victorious. Cuvier's knowledge and skilful advocacy had a few years
before over-powered Geoffroy St. Hilaire's conception of a common
plan of structure pervading the whole animal kingdom, and the new
_Philosophie Anatomique_ was laid on the shelf, side by side with
the _Philosophie Zoologique_ of Lamarck, the _Zoonomia_ of Erasmus
Darwin, the _Théorie de la Terre_ of Buffon, and the _Protogæa_
of Leibnitz. Yet even then a spectator who was fully informed and at the
same time gifted with uncommon foresight might have satisfied himself
that the victory of evolution had become inevitable.
Cuvier's memorable descriptions of the extinct vertebrates of the
Paris basin had founded the new science of Palæontology, and though
neither he nor anyone else was aware of the fact, had made it possible
to trace, very imperfectly no doubt, the descent of a few modern
ungulates. Lyell's _Principles of Geology_ (1830-3) had shaken the
belief in catastrophes repeatedly breaking the succession of life on
the earth. It was rapidly becoming impossible to maintain that the
account of creation given in the book of Genesis was even approximately
accurate. In the year 1828 Baer had almost made up his mind that the
facts of development pointed to a common plan of structure, perhaps
to a common origin, for each of the great types of animal life.[43]
Darwin's _Journal_ had appeared in 1839, and though the explanations
which it offered were not inconsistent with prevalent opinion,
evolutionary suggestions were introduced into the second edition of
1845. Lyell at least was already aware that the voyage of the _Beagle_
had impelled Darwin to examine afresh the accepted philosophy of
creation. Between 1840 and 1850 faint signs of coming change struck
orthodox reasoners with misgiving and gave increased confidence to
free-thinkers. A few German botanists and zoologists declared against
the immutability of species. The _Vestiges of the Natural History of
Creation_, which might be called a premature explosion, dates from
1844. Hofmeister (see supra, p. 109) put forth a detailed comparison
of the flowering plants with the higher cryptogams, which strongly
suggested a theory of descent with modification, and is unintelligible
on any other basis. He indicated no such interpretation himself, being
content to establish the new homologies; but the _Origin of Species_,
as soon as it appeared, commanded his entire sympathy.
Among those who rejected fixity of species and special creation before
1859 none was so clear or so outspoken as Herbert Spencer, who thought
out for himself an evolutionary philosophy which was not shaken by
Darwin. It is impossible to discuss in this place the question whether
or not it was shaken by Weismann.
Agassiz's _Essay on Classification_, which was published in October,
1857, was the last manifesto issued before the _Origin of Species_
by the party which stood out for fixity of species, the last polemic
which made De Maillet, Lamarck, and the _Vestiges_ its targets. It
is an eloquent but inconsiderate defence of an extreme position.
According to Agassiz every branch, class, order, family, genus, and
species represents a distinct creative thought; every mark of affinity,
every appearance of adaptation to surroundings, has been expressly
designed. Extinction and replacement of species are due to the direct
intervention of the Creator; pterodactyls are prophetic types of
birds, and indicate that divine wisdom had foreseen the possibility of
an advance in the organisation of animals which was not immediately
practicable; the mallard and scaup duck occur on both sides of the
Atlantic because they were simultaneously but separately created in
Europe and North America; the teeth of the whale, which never cut the
gum, are the result of obedience to a certain uniformity of fundamental
structure. Explanations like these removed no difficulties and
suggested no inquiries. In the hot debates which ensued the _Essay on
Classification_ was rarely mentioned.
[32] _Cross and Self-Fertilisation of Plants_, chap. xi.
Darwin could exhaust the inquiry. "The veil of secrecy," he goes on,
"is as yet far from lifted."
[33] Cuvier did not himself use the word _palæontology_, which first
came in about 1830. In the same way Buffon writes on the history of
animals, not on _zoology_, and on the theory of the earth, not on
_geology_.
[34] This anecdote has also been related in a rather different form.
