[1878]
In the former half of the eighteenth century, the term "evolution" was
introduced into biological writings, in order to denote the mode in which
some of the most eminent physiologists of that time conceived that the
generations of living things took place; in opposition to the hypothesis
advocated, in the preceding century, by Harvey in that remarkable work
[Footnote: The _Exercitationes de Generatione Animalium_, which Dr.
George Ent extracted from him and published in 1651.] which would give him
a claim to rank among the founders of biological science, even had he not
been the discoverer of the circulation of the blood.
One of Harvey's prime objects is to defend and establish, on the basis of
direct observation, the opinion already held by Aristotle; that, in the
higher animals at any rate, the formation of the new organism by the
process of generation takes place, not suddenly, by simultaneous accretion
of rudiments of all, or of the most important, of the organs of the adult;
nor by sudden metamorphosis of a formative substance into a miniature of
the whole, which subsequently grows; but by _epigenesis_, or
successive differentiation of a relatively homogeneous rudiment into the
parts and structures which are characteristic of the adult.
"Et primò, quidem, quoniam per _epigenesin_ sive partium
superexorientium additamentum pullum fabricari certum est: quænam pars ante
alias omnes exstruatur, et quid de illa ejusque generandi modo observandum
veniat, dispiciemus. Ratum sane est et in ovo manifestè apparet quod
_Aristoteles_ de perfectorum animalium generatione enuntiat: nimirum,
non omnes partes simul fieri, sed ordine aliam post aliam; primùmque
existere particulam genitalem, cujus virtute postea (tanquam ex principio
quodam) reliquæ omnes partes prosiliant. Qualem in plantarum seminibus
(fabis, putà, aut glandibus) gemmam sive apicem protuberantem cernimus,
totius futuræ arboris principium. _Estque hæc particula, velut filius
emancipatus seorsumquc collocatus, et principium per se vivens; unde
postea, membrorum ordo describitur; et quæcunque ad absolvendum animal
pertinent, disponuntur._ [Footnote: _De Generatione Animalium_,
lib. ii. cap. x.] Quoniam enim _nulla pars se ipsam generat; sed postquam
generata est, se ipsam jam auget; ideo eam primùm oriri necesse est, quæ
principium augendi contineat (sive enim planta, sive animal est, æque
omnibus inest quod vim habeat vegetandi, sive nutriendi_), [Footnote:
_De Generatione_, lib. ii. cap. iv.] simulque reliquas omnes partes
suo quamque ordine distinguat et formet; proindeque in eadem primogenita
particula anima primario inest, sensus, motusque, et totius vitæ auctor et
principium." (Exercitatio 51.)
Harvey proceeds to contrast this view with that of the "Medici," or
followers of Hippocrates and Galen, who, "badly philosophising," imagined
that the brain, the heart, and the liver were simultaneously first
generated in the form of vesicles; and, at the same time, while expressing
his agreement with Aristotle in the principle of epigenesis, he maintains
that it is the blood which is the primal generative part, and not, as
Aristotle thought, the heart.
In the latter part of the seventeenth century, the doctrine of epigenesis,
thus advocated by Harvey, was controverted, on the ground of direct
observation, by Malpighi, who affirmed that the body of the chick is to be
seen in the egg, before the _punctum sanguineum_ makes it appearance.
But, from this perfectly correct observation a conclusion which is by no
means warranted was drawn; namely, that the chick, as a whole, really
exists in the egg antecedently to incubation; and that what happens in the
course of the latter process is no addition of new parts, "alias post alias
natas," as Harvey puts it, but a simple expansion, or unfolding, of the
organs which already exist, though they are too small and inconspicuous to
be discovered. The weight of Malpighi's observations therefore fell into
the scale of that doctrine which Harvey terms _metamorphosis_, in
contradistinction to epigenesis.
The views of Malpighi were warmly welcomed, on philosophical grounds, by
Leibnitz, [Footnote: "Cependant, pour revenir aux formes ordinaires ou aux
âmes matérielles, cette durée qu'il leur faut attribuer à la place de celle
qu'on avoit attribuée aux atomes pourroit faire douter si elles ne vont pas
de corps en corps; ce qui seroit la métempsychose, à peu près comme
quelques philosophes ont cru la transmission du mouvement et celle des
espèces. Mais cette imagination est bien éloignée de la nature des choses.
Il n'y a point de tel passage; et c'est ici où les transformations de
Messieurs Swammerdam, Malpighi, et Leewenhoek, qui sont des plus excellens
observateurs de notre tems, sont venues à mon secours, et m'ont fait
admettre plus aisément, que l'animal, et toute autre substance organisée ne
commence point lorsque nous le croyons, et que sa generation apparente
n'est qu'une développement et une espèce d'augmentation. Aussi ai je
remarqué que l'auteur de la _Recherche de la Verité_, M. Regis, M.
