We now come to that branch of the subject which is the most important
and distinctive of our age, and which, in popular estimation, alone
constitutes evolution—the mode of origin of the innumerable species of
animal and plant life which now exist or have ever existed upon the
earth.
The origin of the different forms of life has till quite recent times
been looked upon as an almost insoluble problem, although a few
advanced thinkers, even in the eighteenth century, perceived that it
was probably the result of some natural process of modification or
evolution; but no force or law had been set forth and established
in any way adequate to produce it until the publication of Darwin’s
_Origin of Species_, in 1859. In the later editions of that work,
Darwin has given a historical sketch of the progress of opinion on the
subject. I shall, therefore, now only notice a few great writers which
he has not referred to.
We have seen what an impossible and even ludicrous explanation had
to be given by Lucretius; and from his day down to the middle of the
eighteenth century no advance had been made. Either the problem was
not referred to at all, or the theological doctrine of a special
creation was held to be the only possible one. But in the middle of
the eighteenth century the great French naturalist, Buffon, published
his very important work, _Histoire Naturelle_, in fifteen volumes
(1749–1767), in which, besides describing the characters and habits
of all the animals then known, he introduced much philosophical and
speculative thought, which would probably have been carried much
further had he not felt obliged to conform to the religious prejudices
of the age. We are indebted to Mr. Samuel Butler for having brought
together all the important passages of Buffon’s voluminous and now
little-read works bearing upon the question of evolution, and it is
from his volume that I quote.
Buffon lays stress on the great resemblance of all mammalia in internal
structure, showing that the most unlike creatures may be really alike
structurally. He says: “The horse, for example—what can at first
sight seem more unlike mankind? Yet when we compare man and horse,
point by point and detail by detail, our wonder is excited rather by
the resemblances than by the differences between them.” He then shows
that all the parts of the skeleton agree, and that it is only in
proportions, the increase of some bones and the suppression of others,
that they differ, adding: “If we regard the matter thus, not only the
ass and the horse, but even man himself, the apes, etc., might be
regarded as forming members of one and the same family.” Then, after a
few more illustrations, he remarks: “If we once admit that there are
families of plants and animals, so that the ass may be of the family
of the horse, and that the one may only differ from the other by
degeneration from a common ancestor, we might be driven to admit that
the ape is of the family of man, that he is but a degenerate man, and
that he and man have had a common ancestor.... If it were once shown
that we had right grounds for establishing these families, if the point
were once gained that among plants and animals there have been even a
single species which had been produced in the course of direct descent
from another species, then there is no further limit to be set to the
power of nature, and we should not be wrong in supposing that with
sufficient time she could have evolved all other organized forms from
one primordial type.”
This indicates clearly enough his own opinion, but to save himself from
the ecclesiastical authorities he at once adds this saving clause: “But
no! It is certain, from revelation, that all animals have alike been
favored with the grace of an act of direct creation, and that the first
pair of every species issued full formed from the hands of the Creator.”
Such examples of disarming religious prejudice are frequent, but he
continually recurs to statements as to mutability which neutralize
them. Here, for example, is a broad claim for nature as opposed to
creation. He has been showing how variable are many animals, and how
changes of food, climate, and general surroundings influence both their
forms and their habits; and then he exclaims:
“What cannot nature effect with such means at her disposal? She can do
all except either create matter or destroy it. These two extremes of
power the Deity has reserved for Himself only; creation and destruction
are the action of His omnipotence. To alter and undo, to develop and
renew—these are powers which He has handed over to the charge of
nature.”
Here we have a claim for the power of nature in the modification of
species which fully comes up to the requirements of the most advanced
evolutionist. It is remarkable, too, how clearly he perceived the
great factors so important for the evolution of organisms, rapid
multiplication, great variability, and the struggle for existence.
Thus he remarks: “It may be said that the movement of nature turns
upon two immovable pivots—one, the illimitable fecundity which she
has given to all species; the other, the innumerable difficulties
which reduce the results of that fecundity and leave throughout time
nearly the same quantity of individuals in every species.” Here the
term “difficulties” corresponds to the “positive checks” of Malthus,
and to the “struggle for existence” of Darwin; and he again and again
refers to variability—as when he says: “Hence, when by some chance,
common enough with nature, a variation or special feature makes its
appearance, man has tried to perpetuate it by uniting together the
individuals in which it has appeared.”
