"Naturæ leges et regulæ, secundum quas omnia fiunt et ex unis formis in
alias mutantur, sunt ubique et semper eadem." [Footnote: _Ethices_,
Pars tertia, Præfatio.] Leibnitz's doctrine of continuity necessarily led
him in the same direction; and, of the infinite multitude of monads with
which he peopled the world, each is supposed to be the focus of an endless
process of evolution and involution. In the "Protogæa," xxvi., Leibnitz
distinctly suggests the mutability of species--
"Alii mirantur in saxis passim species videri quas vel in orbe cognito, vel
saltem in vicinis locis frustra quæras. 'Ita Cornua Ammonis,' quæ ex
nautilorum numero habeantur, passim et forma et magnitudine (nam et pedali
diametro aliquando reperiuntur) ab omnibus illis naturis discrepare dicunt,
quas præbet mare. Sed quis absconditos ejus recessus aut subterraneas
abyssos pervestigavit? quam multa nobis animalia antea ignota offert novus
orbis? Et credibile est per magnas illas conversiones etiam animalium
species plurimum immutatas."
Thus, in the end of the seventeenth century, the seed was sown which has,
at intervals, brought forth recurrent crops of evolutional hypotheses,
based, more or less completely, on general reasonings.
Among the earliest of these speculations is that put forward by Benoit de
Maillet in his "Telliamed," which, though printed in 1735, was not
published until twenty-three years later. Considering that this book was
written before the time of Haller, or Bonnet, or Linnæus, or Hutton, it
surely deserves more respectful consideration than it usually receives. For
De Maillet not only has a definite conception of the plasticity of living
things, and of the production of existing species by the modification of
their predecessors; but he clearly apprehends the cardinal maxim of modern
geological science, that the explanation of the structure of the globe is
to be sought in the deductive application to geological phenomena of the
principles established inductively by the study of the present course of
nature. Somewhat later, Maupertuis [Footnote: _Système de la Nature_.
"Essai sur la Formation des Corps Organisés," 1751, xiv.] suggested a
curious hypothesis as to the causes of variation, which he thinks may be
sufficient to account for the origin of all animals from a single pair.
Robinet [Footnote: _Considérations Philosophiques sur la gradation
naturelle des formes de l'être; ou les essais de la nature qui apprend a
faire l'homme,_ 1768.] followed out much the same line of thought as De
Maillet, but less soberly; and Bonnet's speculations in the "Palingénésie,"
which appeared in 1769, have already been mentioned. Buffon (1753-1778), at
first a partisan of the absolute immutability of species, subsequently
appears to have believed that larger or smaller groups of species have been
produced by the modification of a primitive stock; but he contributed
nothing to the general doctrine of evolution.
Erasmus Darwin ("Zoonomia," 1794), though a zealous evolutionist, can
hardly be said to have made any real advance on his predecessors; and,
notwithstanding that Goethe (1791-4) had the advantage of a wide knowledge
of morphological facts, and a true insight into their signification, while
he threw all the power of a great poet into the expression of his
conceptions, it may be questioned whether he supplied the doctrine of
evolution with a firmer scientific basis than it already possessed.
Moreover, whatever the value of Goethe's labours in that field, they were
not published before 1820, long after evolutionism had taken a new
departure from the works of Treviranus and Lamarck--the first of its
advocates who were equipped for their task with the needful large and
accurate knowledge of the phenomena of life, as a whole. It is remarkable
that each of these writers seems to have been led, independently and
contemporaneously, to invent the same name of "Biology" for the science of
the phenomena of life; and thus, following Buffon, to have recognised the
essential unity of these phenomena, and their contradistinction from those
of inanimate nature. And it is hard to say whether Lamarck or Treviranus
has the priority in propounding the main thesis of the doctrine of
evolution; for though the first volume of Treviranus's "Biologie" appeared
only in 1802, he says, in the preface to his later work, the "Erscheinungen
und Gesetze des organischen Lebens," dated 1831, that he wrote the first
volume of the "Biologie" "nearly five-and-thirty years ago," or about 1796.
