A B C D E F G H I J K L M N O P R S T U V W X Z

Darwiniana

T >> Thomas Henry Huxley >> Darwiniana




"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.

I. _The Evolution of the Individual_.

No exception is at this time, known to the general law, established upon an
immense multitude of direct observations, that every living thing is
evolved from a particle of matter in which no trace of the distinctive
characters of the adult form of that living thing is discernible. This
particle is termed a _germ_. Harvey [Footnote: _Execitationes de
Generatione_. Ex. 62, "Ovum esse primordium commune omnibus
animalibus."] says--

"Omnibus viventibus primordium insit, ex quo et a quo proveniant. Liceat
hoc nobis _primordium vegetale_ nominare; nempe substantiam quandam
corpoream vitam habentem potentiâ; vel quoddam per se existens, quod aptum
sit, in vegetativam formam, ab interno principio operante, mutari. Quale
nempe primordium, ovum est et plantarum semen; tale etiam viviparorum
conceptus, et insectorum _vermis_ ab Aristotele dictus: diversa
scilicet diversorum viventium primordia."

The definition of a germ as "matter potentially alive, and having within
itself the tendency to assume a definite living form," appears to meet all
the requirements of modern science. For, notwithstanding it might be justly
questioned whether a germ is not merely potentially, but rather actually,
alive, though its vital manifestations are reduced to a minimum, the term
"potential" may fairly be used in a sense broad enough to escape the
objection. And the qualification of "potential" has the advantage of
reminding us that the great characteristic of the germ is not so much what
it is, but what it may, under suitable conditions, become. Harvey shared
the belief of Aristotle--whose writings he so often quotes and of whom he
speaks as his precursor and model, with the generous respect with which one
genuine worker should regard another--that such germs may arise by a
process of "equivocal generation" out of not-living matter; and the
aphorism so commonly ascribed to him, "_omne vivum ex ovo_" and which
is indeed a fair summary of his reiterated assertions, though incessantly
employed against the modern advocates of spontaneous generation, can be
honestly so used only by those who have never read a score of pages of the
"Exercitationes." Harvey, in fact, believed as implicitly as Aristotle did
in the equivocal generation of the lower animals. But, while the course of
modern investigation has only brought out into greater prominence the
accuracy of Harvey's conception of the nature and mode of development of
germs, it has as distinctly tended to disprove the occurrence of equivocal
generation, or abiogenesis, in the present course of nature. In the immense
majority of both plants and animals, it is certain that the germ is not
merely a body in which life is dormant or potential, but that it is itself
simply a detached portion of the substance of a pre-existing living body;
and the evidence has yet to be adduced which will satisfy any cautious
reasoner that "omne vivum ex vivo" is not as well-established a law of the
existing course of nature as "omne vivum ex ovo."

In all instances which have yet been investigated, the substance of this
germ has a peculiar chemical composition, consisting of at fewest four
elementary bodies, viz., carbon, hydrogen, oxygen, and nitrogen, united
into the ill-defined compound known as protein, and associated with much
water, and very generally, if not always, with sulphur and phosphorus in
minute proportions. Moreover, up to the present time, protein is known only
as a product and constituent of living matter. Again, a true germ is either
devoid of any structure discernible by optical means, or, at most, it is a
simple nucleated cell. [Footnote: In some cases of sexless multiplication
the germ is a cell-aggregate--if we call germ only that which is already
detached from the parent organism.]

In all cases the process of evolution consists in a succession of changes
of the form, structure, and functions of the germ, by which it passes, step
by step, from an extreme simplicity, or relative homogeneity, of visible
structure, to a greater or less degree of complexity or heterogeneity; and
the course of progressive differentiation is usually accompanied by growth,
which is effected by intussusception. This intussusception, however, is a
very different process from that imagined either by Buffon or by Bonnet.
The substance by the addition of which the germ is enlarged is in no case
simply absorbed, ready-made, from the not-living world and packed between
the elementary constituents of the germ, as Bonnet imagined; still less
does it consist of the "molecules organiques" of Buffon. The new material
is, in great measure, not only absorbed but assimilated, so that it becomes
part and parcel of the molecular structure of the living body into which it
enters. And, so far from the fully developed organism being simply the germ
_plus_ the nutriment which it has absorbed, it is probable that the
adult contains neither in form, nor in substance, more than an
inappreciable fraction of the constituents of the germ, and that it is
almost, if not wholly, made up of assimilated and metamorphosed nutriment.
In the great majority of cases, at any rate, the full-grown organism
becomes what it is by the absorption of not-living matter, and its
conversion into living matter of a specific type. As Harvey says (Ex. 45),
all parts of the body are nourished "ab eodem succo alibili, aliter
aliterque cambiato," "ut plantæ omnes ex eodem communi nutrimento (sive
rore seu terræ humore)."