[35] The same process of "embryonic fission" occurs in other animals
also, one of which is a mammal (Praopus).
[36] Linnæus (Fund. Bot. § 134, and Sponsalia Plantarum) gives it as
above; Harvey has "Ex ovo omnia"; "ovum esse primordium commune omnibus
animalibus," etc.
[37] Harvey need not have gone outside the writings of Aristotle to
get the substance of his generalisation. He would have found there
that the chief task of both plants and animals is propagation, either
by seeds, or eggs, which Aristotle believed to be equivalent to seeds
(_Hist. anim._, VIII., i.; _De anim. gen._, I., iv.; I., xxiii.).
Aristotle excepted the "imperfect animals," such as insects, and the
seedless plants, concerning both of which his knowledge was misty and
inaccurate; there is no indication that Harvey was better informed.
[38] Hooke figured a thin section of dry cork in his _Micrographia_
(1665), remarking that it was divided into "little boxes or cells."
The word _cell_ was suggested by the resemblance of the tissue to a
honeycomb; since 1838 it has been thoughtlessly extended from the
skeleton to the particle of living matter enclosed within it. Robert
Brown (1831) showed that a nucleus is usual in plant-cells; it had been
figured by Fontana and others long before. Down to 1838 no results of
biological interest followed from the discovery.
[39] Parkinson (1629) speaks of a stove or hothouse, "such as are used
in Germany."
[40] The graceful practice of naming genera of plants after benefactors
to botany or horticulture was introduced by Father Plumier (1646-1704),
who gave the names of L'Obel and Fuchs to the Lobelia and Fuchsia, and
whose own name is appropriately borne by the frangipane (Plumeria).
[41] See the account of Cartagena in the _Personal Narrative_.
[42] See particularly his _Essai sur la géographie des plantes_ (1805).
[43] Baer's expressions are so guarded that his real opinions in 1828
can only be surmised. He never accepted a consistent theory of organic
evolution.
PERIOD V.
(1859 AND LATER)
Period V.
We do not attempt to characterise our last period, nor to describe its
biological achievement. It seems better to devote the whole of our
scanty space to the scientific careers of Darwin and Pasteur, in which
so much past effort culminated, and from which so much progress was to
spring.
Darwin on the Origin of Species.
Setting aside as superfluous and we might say impossible, under our
conditions of space, all attempt to restate the evidence on which
Darwin based his great argument, we shall here try to show that the
_Origin of Species_ shed a new light upon many biological facts,
combined many partial truths into one consistent theory, and gave a
great stimulus to further inquiry.
1. _Classification and Affinity._—The sixteenth-century herbalists
and still earlier writers (see p. 17) recognised a property of
_affinity_, by which plants were associated in natural groups. Bock (
1546) tried to bring together all plants which are related (verwandt)
to one another, but similarity of any kind was with him a proof of
affinity; it did not shock him to place the dead nettles next to the
stinging nettles. L'Obel gave names to several families of flowering
plants which are still admitted as natural. Ray spoke of the _affinity_
(cognatio) between plants, and his affinity was a thing not to be
violated for the sake of practical convenience or logical rules, but he
was unable to explain what he meant by it. Linnæus tried to illustrate
affinity between plants by contiguous provinces on a map, a better
metaphor than the linear scale, for the scale can only express affinity
on two sides, while the map can express affinity on many. His practical
experience of classification taught him a truth, shocking at first sight
to the logician[44]—viz., that the characters which serve for the
definition of one genus may be useless for the definition of the next,
and he laid it down that the characters do not make the genus, but the
genus the characters. After Linnæus we find for a long time no advance
in the philosophy of natural classification. Cuvier (1816) is even
retrograde, for he sets aside the maxims of Linnæus, maintains that
adaptive characters (characters closely related to the conditions of
life) are relatively constant, and that large groups should be defined
by characters drawn from organs of great physiological importance.