Now, in 1794, there is evidence that Lamarck held doctrines which present a
striking contrast to those which are to be found in the "Philosophie
Zoologique," as the following passages show:--
"685. Quoique mon unique objet dans cet article n'ait été que de traiter de
la cause physique de l'entretien de la vie des êtres organiques, malgré
cela j'ai osé avancer en débutant, que l'existence de ces êtres étonnants
n'appartiennent nullement à la nature; que tout ce qu'on peut entendre par
le mot _nature_, ne pouvoit donner la vie, c'est-à-dire, que toutes
les qualités de la matière, jointes à toutes les circonstances possibles,
et même à l'activité répandue dans l'univers, ne pouvaient point produire
un être muni du mouvement organique, capable de reproduire son semblable,
et sujet à la mort.
"686. Tous les individus de cette nature, qui existent, proviennent
d'individus semblables qui tous ensemble constituent l'espèce entière. Or,
je crois qu'il est aussi impossible à l'homme de connôitre la cause
physique du premier individu de chaque espèce, que d'assigner aussi
physiquement la cause de l'existence de la matière ou de l'univers entier.
C'est au moins ce que le résultat de mes connaissances et de mes réflexions
me portent à penser. S'il existe beaucoup de variétés produites par l'effet
des circonstances, ces variétés ne denaturent point les espèces; mais on se
trompe, sans doute souvent, en indiquant comme espèce, ce qui n'est que
variété; et alors je sens que cette erreur peut tirer à conséquence dans
les raisonnements que l'on fait sur cette matière." [Footnote:
_Recherches sur les causes des principaux faits physiques_, par J.B.
Lamarck. Paris. Seconde année de la République. In the preface, Lamarck
says that the work was written in 1776, and presented to the Academy in
1780; but it was not published before 17994, and, at that time, it
presumably expressed Lamarck's mature views. It would be interesting to
know what brought about the change of opinion manifested in the
_Recherches sur l'organisation des corps vivants_, published only
seven years later.]
The first three volumes of Treviranus's "Biologie," which contain his
general views of evolution, appeared between 1802 and 1805. The "Recherches
sur l'organisation des corps vivants," in which the outlines of Lamarck's
doctrines are given, was published in 1802, but the full development of his
views, in the "Philosophie Zoologique," did not take place until 1809.
The "Biologie" and the "Philosophie Zoologique" are both very remarkable
productions, and are still worthy of attentive study, but they fell upon
evil times. The vast authority of Cuvier was employed in support of the
traditionally respectable hypotheses of special creation and of
catastrophism; and the wild speculations of the "Discours sur les
Révolutions de la Surface du Globe" were held to be models of sound
scientific thinking, while the really much more sober and philosophical
hypotheses of the "Hydrogeologie" were scouted. For many years it was the
fashion to speak of Lamarck with ridicule, while Treviranus was altogether
ignored.
Nevertheless, the work had been done. The conception of evolution was
henceforward irrepressible, and it incessantly reappears, in one shape or
another, [Footnote: See the "Historical Sketch" prefixed to the last
edition of the _Origin of Species_.] up to the year 1858, when Mr.
Darwin and Mr. Wallace published their "Theory of Natural Selection." The
"Origin of Species" appeared in 1859; and it is within the knowledge of all
whose memories go back to that time, that, henceforward, the doctrine of
evolution has assumed a position and acquired an importance which it never
before possessed. In the "Origin of Species," and in his other numerous and
important contributions to the solution of the problem of biological
evolution, Mr. Darwin confines himself to the discussion of the causes
which have brought about the present condition of living matter, assuming
such matter to have once come into existence. On the other hand, Mr.
Spencer [Footnote: _First Principles_. and _Principles of
Biology_, 1860-1864.] and Professor Haeckel [Footnote: _Generelle
Marphologie_, 1866.] have dealt with the whole problem of evolution. The
profound and vigorous writings of Mr. Spencer embody the spirit of
Descartes in the knowledge of our own day, and may be regarded as the
"Principes de la Philosophie" of the nineteenth century; while, whatever
hesitation may not unfrequently be felt by less daring minds, in following
Haeckel in many of his speculations, his attempt to systematise the
doctrine of evolution and to exhibit its influence as the central thought
of modern biology, cannot fail to have a far-reaching influence on the
progress of science.