Hartsoeker, et d'autres habiles hommes n'ont pas été fort éloignés de ce
sentiment." Leibnitz, _Système Nouveau de la Nature_, 1695. The
doctrine of "Embôitement" is contained in the _Considérations sur le
Principe de Vie_, 1705; the preface to the _Theodicée_, 1710; and
the _Principes de la Nature et de la Grace_ (§ 6), 1718.] who found in
them a support to his hypothesis of monads, and by Malebranche; [Footnote:
"Il est vrai que la pensée la plus raisonnable et la plus conforme à
l'experience sur cette question très difficile de la formation du foetus;
c'est que les enfans sont déja presque tout formés avant même l'action par
laquelle ils sont conçus; et que leurs mères ne font que leur donner
l'accroissement ordinaire dans le temps de la grossesse." _De la
Recherche de la Verité_, livre ii. chap. vii. p. 334, 7th ed., 1721.]
while, in the middle of the eighteenth century, not only speculative
considerations, but a great number of new and interesting observations on
the phenomena of generation, led the ingenious Bonnet, and Haller,
[Footnote: The writer is indebted to Dr. Allen Thomson for reference to the
evidence contained in a note to Haller's edition of Boerhaave's
_Prælectiones Academicæ_, vol. v. pt. ii. p. 497, published in 1744,
that Haller originally advocated epigenesis.] the first physiologist of the
age, to adopt, advocate, and extend them.
Bonnet affirms that, before fecundation, the hen's egg contains an
excessively minute but complete chick; and that fecundation and incubation
simply cause this germ to absorb nutritious matters, which are deposited in
the interstices of the elementary structures of which the miniature chick,
or germ, is made up. The consequence of this intussusceptive growth is the
"development" or "evolution" of the germ into the visible bird. Thus an
organised individual (_tout organisé_) "is a composite body consisting
of the original, or _elementary_, parts and of the matters which have
been associated with them by the aid of nutrition;" so that, if these
matters could be extracted from the individual (_tout_), it would, so
to speak, become concentrated in a point, and would thus be restored to its
primitive condition of a _germ_; "just as by extracting from a bone
the calcareous substance which is the source of its hardness, it is reduced
to its primitive state of gristle or membrane." [Footnote:
_Considérations sur les Corps organisés, chap. x.] "Evolution" and
"development" are, for Bonnet, synonymous terms; and since by "evolution"
he means simply the expansion of that which was invisible into visibility,
he was naturally led to the conclusion, at which Leibnitz had arrived by a
different line of reasoning, that no such thing as generation, in the
proper sense of the word, exists in Nature. The growth of an organic being
is simply a process of enlargement as a particle of dry gelatine may be
swelled up by the intussusception of water; its death is a shrinkage, such
as the swelled jelly might undergo on desiccation. Nothing really new is
produced in the living world, but the germs which develop have existed
since the beginning of things; and nothing really dies, but, when what we
call death takes place, the living thing shrinks back into its germ state.
[Footnote: Bonnet had the courage of his opinions, and in the
_Palingénésie Philosophique_, part vi. chap, iv., he develops a
hypothesis which he terms "évolution naturelle;" and which, making
allowance for his peculiar views of the nature of generation, bears no
small resemblance to what is understood by "evolution" at the present
day:--
"Si la volonté divine a créé par un seul Acte l'Universalité des êtres,
d'où venoient ces plantes et ces animaux dont Moyse nous decrit la
Production au troisieme et au cinquieme jour du renouvellement de notre
monde?
"Abuserois-je de la liberté de conjectures si je disois, que les Plantes et
les Animaux qui existent aujourd'hui sont parvenus par une sorte
d'evolution naturelle des Etres organises qui peuplaient ce premier Monde,
sorti immédiatement des MAINS du CREATEUR?...
"Ne supposons que trois révolutions. La Terre vient de sortir des MAINS du
CREATEUR. Des causes preparées par sa SAGESSE font développer de toutes
parts les Germes. Les Etres organisés commencent à jouir de l'existence.
Ils étoient probablement alors bien différens de ce qu'ils sont
aujourd'hui. Ils l'etoient autant que ce premier Monde différoit de celui
que nous habitons. Nous manquons de moyens pour juger de ces dissemblances,
et peut-être que le plus habile Naturaliste qui auroit été placé dans ce
premier Monde y auroit entièrement méconnu nos Plantes et nos Animaux."]
The two parts of Bonnet's hypothesis, namely, the doctrine that all living
things proceed from pre-existing germs, and that these contain, one
inclosed within the other, the germs of all future living things, which is
the hypothesis of "_emboîtement_;" and the doctrine that every germ
contains in miniature all the organs of the adult, which is the hypothesis
of evolution or development, in the primary senses of these words, must be
carefully distinguished. In fact, while holding firmly by the former,
Bonnet more or less modified the latter in his later writings, and, at
length, he admits that a "germ" need not be an actual miniature of the
organism; but that it may be merely an "original preformation" capable of
producing the latter. [Footnote: "Ce mot (germe) ne désignera pas seulement
un corps organisé _réduit en petit_; il désignera encore toute espèce
de _préformation originelle dont un Tout organique peut résulter comme de
son principe immédiat."--Palingénésie Philosophique_, part X. chap. II.]
