie in the immense
amount of individual facts that may be adduced as proofs, but in the
harmonious connection of all the great and general phenomena of organic
nature, which agree in bearing testimony to the truth of the Theory of
Selection.
Darwin, at first, intentionally did not notice the important conclusion
from his Theory of Descent, namely, the descent of the human race from
other mammals. It was not till this highly important conclusion had been
definitely established by other naturalists as the necessary sequel of
the doctrine of descent, that Darwin himself expressly endorsed it, and
thereby completed his system. This was done in the highly interesting
work, “The Descent of Man, and Sexual Selection,” which appeared as late
as 1871, and has likewise been translated into German by Victor
Carus.(48)
The careful study which Darwin devoted to _domestic animals and
cultivated plants_ was of the greatest importance in establishing the
Theory of Selection. The infinitely varied changes of form which man has
produced in these domesticated organisms by artificial selection are of
the very highest importance for a right understanding of animal and
vegetable forms; and yet this study has, down to the most recent times,
been most grossly neglected by zoologists and botanists. Without
entering upon the discussion of the significance to be attached to the
idea of species itself, they have filled not only bulky volumes, but
whole libraries, with descriptions of individual species, and with most
childish controversies as to whether these species are good, or
tolerably good, and bad, or tolerably bad. If naturalists instead of
spending their time on these useless fancies had duly studied cultivated
organisms, and had examined the transmutation of the living forms,
instead of the individual dead ones, they would not have been led
captive so long by the fetters of Cuvier’s dogma. But as cultivated
organisms are so extremely inconvenient to the dogmatic conception of
the permanence of species, naturalists to a great extent intentionally
did not concern themselves about them, and even celebrated naturalists
have often expressed the opinion that cultivated organisms, domesticated
animals and garden plants, are artificial productions of man, and that
their formation and transformation could not decide anything about the
nature of species and about the origin of the forms of species that live
in a natural state.
This perverse view went so far that, for example, Andreas Wagner, a
zoologist of Munich, quite seriously made the following ridiculous
assertion:—“Animals and plants in their wild state have been called
into being by the Creator as distinctly different and unchangeable
species; but in the case of domestic animals and cultivated plants this
was not necessary, because he formed them from the beginning for the use
of man. The Creator formed man out of a clod of earth, breathed the
living breath into his nostrils, and then created for him the different
useful domestic animals and garden plants, among which he thought well
to save himself the trouble of distinguishing species.” Unfortunately,
Andreas Wagner does not tell us whether the _Tree of Knowledge_ in
Paradise was a “good” wild species, or, as a cultivated plant, “no
species” at all. As the Tree of Knowledge was placed by the Creator in
the centre of Paradise, we might be inclined to believe that it was a
highly favoured cultivated plant, and therefore no species at all. But
since, on the other hand, the fruit of the Tree of Knowledge was
forbidden to man, and since many men, as Wagner himself clearly shows,
have never eaten of the fruit, it was evidently not created for the use
of man, and therefore in all probability was a _real species!_ What a
pity Wagner has not given us any information about this important and
difficult problem!
Now, however ridiculous this view may appear to us, it is only the
logical sequence of a false view (which is widely spread) of the special
nature of cultivated organisms, and one may occasionally hear similar
objections from naturalists of great reputation. I must most decidedly,
and at once, condemn this utterly false conception. It is the same
perverseness which is committed by physicians who maintain that diseases
are artificial productions, and not natural phenomena. It has been a
work of hard labour to combat this prejudice, and it is only in recent
times that men have generally adopted the view that diseases are nothing
but natural changes of the organisms, or really natural phenomena of
life, which are produced by changed and abnormal conditions of
existence. Disease, therefore, is not a life beyond Nature’s realm (vita
præter naturam), as the early physicians used to say, but a natural life
under conditions which produce illness and threaten the body with
danger. Just in the same manner, cultivated organic forms are not
artificial works of man, but natural productions which have arisen under
the influence of peculiar conditions of life. Man by his culture can
never directly produce a new organic form, but he can breed organisms
under new conditions of life, which are such as to influence and
transform them. All domestic animals and all garden plants are
originally descended from wild species, which have been transformed by
the peculiar conditions of culture.