If we seek for the reason of the difference between the scientific position
of the doctrine of evolution a century ago, and that which it occupies now,
we shall find it in the great accumulation of facts, the several classes of
which have been enumerated above, under the second to the eighth heads. For
those which are grouped under the second to the seventh of these classes,
respectively, have a clear significance on the hypothesis of evolution,
while they are unintelligible if that hypothesis be denied. And those of
the eighth group are not only unintelligible without the assumption of
evolution, but can be proved never to be discordant with that hypothesis,
while, in some cases, they are exactly such as the hypothesis requires. The
demonstration of these assertions would require a volume, but the general
nature of the evidence on which they rest may be briefly indicated.
2. The accurate investigation of the lowest forms of animal life, commenced
by Leeuwenhoek and Swammerdam, and continued by the remarkable labours of
Reaumur, Trembley, Bonnet, and a host of other observers, in the latter
part of the seventeenth and the first half of the eighteenth centuries,
drew the attention of biologists to the gradation in the complexity of
organisation which is presented by living beings, and culminated in the
doctrine of the "échelle des êtres," so powerfully and clearly stated by
Bonnet; and, before him, adumbrated by Locke and by Leibnitz. In the then
state of knowledge, it appeared that all the species of animals and plants
could be arranged in one series; in such a manner that, by insensible
gradations, the mineral passed into the plant, the plant into the polype,
the polype into the worm, and so, through gradually higher forms of life,
to man, at the summit of the animated world.
But, as knowledge advanced, this conception ceased to be tenable in the
crude form in which it was first put forward. Taking into account existing
animals and plants alone, it became obvious that they fell into groups
which were more or less sharply separated from one another; and, moreover,
that even the species of a genus can hardly ever be arranged in linear
series. Their natural resemblances and differences are only to be expressed
by disposing them as if they were branches springing from a common
hypothetical centre.
Lamarck, while affirming the verbal proposition that animals form a single
series, was forced by his vast acquaintance with the details of zoology to
limit the assertion to such a series as may be formed out of the
abstractions constituted by the common characters of each group. [Footnote:
"Il s'agit donc de prouver que la série qui constitue l'échelle animale
réside essentiellement dans la distribution des masses principales qui la
composent et non dans celle des espèces ni même toujours dans celle des
genres."--_Philosophie Zoologique_. chap. v.]
Cuvier on anatomical, and Von Baer on embryological grounds, made the
further step of proving that, even in this limited sense, animals cannot be
arranged in a single series, but that there are several distinct plans of
organisation to be observed among them, no one of which, in its highest and
most complicated modification, leads to any of the others.
The conclusions enunciated by Cuvier and Von Baer have been confirmed, in
principle, by all subsequent research into the structure of animals and
plants. But the effect of the adoption of these conclusions has been rather
to substitute a new metaphor for that of Bonnet than to abolish the
conception expressed by it. Instead of regarding living things as capable
of arrangement in one series like the steps of a ladder, the results of
modern investigation compel us to dispose them as if they were the twigs
and branches of a tree. The ends of the twigs represent individuals, the
smallest groups of twigs species, larger groups genera, and so on, until we
arrive at the source of all these ramifications of the main branch, which
is represented by a common plan of structure. At the present moment, it is
impossible to draw up any definition, based on broad anatomical or
developmental characters, by which any one of Cuvier's great groups shall
be separated from all the rest. On the contrary, the lower members of each
tend to converge towards the lower members of all the others. The same may
be said of the vegetable world. The apparently clear distinction between
flowering and flowerless plants has been broken down by the series of
gradations between the two exhibited by the _Lycopodiaceæ,
Rhizocarpeæ_, and _Gymnospermeæ_. The groups of _Fungi_,
_Lichenes_, and _Algæ_ have completely run into one another, and,
when the lowest forms of each are alone considered, even the animal and
vegetable kingdoms cease to have a definite frontier.