As Buffon thus clearly understood artificial selection, thoroughly
appreciated the rapid increase of all organisms, and equally well saw
that their inordinate increase was wholly neutralized through such
destructive agencies as hunger, disease, and enemies, and as, at the
same time, he had such unbounded faith in the power of nature to modify
animal and vegetable forms, we feel assured that this great writer and
original thinker only needed freedom to pursue this train of thought a
little further and he would certainly have anticipated Darwin’s great
discovery of natural selection by a whole century. Even as it is we
must class him as one of the great pioneers of organic evolution.
The next distinct step towards a theory of organic evolution was made
by the poet Goethe at the very end of the eighteenth century, in his
views of the metamorphosis of plants. He pointed out the successive
modifications of the leaf which produced all the other essential parts
of the higher plants—the simple cotyledons or seed leaves became
modified into the variously formed leaves of the fully grown plants;
these again were successively modified into the calyx, corolla,
stamens, and ovary of the flower. He supposed this to be due to the
increased refinement of the sap under the influence of light and air,
and to indicate the steps by which the various parts of the flower
had been developed. It was, therefore, a theory of evolution; but it
was very unsatisfactory, inasmuch as it in no way accounted for the
wonderful variety of the floral organs, or indicated any purpose served
by the most prominent and conspicuous part of the flower, the highly
colored and often strangely formed corolla. It was also erroneous in
supposing that the corolla was a modified calyx, whereas it is now
known to be a modification of the stamens.
Next came the great work of Lamarck in the first decade of the
nineteenth century, in which he proposed a general system of
evolution of the whole animal world. Hence he may be termed the
first systematic evolutionist. His system has been rather fully
described by Lyell, who, in his _Principles of Geology_, devotes
a whole chapter to a summary of his doctrines; while Mr. Butler
gives copious quotations in three chapters of his _Evolution Old and
New_; and any one who is not acquainted with the original work of
Lamarck should read these two authors in order to understand how wide
was his knowledge, how ingenious his explanations, and in how many
important points he anticipated the views both of Lyell and Darwin.
But he was half a century in advance of his age, and his only alleged
causes of modification—changed conditions, use and disuse, habit
and effort—were wholly insufficient to account for the vast range
of the phenomena presented by the innumerable minute adaptations of
living organisms to their conditions of life. He even imputed all the
modifications of domestic animals to the changed conditions of food and
habits to which they have been subjected by man, making no reference to
the use of selection by breeders, in this respect falling short of his
great predecessor, Buffon.
The general laws which Lamarck deduces from his elaborate study of
nature are these:
“Firstly. That in every animal which has not passed its limit of
development, the more frequent and sustained employment of any organ
develops and aggrandizes it, giving it a power proportionate to the
duration of its employment, while the same organ, in default of
constant use, becomes insensibly weakened and deteriorated, decreasing
imperceptibly in power until it finally disappears.
“Secondly. That these gains or losses of organic development, due to
use or disuse, are transmitted to offspring, provided they have been
common to both sexes, or to the animals from which the offspring have
descended.”
The whole force of this argument depends upon the second clause—the
inheritance of those individual modifications due to use and disuse.
But no direct evidence of this has ever been found, while there is
a good deal of evidence showing that it does not occur. Again, there
are many structures which cannot have been produced by use, such, for
example, as the feathers of the peacock’s train, the poison in the
serpent’s fangs, the hard shells of nuts, the prickly covering of many
fruits, the varied armor of the turtle, porcupine, crocodile, and many
others. For these reasons Lamarck’s views gained few converts; and
although some of his arguments have been upheld in recent years, the
fatal objections to his general principle as a means of explaining the
evolution of organic forms has never been overcome.
Between the periods of Lamarck and Darwin many advances were made
which clearly pointed to a general law of evolution in nature. Such
were Sir William Grove’s lectures on the “Correlation of the Physical
Forces,” in 1842; Helmholtz on the “Conservation of Energy,” in 1847;
and Herbert Spencer’s essay on “The Development Hypothesis,” in 1852.
This latter work was a complete and almost unanswerable argument for a
natural process of continuous evolution of the whole visible universe,
including organic nature, man, and social phenomena. It is further
extended in the later editions of the author’s _First Principles_,
which, as a coherent exposition of philosophy, co-ordinating and
explaining all human knowledge of the universe into one great system of
evolution everywhere conforming to the same general principles, must
be held to be one of the greatest intellectual achievements of the
nineteenth century. It left, however, the exact method of evolution
of organisms untouched, and thus failed to account for those complex
adaptations and appearances of design in the various species of animals
and plants which have always been the stronghold of those who advocated
special creation. This difficulty was met by Darwin’s theory of _The
Origin of Species by Means of Natural Selection_, published in 1859,
and the series of works that succeeded it; and to a brief sketch of
this theory the remainder of our space must be devoted.