Harvey, has definitively triumphed over the doctrine of evolution,
as understood by his opponents of the eighteenth century, it is not
impossible that, when the analysis of the process of development is
carried still farther, and the origin of the molecular components of
the physically gross, though sensibly minute, bodies which we term
germs is traced, the theory of development will approach more nearly
to metamorphosis than to epigenesis. Harvey thought that impregnation
influenced the female organism as a contagion; and that the blood,
which he conceived to be the first rudiment of the germ, arose in
the clear fluid of the “colliquamentum” of the ovum by a process of
concrescence, as a sort of living precipitate. We now know, on the
contrary, that the female germ or ovum, in all the higher animals
and plants, is a body which possesses the structure of a nucleated
cell; that impregnation consists in the fusion of the substance[79]
of another more or less modified nucleated cell, the male germ, with
the ovum; and that the structural components of the body of the
embryo are all derived, by a process of division, from the coalesced
male and female germs. Hence it is conceivable, and indeed probable,
that every part of the adult contains molecules, derived both from
the male and from the female parent; and that, regarded as a mass
of molecules, the entire organism may be compared to a web of which
the warp is derived from the female and the woof from the male. And
each of these may constitute one individuality, in the same sense
as the whole organism is one individual, although the matter of the
organism has been constantly changing. The primitive male and female
molecules may play the part of Buffon’s “moules organiques,” and
mould the assimilated nutriment, each according to its own type, into
innumerable new molecules. From this point of view the process, which,
in its superficial aspect, is epigenesis, appears in essence, to be
evolution, in the modified sense adopted in Bonnet’s later writings;
and development is merely the expansion of a potential organism or
“original preformation” according to fixed laws.
II. _The Evolution of the Sum of Living Beings._
The notion that all the kinds of animals and plants may have come into
existence by the growth and modification of primordial germs is as old
as speculative thought; but the modern scientific form of the doctrine
can be traced historically to the influence of several converging lines
of philosophical speculation and of physical observation, none of which
go farther back than the seventeenth century. These are:—
1. The enunciation by Descartes of the conception that the physical
universe, whether living or not living, is a mechanism, and that, as
such, it is explicable on physical principles.
2. The observation of the gradations of structure, from extreme
simplicity to very great complexity, presented by living things, and of
the relation of these graduated forms to one another.
3. The observation of the existence of an analogy between the series
of gradations presented by the species which compose any great group
of animals or plants, and the series of embryonic conditions of the
highest members of that group.
4. The observation that large groups of species of widely different
habits present the same fundamental plan of structure; and that parts
of the same animal or plant, the functions of which are very different,
likewise exhibit modifications of a common plan.
5. The observation of the existence of structures, in a rudimentary
and apparently useless condition, in one species of a group, which are
fully developed and have definite functions in other species of the
same group.
6. The observation of the effects of varying conditions in modifying
living organisms.
7. The observation of the facts of geographical distribution.
8. The observation of the facts of the geological succession of the
forms of life.
1. Notwithstanding the elaborate disguise which fear of the powers that
were led Descartes to throw over his real opinions, it is impossible to
read the “Principes de la Philosophie” without acquiring the conviction
that this great philosopher held that the physical world and all things
in it, whether living or not living, have originated by a process of
evolution, due to the continuous operation of purely physical causes,
out of a primitive relatively formless matter.[80]
The following passage is especially instructive:—
“Et tant s’en faut que je veuille que l’on croie toutes
les choses que j’écrirai, que même je pretends en
proposer ici quelques unes que je crois absolument être
fausses; à savoir, je ne doute point que le monde n’ait
été créé au commencement avec autant de perfection qu’il
en a; en sorte que le soleil, la terre, la lune, et
les étoiles ont été dès lors; et que la terre n’a pas
eu seulement en soi les semences des plantes, mais que
les plantes même en ont couvert une partie; et qu’Adam
et Eve n’ont pas été créés enfans mais en âge d’hommes
parfaits. La religion chrétienne veut que nous le croyons
ainsi, et la raison naturelle nous persuade entièrement
cette vérité; car si nous considérons la toute puissance
de Dieu, nous devons juger que tout ce qu’il a fait a
eu dès le commencement toute la perfection qu’il devoit
avoir. Mais néanmoins, comme on connôitroit beaucoup
mieux quelle a été la nature d’Adam et celle des arbres
de Paradis si on avoit examiné comment les enfants se
forment peu à peu dans le ventre de leurs mères et
comment les plantes sortent de leurs semences, que si on
avoit seulement considéré quels ils ont été quand Dieu
les a créés: tout de même, nous ferons mieux entendre
quelle est généralement la nature de toutes les choses
qui sont au monde si nous pouvons imaginer quelques
principes qui soient fort intelligibles et fort simples,
desquels nous puissions voir clairement que les astres et
la terre et enfin tout ce monde visible auroit pu être
produit ainsi que de quelques semences (bien que nous
sachions qu’il n’a pas été produit en cette façon) que si
nous la decrivions seulement comme il est, ou bien comme
nous croyons qu’il a été créé. Et parceque je pense avoir
trouvé des principes qui sont tels, je tacherai ici de
les expliquer.”[81]
If we read between the lines of this singular exhibition of force of
one kind and weakness of another, it is clear that Descartes believed
that he had divined the mode in which the physical universe had been
evolved; and the “Traité de l’homme,” and the essay “Sur les Passions”
afford abundant additional evidence that he sought for, and thought
he had found, an explanation of the phenomena of physical life by
deduction from purely physical laws.
Spinoza abounds in the same sense, and is as usual perfectly candid—
“Naturæ leges et regulæ, secundum quas omnia fiunt et
ex unis formis in alias mutantur, sunt ubique et semper
eadem.”[82]
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[83] 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[84] 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
“Philosophic 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
dénaturent 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.”[85]
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,[86] 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[87] and Professor Haeckel[88] 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.[89]
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—[90]
“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
rameuse, 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[91] 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
drawn 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 perenni-branchiate _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
over-statement 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.