If it is permissible to speak of the relations of living forms to one
another metaphorically, the similitude chosen must undoubtedly be that of a
common root, whence two main trunks, one representing the vegetable and one
the animal world, spring; and, each dividing into a few main branches,
these subdivide into multitudes of branchlets and these into smaller groups
of twigs.
As Lamarck has well said--[Footnote: _Philosophie Zoologique_,
première partie, chap. iii.] "Il n'y a que ceux qui se sont longtemps et
fortement occupés de la détermination des espèces, et qui ont consulté de
riches collections, qui peuvent savoir jusqu'à quel point les
_espèces_, parmi les corps vivants se fondent les unes dans les
autres, et qui ont pu se convaincre que, dans les parties où nous voyons
des _espèces_ isolès, cela n'est ainsi que parcequ'il nous en manque
d'autres qui en sont plus voisines et que nous n'avons pas encore
recueillies.
"Je ne veux pas dire pour cela que les animaux qui existent forment une
série très-simple et partout également nuancée; mais je dis qu'ils forment
une série ramense, irréguliérement graduée et qui n'a point de
discontinuité dans ses parties, ou qui, du moins, n'en a toujours pas eu,
s'il est vrai que, par suite de quelques espèces perdues, il s'en trouve
quelque part. Il en resulte que les _espèces_ qui terminent chaque
rameau de la série générale tiennent, au moins d'un côté, à d'autres
espèces voisines qui se nuancent avec elles. Voilà ce que l'état bien connu
des choses me met maintenant à portée de demontrer. Je n'ai besoin d'aucune
hypothèse ni d'aucune supposition pour cela: j'en atteste tous les
naturalistes observateurs."
3. In a remarkable essay [Footnote: "Entwurf einer Darstellung der zwischen
dem Embryozustände der höheren Thiere und dem permanenten der niederen
stattfindenden Parallele," _Beyträge zur Vergleichenden Anatomie_, Bd.
ii. 1811.] Meckel remarks--
"There is no good physiologist who has not been struck by the observation
that the original form of all organisms is one and the same, and that out
of this one form, all, the lowest as well as the highest, are developed in
such a manner that the latter pass through the permanent forms of the
former as transitory stages. Aristotle, Haller, Harvey, Kielmeyer,
Autenrieth, and many others, have either made this observation
incidentally, or, especially the latter, have drawn particular attention to
it, and deduced therefrom results of permanent importance for physiology."
Meckel proceeds to exemplify the thesis, that the lower forms of animals
represent stages in the course of the development of the higher, with a
large series of illustrations.
After comparing the Salamanders and the perennibranchiate _Urodela_
with the Tadpoles and the Frogs, and enunciating the law that the more
highly any animal is organised the more quickly does it pass through the
lower stages, Meckel goes on to say--
"From these lowest Vertebrata to the highest, and to the highest forms
among these, the comparison between the embryonic conditions of the higher
animals and the adult states of the lower can be more completely and
thoroughly instituted than if the survey is extended to the Invertebrata,
inasmuch as the latter are in many respects constructed upon an altogether
too dissimilar type; indeed they often differ from one another far more
than the lowest vertebrate does from the highest mammal; yet the following
pages will show that the comparison may also be extended to them with
interest. In fact, there is a period when, as Aristotle long ago said, the
embryo of the highest animal has the form of a mere worm; and, devoid of
internal and external organisation, is merely an almost structureless lump
of polype substance. Notwithstanding the origin of organs, it still for a
certain time, by reason of its want of an internal bony skeleton, remains
worm and mollusk, and only later enters into the series of the Vertebrata,
although traces of the vertebral column even in the earliest periods
testify its claim to a place in that series."--_Op, cit_ pp. 4, 5.