A thorough comparison of cultivated forms (races and varieties) with
organisms not altered by cultivation (species and varieties), is of the
utmost importance to the theory of selection. What is most surprising in
such a comparison is the remarkably short time in which man can produce
a new form, and the high degree in which this form, produced by man, can
deviate from the original form. While wild animals and plants, one year
after another, appear to the zoologist and botanist approximately in the
same form, so as to have given rise to the false doctrine of the
constancy of species, domestic animals and garden plants, on the other
hand, display the greatest changes within a few years. The perfection
which gardeners and farmers have attained in the art of selection now
enables them, in the space of a few years, arbitrarily to create
entirely new animal and vegetable forms. For this purpose it is only
necessary to keep and propagate the organism under the influence of
special conditions—which are capable of producing new formations—and
even at the end of a few generations new species may be obtained, which
differ from the original form in a much higher degree than so-called
good species in a wild state differ from one another. This fact is
extremely important, and we cannot lay sufficient stress upon it. The
assertion is not true that cultivated forms descended from one and the
same primary form do not differ from one another as much as wild animal
and vegetable species differ among themselves. If we only make
comparisons, without prejudice, we can very easily perceive that a
number of races or varieties which have been derived from a single
cultivated form, within a short series of years, differ from one another
in a higher degree than so-called good species (bonæ species), or even
different genera of one family, in the wild state.
In order to establish this extremely important fact as firmly as
possible by experiments, Darwin decided to make a special study of the
whole extent of variation in form in a single group of domesticated
animals, and for this purpose he chose the _domestic pigeons_, which are
in many respects especially suited for such a study. For a long time he
kept on his estate all possible races and varieties of pigeons which he
was able to procure, and he was helped in this by rich contributions
from all parts of the world. He also joined two London pigeon clubs, the
members of which passionately, and with truly artistic skill, carry on
the breeding of the different forms of pigeons. Lastly, he formed
connections with some of the most celebrated pigeon-fanciers; so that he
could command the richest experimental material.
The art of, and fancy for, pigeon breeding is very ancient. Even more
than 3,000 years before Christ, it was carried on by the Egyptians. The
Romans, under the emperors, laid out enormous sums upon the breeding of
pigeons, and kept accurate pedigrees of their descent, just as the Arabs
keep genealogical pedigrees of their horses, and the Mecklenburg
aristocracy of their own ancestors. In Asia, too, among the wealthy
princes, pigeon breeding was a very ancient fancy; in 1600, the court of
Akber Khan possessed more than 20,000 pigeons. Thus in the course of
several centuries, and in consequence of the various methods of breeding
practised in the different parts of the world, there has arisen out of
one single originally tamed form, an immense number of different races
and varieties, which in their most divergent forms are extremely
different from one another, and are often curiously characterized.
One of the most striking races of pigeons is the well-known fan-tailed
pigeon, which spreads its tail like the peacock, and carries a number of
(from thirty to forty) feathers placed in the form of radii, while other
pigeons possess much fewer tail feathers—generally twelve. We may here
mention that the number of feathers on the tails of birds is considered
by naturalists of great value as a systematic distinction, so that whole
orders can thereby be distinguished. For example, singing birds, almost
without exception, possess twelve tail feathers; chirping birds
(Strisores) ten, etc. Several races of pigeons, moreover, are
characterized by a tuft of neck feathers, which form a kind of periwig;
others by grotesque transformation of their beaks and feet, by peculiar
and often very remarkable decorations, as, for example, skinny lappets,
which develop on the head; by a large crop, which is formed by the
gullet being strongly inclined forward, etc. Remarkable, also, are the
strange habits which many pigeons have acquired; for example, the turtle
pigeons and the trumpeters with their musical accomplishments, the
carriers with their topographical instinct. The tumblers have the
strange habit of ascending into the air in great numbers, then turning
over and falling down through the air as if dead. The ways and habits of
these endless races of pigeons—the form, size, and colour of the
individual parts of their bodies, and their proportions, differ in a
most astonishing degree from one another; in a much higher degree than
is the case with the so-called good species, or even with the perfectly
distinct genera, of wild pigeons. And what is of the greatest
importance, is the fact that these differences are not confined to the
external form, but extend even to the most important internal parts;
there even occur great modifications of the skeleton and of the
muscular tissues. For example, we find great differences in the number
of vertebræ and ribs, in the size and shape of the gaps in the
breast-bones, in the size and shape of the merry-thought, in the lower
jaw, in the facial bones, etc. In short, the bony skeleton, which
morphologists consider a very permanent part of the body, and which
never varies to such an extent as the external parts—shows such great
changes, that many races of pigeons might be described as special
genera, and this would doubtless be done if all these different forms
had been found in a wild and natural state.