These decisions of his are repudiated by later naturalists.
The key to the affinity puzzle which had so long baffled thinking
naturalists was at last supplied by Darwin, who explained that "the
natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from
a common parent, all true classification being genealogical; that
community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike."[45]
Natural groups, large or small, result from the long-continued
operation of divergence, the survival of some, and the extinction of
others; they are to be respected as facts; they are not created by
definitions, which only serve to indicate and remind; any character,
however trifling, will suffice, if only it is constant and distinctive.
The conflict between natural classification and logic is apparent
only. Logicians say that in classifying books, for instance, you may
take any property you please, subject, size, etc., as the basis of
your arrangement, but having made your choice, you must adhere to it
for all divisions of the same rank. Naturalists seem to say something
different, for they are agreed that what they call "single-character
classifications," in which one property is adhered to throughout, are
unnatural. The fact is that a natural classification always rests upon
one and the same property—viz. _affinity_, _i.e._ relative
nearness of descent from some common ancestor. Every natural
classification, like every logical classification, proceeds upon a
single basis, and the failure of the single-character classifications
is due to their displacing affinity by some definition.
The effect of the _Origin of Species_ upon zoological and botanical
systems has been revolutionary. Furnished with a new and intelligible
meaning of the word natural, and with new criteria of naturalness,
systematists have during the last fifty years worked hard to
create classifications which admit of being thrown into the form
of genealogical trees. Wide gaps in the geological history of life
render the task difficult beyond expression, but much has already
been accomplished. Newly discovered forms (especially the fossil
Archæopteryx and the Cycadofilices) and more fully investigated forms,
far too numerous to be specified, have established links between groups
which formerly seemed to be wholly independent. Unnatural assemblages
based on pre-determined characters (Radiates, Entozoa, Birds of prey,
etc.) have been replaced by groups which are at least possible on
evolutionary principles. Almost every working naturalist will admit
that the progress of zoological and botanical system during the last
two generations has done much to fortify the Darwinian position.
2. _Embryology._—Baer in 1828 was possessed of all the embryological
facts which Darwin used in support of his theory of evolution; in
particular, he was well acquainted with the most striking fact of
all—viz., the presence in embryo mammals and birds of a series of
paired clefts along the sides of the neck, between which run vessels
arranged as in gill-breathing vertebrates. The vessels had been figured
by Malpighi; the clefts had been discovered by Rathke, who had no
hesitation in calling them _gill-clefts_ and the vessels
_gill-arches_. Nor had Baer, who nevertheless to the end of his long
life refused to accept the one explanation which gives meaning to the
facts—viz., that remote progenitors of mammals and birds breathed by
gills. Few embryologists have since felt such a scruple. The adaptation
to gill-breathing is obvious; is gill-breathing _now_ practised by any
mammal or bird? Certainly not. Is it destined to be practised by their
descendants at some _future_ time? To say nothing of the danger of
putting forth any such prophecy, it involves all the consequences of
descent with modification. The opponent of evolution may as well admit
at once that the gill-breathing was practised in time _past_. As an
example of the same kind taken from plants, we may quote the trifoliate
leaves of the furze-seedling, which, though absent from the full-grown
furze, are frequent in the family (Leguminosæ) to which it belongs.
The general similarity of vertebrate embryos, of insect-embryos and of
dicotyledonous seedlings, is also worthy of note. We may suppose that
early embryos, being largely or wholly dependent on food supplied by
the parent, and perhaps protected by the parent as well, escape the
pressure of the struggle for existence, and are often not urgently
impelled to produce adaptations of their own. In these circumstances it
is intelligible that features inherited from remote ancestors should
persist. If, however, early independence is demanded by the conditions
of life, the embryo may develop temporary adaptations, wanting in the
parent and in embryos of allied groups. Larval adaptation is as much a
part of the economy of nature as the retention of ancestral structures
which have been lost by the adult.