If Meckel's proposition is so far qualified, that the comparison of adult
with embryonic forms is restricted within the limits of one type of
organisation; and, if it is further recollected that the resemblance
between the permanent lower form and the embryonic stage of a higher form
is not special but general, it is in entire accordance with modern
embryology; although there is no branch of biology which has grown so
largely, and improved its methods so much, since Meckel's time, as this. In
its original form, the doctrine of "arrest of development," as advocated by
Geoffroy Saint-Hilaire and Serres, was no doubt an overstatement of the
case. It is not true, for example, that a fish is a reptile arrested in its
development, or that a reptile was ever a fish: but it is true that the
reptile embryo, at one stage of its development, is an organism which, if
it had an independent existence, must be classified among fishes; and all
the organs of the reptile pass, in the course of their development, through
conditions which are closely analogous to those which are permanent in some
fishes.
4. That branch of biology which is termed Morphology is a commentary upon,
and expansion of, the proposition that widely different animals or plants,
and widely different parts of animals or plants, are constructed upon the
same plan. From the rough comparison of the skeleton of a bird with that of
a man by Belon, in the sixteenth century (to go no farther back), down to
the theory of the limbs and the theory of the skull at the present day; or,
from the first demonstration of the homologies of the parts of a flower by
C. F. Wolff, to the present elaborate analysis of the floral organs,
morphology exhibits a continual advance towards the demonstration of a
fundamental unity among the seeming diversities of living structures. And
this demonstration has been completed by the final establishment of the
cell theory, which involves the admission of a primitive conformity, not
only of all the elementary structures in animals and plants respectively,
but of those in the one of these great divisions of living things with
those in the other. No _à priori_ difficulty can be said to stand in
the way of evolution, when it can be shown that all animals and all plants
proceed by modes of development, which are similar in principle, from a
fundamental protoplasmic material.
5. The innumerable cases of structures, which are rudimentary and
apparently useless, in species, the close allies of which possess
well-developed and functionally important homologous structures, are
readily intelligible on the theory of evolution, while it is hard to
conceive their _raison d'être_ on any other hypothesis. However, a
cautious reasoner will probably rather explain such cases deductively from
the doctrine of evolution than endeavour to support the doctrine of
evolution by them. For it is almost impossible to prove that any structure,
however rudimentary, is useless--that is to say, that it plays no part
whatever in the economy; and, if it is in the slightest degree useful,
there is no reason why, on the hypothesis of direct creation, it should not
have been created. Nevertheless, double-edged as is the argument from
rudimentary organs, there is probably none which has produced a greater
effect in promoting the general acceptance of the theory of evolution.
6. The older advocates of evolution sought for the causes of the process
exclusively in the influence of varying conditions, such as climate and
station, or hybridisation, upon living forms. Even Treviranus has got no
farther than this point. Lamarck introduced the conception of the action of
an animal on itself as a factor in producing modification. Starting from
the well-known fact that the habitual use of a limb tends to develop the
muscles of the limb, and to produce a greater and greater facility in using
it, he made the general assumption that the effort of an animal to exert an
organ in a given direction tends to develop the organ in that direction.
But a little consideration showed that, though Lamarck had seized what, as
far it goes, is a true cause of modification, it is a cause the actual
effects of which are wholly inadequate to account for any considerable
modification in animals, and which can have no influence at all in the
vegetable world; and probably nothing contributed so much to discredit
evolution, in the early part of this century, as the floods of easy
ridicule which were poured upon this part of Lamarck's speculation. The
theory of natural selection, or survival of the fittest, was suggested by
Wells in 1813, and further elaborated by Matthew in 1831. But the pregnant
suggestions of these writers remained practically unnoticed and forgotten,
until the theory was independently devised and promulgated by Darwin and
Wallace in 1858, and the effect of its publication was immediate and
profound.
Those who were unwilling to accept evolution, without better grounds than
such as are offered by Lamarck, or the author of that particularly
unsatisfactory book, the "Vestiges of the Natural History of the Creation,"
and who therefore preferred to suspend their judgment on the question,
found in the principle of selective breeding, pursued in all its
applications with marvellous knowledge and skill by Mr. Darwin, a valid
explanation of the occurrence of varieties and races; and they saw clearly
that, if the explanation would apply to species, it would not only solve
the problem of their evolution, but that it would account for the facts of
teleology, as well as for those of morphology; and for the persistence of
some forms of life unchanged through long epochs of time, while others
undergo comparatively rapid metamorphosis.