How far the differences of the races of pigeons have been carried is
best shown by the fact that all pigeon breeders are unanimously of
opinion that each peculiar or specially marked race of pigeons must be
derived from a corresponding wild original species. It is true every one
assumes a different number of original species. Yet Darwin has most
convincingly and acutely proved that all these pigeons, without
exception, must be derived from a single wild primary species—from the
blue rock-pigeon (_Columba livia_). In like manner, it can be proved of
most of the domestic animals and cultivated plants, that all the
different races are descendants of a single original wild species which
has been brought by man into a cultivated condition.
An example similar to that of the domestic pigeons is furnished among
mammals by our tame _rabbit_. All zoologists, without exception, have
long considered it proved that all its races and varieties are descended
from the common wild rabbit, that is, from a single primary species. And
yet the extreme forms of these races differ to such a degree from one
another, that every zoologist, if he met with them in a wild state,
would unhesitatingly designate them not only as an entirely distinct
“good species,” but even as species of entirely different genera of the
Leporid family. Not only does the colour, length of hair, and other
qualities of the fur of the different tame races of rabbits vary
exceedingly, and form extremely broad contrasts, but, what is still more
important, the typical form of the skeleton and its individual parts do
so also, especially the form of the skull and the jaw (which is of such
importance in systematic arrangement); further, the relative proportion
of the length of the ears, legs, etc. In all these respects the races of
tame rabbits avowedly differ from one another far more than all the
different forms of wild rabbits and hares which are scattered over all
the earth, and are the recognized “good species” of the genus _Lepus_.
And yet, in the face of these clear facts, the opponents of the theory
of development maintain that the wild species are not descended from a
common prototype, although they at once admit it in the case of the tame
races. With opponents who so intentionally close their eyes against the
clear light of truth, no further dispute can be carried on.
While in this manner it appears certain that the domestic races of
pigeons, of tame rabbits, of horses, etc., notwithstanding the
remarkable difference of their varieties, are descended in each case
from but one wild, so-called “species”; yet, on the other hand, it is
certainly probable that the great variety of races of some of the
domestic animals, especially dogs, pigs, and oxen, must be ascribed to
the existence of several wild prototypes, which have become mixed. It
is, however, to be observed that the number of these originally wild
primary species is always much smaller than that of the cultivated forms
proceeding from their mingling and selection, and naturally they were
originally derived from a single primary ancestor, common to the whole
genus. In no case is each separate cultivated race descended from a
distinct wild species.
In opposition to this, almost all farmers and gardeners maintain, with
the greatest confidence, that each separate race bred by them must be
descended from a separate wild primary species, because they clearly
perceive the differences of the races, and attach very high importance
to the inheritance of their qualities; but they do not take into
consideration the fact that these qualities have arisen only by the slow
accumulation of small and scarcely observable changes. In this respect
it is extremely instructive to compare cultivated races with wild
species.
Many naturalists, and especially the opponents of the Theory of
Development, have taken the greatest trouble to discover some
morphological or physiological mark, some characteristic property,
whereby the artificially bred and cultivated races may be clearly and
thoroughly distinguished from wild species which have arisen naturally.
All these attempts have completely failed, and have led only with
increasing certainty to the result, that such a distinction is
altogether impossible. I have minutely discussed this fact, and
illustrated it by examples in my criticism of the idea of species. (Gen.
Morph. ii. 323-364.)
I may here briefly touch on yet another side of this question, because
not only the opponents, but even a few of the most distinguished
followers of Darwin—for example, Huxley—have regarded the phenomena of
_bastard-breeding_, or _hybridism_, as one of the weakest points of
Darwinism. Between cultivated races and wild species, they say, there
exists this difference, that the former are capable of producing
fruitful bastards, but that the latter are not. Two different cultivated
races, or wild _varieties of one species_, are said in all cases to
possess the power of producing bastards which can fruitfully mix with
one another, or with one of their parent forms, and thus propagate
themselves; on the other hand, _two really different species_, two
cultivated or wild _species_ of one genus, are said _never_ to be able
to produce from one another bastards which can be fruitfully crossed
with one another, or with one of their parent species.