3. _Morphology._—Let us next consider the light which the Origin of
Species throws upon homologous parts. No example will serve our purpose
better than the very familiar one of the fore-limbs of different
vertebrates, the arm and hand of man, the wing of the bat, the wing of
the bird, the pectoral fin of the fish, and the paddle of the whale.
These limbs, adapted for actions so diverse as grasping, running,
flying, and swimming, nevertheless exhibit a common plan, evident at a
glance, except in the pectoral fin of the fish. But why a common plan?
Of what advantage is it to an animal that its wing, paddle, or hand
should reproduce the general plan of a fore-foot? Why should the digits
of the land vertebrates never exceed five? Why should the thumb never
have more than two free joints? It would be hard to find a satisfactory
answer to these questions in any book earlier than the _Origin of
Species_; no student of the _Origin of Species_ finds any
difficulty in answering them all. The common plan has been transmitted
from type to type by inheritance, and its features are derived from an
unknown common ancestor.
The new conception, that structures inherited from remote ancestors
may be incessantly modified by the conditions of life and by mutual
competition, is the key to the chief problems of morphology. No limited
collection of examples can substantiate so wide a proposition as
this. Those who have made themselves familiar with old text-books of
comparative anatomy will recollect how dry, or else how inconclusive,
was pre-evolutionary morphology, how vague were the references to some
ideal archetype, or to climate, or to the ancient conditions of the
earth's surface; how often exclamations of admiration for the marvels
of nature or Providence were substituted for clear explanations.
Cuvier, it is true, was both precise and reasonable; but how little he
was in a position to explain! His "empirical" comparative anatomy could
throw no direct light upon origins or transformations; his "rational"
comparative anatomy was practicable only in a few easy cases.
4. _Geographical Distribution._—The facts of distribution were
handled in the _Origin of Species_ with great originality. It was
shown that they support, and indeed require, some doctrine of organic
evolution. The succession in the same area of the same types—armadillos
succeeding armadillos in South America, marsupials succeeding
marsupials in Australia—was enough of itself to render independent
creation highly improbable. This was not all. Darwin's mind being
charged with facts and reasonings, the accumulations of many years of
travel and meditation, he sketched in rapid outline conclusions which
have given a new form to the distribution question. The subject had
hitherto been treated by collecting masses of facts and interpreting
them by recent physical geography; Darwin showed that the _history_
of the continents and islands may be far more influential than soil,
elevation, or climate.
The scientific discussion of the facts of distribution is as old as the
sixteenth century, when L'Obel pointed out that the mountain plants of
warm countries descend to low levels in the north. Linnæus remarked
that fresh-water plants and alpine plants are often cosmopolitan.
Another early and well-founded generalisation is the statement of
Linnæus that the plants common to the old and the new world are all
of northern range. Buffon made the same remark about the animals,
and offered the probable explanation—viz., that since the two great
land-masses approach one another only in high latitudes, it is only
there that animals have been able to cross from one to the other.
In the nineteenth century theories involving prodigious changes of
land and sea were much in the minds of naturalists. Darwin lost his
temper (a rare thing with him) over the land-bridges, hundreds, or
even thousands, of miles long, which were created in order to explain
trifling correspondences in the population of distant countries. A
belief in the comparative stability of the great continents and
oceans has since prevailed, and it is now recognised that the means of
dispersal of species are greater than was once supposed.
The discovery, about the year 1846, of the marks of ancient glaciers
in all parts of northern Europe, and the acceptance of an Ice Age, had
a still greater influence upon the teaching of naturalists. Edward
Forbes[46] put forth a glacial theory to account for the present
distribution of plants of northern origin. Glacial cold, he maintained,
had driven the arctic flora far southward. When more genial conditions
returned, most of the northern plants retreated towards the Pole, but
some climbed the mountains and gave rise to an isolated alpine flora.