How far "natural selection" suffices for the production of species remains
to be seen. Few can doubt that, if not the whole cause, it is a very
important factor in that operation; and that it must play a great part in
the sorting out of varieties into those which are transitory and those
which are permanent.
But the causes and conditions of variation have yet to be thoroughly
explored; and the importance of natural selection will not be impaired,
even if further inquiries should prove that variability is definite, and is
determined in certain directions rather than in others, by conditions
inherent in that which varies. It is quite conceivable that every species
tends to produce varieties of a limited number and kind, and that the
effect of natural selection is to favour the development of some of these,
while it opposes the development of others along their predetermined lines
of modification.
7. No truths brought to light by biological investigation were better
calculated to inspire distrust of the dogmas intruded upon science in the
name of theology, than those which relate to the distribution of animals
and plants on the surface of the earth. Very skilful accommodation was
needful, if the limitation of sloths to South America, and of the
ornithorhynchus to Australia, was to be reconciled with the literal
interpretation of the history of the deluge; and with the establishment of
the existence of distinct provinces of distribution, any serious belief in
the peopling of the world by migration from Mount Ararat came to an end.
Under these circumstances, only one alternative was left for those who
denied the occurrence of evolution--namely, the supposition that the
characteristic animals and plants of each great province were created as
such, within the limits in which we find them. And as the hypothesis of
"specific centres," thus formulated, was heterodox from the theological
point of view, and unintelligible under its scientific aspect, it may be
passed over without further notice, as a phase of transition from the
creational to the evolutional hypothesis.
8. In fact, the strongest and most conclusive arguments in favour of
evolution are those which are based upon the facts of geographical, taken
in conjunction with those of geological, distribution.
Both Mr. Darwin and Mr. Wallace lay great stress on the close relation
which obtains between the existing fauna of any region and that of the
immediately antecedent geological epoch in the same region; and rightly,
for it is in truth inconceivable that there should be no genetic connection
between the two. It is possible to put into words the proposition that all
the animals and plants of each geological epoch were annihilated and that a
new set of very similar forms was created for the next epoch; but it may be
doubted if any one who ever tried to form a distinct mental image of this
process of spontaneous generation on the grandest scale, ever really
succeeded in realising it.
Within the last twenty years, the attention of the best palæontologists has
been withdrawn from the hodman's work of making "new species" of fossils,
to the scientific task of completing our knowledge of individual species,
and tracing out the succession of the forms presented by any given type in
time.
Those who desire to inform themselves of the nature and extent of the
evidence bearing on these questions may consult the works of Rütimeyer,
Gaudry, Kowalewsky, Marsh, and the writer of the present article. It must
suffice, in this place, to say that the successive forms of the Equine type
have been fully worked out; while those of nearly all the other existing
types of Ungulate mammals and of the _Carnivora_ have been almost as
closely followed through the Tertiary deposits; the gradations between
birds and reptiles have been traced; and the modifications undergone by the
_Crocodilia_, from the Triassic epoch to the present day, have been
demonstrated. On the evidence of palæontology, the evolution of many
existing forms of animal life from their predecessors is no longer an
hypothesis, but an historical fact; it is only the nature of the
physiological factors to which that evolution is due which is still open to
discussion.
[At page 209, the reference to Erasmus Darwin does not do justice to that
ingenious writer, who, in the 39th section of the _Zoonomia_, clearly
and repeatedly enunciates the theory of the inheritance of acquired
modifications. For example "From their first rudiment, or primordium, to
the termination of their lives, all animals undergo perpetual
transformations; which are in part produced by their own exertions in
consequence of their desires and aversions, of their pleasures and their
pains, or of irritation, or of associations; and many of these acquired
forms or propensities are transmitted to their posterity." _Zoonomia_
I., p. 506. 1893.]