As regards the first of these assertions, it is simply refuted by the
fact that there are organisms which do not mix at all with their own
ancestors, and therefore can produce no fruitful descendants. Thus, for
example, our cultivated guinea-pig does not bear with its wild Brazilian
ancestor; and again, the domestic cat of Paraguay, which is descended
from our European domestic cat, no longer bears with the latter. Between
different races of our domestic dogs, for example, between the large
Newfoundland dogs and the dwarfed lap-dogs, breeding is impossible, even
for simple mechanical reasons. A particularly interesting instance is
afforded by the Porto-Santo rabbit (Lepus Huxleyi). In the year 1419, a
few rabbits, born on board ship of a tame Spanish rabbit, were put on
the island of Porto Santo, near Madeira. These little animals, there
being no beasts of prey, in a short time increased so enormously that
they became a pest to the country, and even compelled a colony to remove
from the island. They still inhabit the island in great numbers; but in
the course of four hundred and fifty years they have developed into a
quite peculiar variety—or if you will have it, into a “good
species”—which is distinguished by a peculiar colour, a rat-like shape,
small size, nocturnal life, and extraordinary wildness. The most
important fact, however, is that this new species, which I call _Lepus
Huxleyi_, no longer pairs with its European parent rabbit, and no longer
produces bastards with it.
On the other hand, we now know of numerous examples of fruitful genuine
bastards; that is, of mixings that have proceeded from the crossing of
two entirely different species, and yet propagate themselves with one
another as well as with one of their parent species. A number of such
bastard species (species Hybridæ) have long been known to botanists; for
example, among the genera of the thistle (Cirsium), the laburnum
(Cytisus), the bramble (Rubus), etc. Among animals also they are by no
means rare, perhaps even very frequent. We know of fruitful bastards
which have arisen from the crossing of two different species of a genus,
as among several genera of butterflies (Zygæna, Saturnia), the family of
carps, finches, poultry, dogs, cats, etc. One of the most interesting is
the hare-rabbit (Lepus Darwinii), the bastard of our indigenous hare and
rabbit, many generations of which have been bred in France, since 1850,
for gastronomic purposes. I myself possess such hybrids, the products of
pure in-breeding, that is, both parents of which are themselves hybrids
by a hare-father and a rabbit-mother. I possess them through the
kindness of Professor Conrad, who has repeatedly made these experiments
in breeding on his estate. The half-blood hybrid thus bred, which I name
in honour of Darwin, appears to propagate itself through many
generations by pure in-breeding, just as well as any genuine species.
Although on the whole it is more like its mother (rabbit), still in the
formation of the ears and of the hind-legs, it possesses distinct
qualities of its father (hare). Its flesh has an excellent taste, rather
resembling that of a hare, though the colour is more like that of a
rabbit. But the hare (Lepus timidus) and the rabbit (Lepus cuniculus)
are two species of the genus Lepus, so different that no systematic
zoologist will recognize them as varieties of one species. Both species,
moreover, live in such different ways, and in their wild state entertain
so great an aversion towards one another, that they do not pair so long
as they are left free. If, however, the newly-born young ones of both
species are brought up together, this aversion is not developed; they
pair with one another and produce the _Lepus Darwinii_.
Another remarkable instance of the crossing of different species (where
the two species belong even to different genera!) is furnished by the
fruitful hybrids of sheep and goats which have for a long time been bred
in Chili for industrial purposes. On what unessential circumstances in
the sexual mingling the fertility of the different species depend, is
shown by the fact that he-goats and sheep in their mingling produce
fruitful hybrids, while the ram and she-goat pair very rarely, and then
without result. The phenomena of hybridism to which undue importance has
been erroneously attributed are thus utterly unmeaning, so far as the
idea of species is concerned. The breeding of hybrids does not enable
us, any more than other phenomena, thoroughly to distinguish cultivated
races from wild species; and this circumstance is of the greatest
importance in the Theory of Selection.