But, thus defined, the germ is neither more nor less than the "particula
genitalis" of Aristotle, or the "primordium vegetale" or "ovum" of Harvey;
and the "evolution" of such a germ would not be distinguishable from
"epigenesis."
Supported by the great authority of Haller, the doctrine of evolution, or
development, prevailed throughout the whole of the eighteenth century, and
Cuvier appears to have substantially adopted Bonnet's later views, though
probably he would not have gone all lengths in the direction of
"emboîtement." In a well-known note to Laurillard's "Éloge," prefixed to
the last edition of the "Ossemens fossiles," the "radical de l'être" is
much the same thing as Aristotle's "particula genitalis" and Harvey's
"ovum." [Footnote: "M. Cuvier considérant que tous les êtres organisés sont
dérivés de parens, et ne voyant dans la nature aucune force capable de
produire l'organisation, croyait à la pré-existence des germes; non pas à
la pré-existence d'un être tout formé, puisqu'il est bien évident que ce
n'est que par des développemens successifs que l'être acquiert sa forme;
mais, si l'on peut s'exprimer ainsi, à la pré-existence du _radical de
l'être_, radical qui existe avant que la série des évolutions ne
commence, et qui remonte certainement, suivant la belle observation de
Bonnet, à plusieurs generations."--Laurillard, _Éloge de Cuvier_, note
12.]
Bonnet's eminent contemporary, Buffon, held nearly the same views with
respect to the nature of the germ, and expresses them even more
confidently.
"Ceux qui ont cru que le coeur étoit le premier formé, se sont trompés;
ceux qui disent que c'est le sang se trompent aussi: tout est formé en même
temps. Si l'on ne consulte que l'observation, le poulet se voit dans l'oeuf
avant qu'il ait été couvé." [Footnote: _Histoire Naturelle_, tom. ii.
ed. ii. 1750, p. 350.]
"J'ai ouvert une grande quantité d'oeufs à differens temps avant et après
l'incubation, et je me suis convaincu par mes yeux que le poulet existe en
entier dans le milieu de la cicatricule au moment qu'il sort du corps de la
poule." [Footnote: _Ibid_., p. 351.]
The "moule intérieur" of Buffon is the aggregate of elementary parts which
constitute the individual, and is thus the equivalent of Bonnet's germ,
[Footnote: See particularly Buffon, _l. c._ p. 41.] as defined in the
passage cited above. But Buffon further imagined that innumerable
"molecules organiques" are dispersed throughout the world, and that
alimentation consists in the appropriation by the parts of an organism of
those molecules which are analogous to them. Growth, therefore, was, on
this hypothesis, a process partly of simple evolution, and partly of what
has been termed "syngenesis." Buffon's opinion is, in fact, a sort of
combination of views, essentially similar to those of Bonnet, with others,
somewhat similar to those of the "Medici" whom Harvey condemns. The
"molecules organiques" are physical equivalents of Leibnitz's "monads."
It is a striking example of the difficulty of getting people to use their
own powers of investigation accurately, that this form of the doctrine of
evolution should have held its ground so long; for it was thoroughly and
completely exploded, not long after its enunciation, by Casper Friederich
Wolff, who in his "Theoria Generationis," published in 1759, placed the
opposite theory of epigenesis upon the secure foundation of fact, from
which it has never been displaced. But Wolff had no immediate successors.
The school of Cuvier was lamentably deficient in embryologists; and it was
only in the course of the first thirty years of the present century, that
Prévost and Dumas in France, and, later on, Döllinger, Pander, Von Bär,
Rathke, and Remak in Germany, founded modern embryology; while, at the same
time, they proved the utter incompatibility of the hypothesis of evolution,
as formulated by Bonnet and Haller, with easily demonstrable facts.
Nevertheless, though the conceptions originally denoted by "evolution" and
"development" were shown to be untenable, the words retained their
application to the process by which the embryos of living beings gradually
make their appearance; and the terms "Development," "Entwickelung," and
"Evolutio," are now indiscriminately used for the series of genetic changes
exhibited by living beings, by writers who would emphatically deny that
"Development" or "Entwickelung" or "Evolutio," in the sense in which these
words were usually employed by Bonnet or by Haller, ever occurs.
Evolution, or development, is, in fact, at present employed in biology as a
general name for the history of the steps by which any living being has
acquired the morphological and the physiological characters which
distinguish it. As civil history may be divided into biography, which is
the history of individuals, and universal history, which is the history of
the human race, so evolution falls naturally into two categories--the
evolution of the individual, and the evolution of the sum of living beings.
It will be convenient to deal with the modern doctrine of evolution under
these two heads.