In all animals and plants above the lowest the germ is a nucleated cell,
using that term in its broadest sense; and the first step in the process of
the evolution of the individual is the division of this cell into two or
more portions. The process of division is repeated, until the organism,
from being unicellular, becomes multicellular. The single cell becomes a
cell-aggregate; and it is to the growth and metamorphosis of the cells of
the cell-aggregate thus produced, that all the organs and tissues of the
adult owe their origin.

In certain animals belonging to every one of the chief groups into which
the _Metazoa_ are divisible, the cells of the cell-aggregate which
results from the process of yelk-division, and which is termed a
_morula_, diverge from one another in such a manner as to give rise to
a central space, around which they dispose themselves as a coat or
envelope; and thus the morula becomes a vesicle filled with fluid, the
_planula_. The wall of the planula is next pushed in on one side, or
invaginated, whereby it is converted into a double-walled sac with an
opening, the _blastopore_, which leads into the cavity lined by the
inner wall. This cavity is the primitive alimentary cavity or
_archenteron_; the inner or invaginated layer is the _hypoblast_;
the outer the _epiblast_; and the embryo, in this stage, is termed a
_gastrula_. In all the higher animals a layer of cells makes its
appearance between the hypoblast and the epiblast, and is termed the
_mesoblast_. In the further course of development the epiblast becomes
the ectoderm or epidermic layer of the body; the hypoblast becomes the
epithelium of the middle portion of the alimentary canal; and the mesoblast
gives rise to all the other tissues, except the central nervous system,
which originates from an ingrowth of the epiblast.

With more or less modification in detail, the embryo has been observed to
pass through these successive evolutional stages in sundry Sponges,
Coelenterates, Worms, Echinoderms, Tunicates, Arthropods, Mollusks, and
Vertebrates; and there are valid reasons for the belief that all animals of
higher organisation than the _Protozoa_, agree in the general
character of the early stages of their individual evolution. Each, starting
from the condition of a simple nucleated cell, becomes a cell-aggregate;
and this passes through a condition which represents the gastrula stage,
before taking on the features distinctive of the group to which it belongs.
Stated in this form, the "gastræa theory" of Haeckel appears to the present
writer to be one of most important and best founded of recent
generalisations. So far as individual plants and animals are concerned,
therefore, evolution is not a speculation but a fact; and it takes place by
epigenesis.

"Animal...per _epigenesin_ procreatur, materiam simul attrahit, parat,
concoquit, et eâdem utitur; formatur simul et augetur ... primum futuri
corporis concrementum ... prout augetur, dividitur sensim et distinguitur
in partes, non simul omnes, sed alias post alias natas, et ordine quasque
suo emergentes." [Footnote: Harvey, _Exercitationes de Generatione_.
Ex. 45, "Quænam sit pulli materia et quomodo fiat in Ovo."] In these words,
by the divination of genius, Harvey, in the seventeenth century, summed up
the outcome of the work of all those who, with appliances he could not
dream of, are continuing his labours in the nineteenth century.

Nevertheless, though the doctrine of epigenesis, as understood by 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 further, 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 [Footnote: [At
any rate of the nuclei of the two germ-cells. 1893]] 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 he 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. [Footnote: As Buffon has well
said:--"L'idée de ramener l'explication de tous les phénomènes à des
principes mecaniques est assurement grande et belle, ce pas est le plus
hardi qu'on peut faire en philosophie, et c'est Descartes qui l'a
fait."--_l. c._ p. 50.]

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 quo le monde n'ait été
créé au commencement avec autant de perfection qu'il eu 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." [Footnote: _Principes de la Philosophie_, Troisième
partie, § 45.]

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." [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.

Copyright (c) 2007. famouswriterz.com. All rights reserved.