Darwin, whose unpublished manuscripts had anticipated Forbes's theory,
believed that the whole earth became chilled during the Ice Age, and
that the fauna and flora of the temperate zone reached the tropics. His
argument, which is contained in chap. xi. of the _Origin of Species_,
is now generally accepted in principle, though opinions differ on many
points of detail. Some think that he extended too widely the effects
of glacial cold, exaggerated the resemblance of the arctic and alpine
fauna and flora, and attributed the extinction of the northern species
in the intermediate plains too exclusively to climatic causes.
One paragraph in the extremely condensed discussion on geographical
distribution which we find in the _Origin of Species_ calls attention
to the dominance of forms of life "generated in the larger areas
and more efficient workshops[47] of the north." The power which
inhabitants of the great northern land-mass of the old world, and
in a less degree those of North America, possess, and have long
possessed, of driving out the inhabitants of the southern continents
is one of the most important factors in the peopling of the earth with
new races of land-plants and land-animals. Races of men, modes of
civilisation, religious faiths, all follow the same rule, which has no
doubt prevailed ever since land came to predominate in the northern
hemisphere and water in the southern hemisphere. In the life of the sea
and the fresh waters no dominance of northern forms has been detected.
5. _Palæontology._—We must not claim for Darwin more than a modest
share in the vast extension of palæontological knowledge which the
last fifty years have created. A profusion of new materials has been
acquired by the diligence of collectors working on a scale previously
unattempted. But though the accumulation of materials is the work
of others, the interpretation has been guided by the principles of
Darwin. The evolution of the horse has now been so fully worked out
that it would bear the whole weight of a doctrine of descent with
modification, though it could not by itself reveal the process by which
the modification had been effected.
_Darwin on Adaptations._—The adaptation of living things to their
surroundings has always been a favourite branch of natural history,
underrated only by those whose studies are little calculated to inflame
the curiosity. Many eminent naturalists have made the interpretation
of natural contrivances their chief aim. Darwin equalled the best of
his predecessors in accuracy, range, and ingenuity, while he surpassed
them all in candour. No one has done so much to vindicate the study
of adaptations from all suspicion of triviality, for no one before
him had seen so clearly how all new species arise by adaptation of
pre-existing ones. It is by adaptation that new forms of life arise; it
is inheritance which preserves old ones.
Socrates, Swammerdam, and Paley had drawn from the adaptations of
nature proofs of the omnipotence and beneficence of the Creator.
Darwin, while admitting that every organism is exquisitely adapted
to its own mode of life, believed that the adaptations have been
perfected by slow degrees, and that they cannot be proved to have
been consciously devised. This interpretation deprives the theologian
of valued arguments, but at the same time rids him of difficulties.
Even before Darwin's day some few natural theologians had the courage
to bring forward instances of the harshness of nature. Kirby and
Spence[48] thought that no injustice was done to certain predatory
insects by comparing them to devils. Others blessed the mercy of
heaven, which, after creating noxious animals, created others to keep
them in check. Darwin, when reflecting upon the odious instincts which
urge the young cuckoo to eject its foster-brothers, some species of
ants to enslave others, and a multitude of ichneumons to lay their eggs
in the bodies of live caterpillars, found it a relief to be able to
shift the responsibility to an unconscious natural process.[49]
In his autobiography Darwin remarks that he had thought it almost
useless to endeavour to prove by indirect evidence that species had
been modified until he was able to show how the adaptations could be
explained. Some of them alarmed him by their difficulty; to suppose
that the eye, with all its inimitable adjustments, had been formed
by an unconscious natural process seemed to him absurd until he had
traced a good many intermediate steps between the mere colour-spot and
the eye of the eagle. He writes to Asa Gray (September 5, 1857) that
the facts which had done most to keep him scientifically orthodox were
facts of adaptation, the pollen-masses of Asclepias, the mistletoe with
its pollen carried by insects and its seeds by birds, the woodpecker
exquisitely fitted by feet, tail, beak, and tongue to climb trees and
capture insects.