of the brain into a sort of bag, or sac, with a narrow neck, the convex
bottom of which is turned outwards, or towards the crystalline lens.
As the development of the eye proceeds, the convex bottom of the bag
becomes pushed in, so that it gradually obliterates the cavity of the
sac, the previously convex wall of which becomes deeply concave. The
sac of the brain is now like a double nightcap ready for the head, but
the place which the head would occupy is taken by the vitreous humour,
while the layer of nightcap next it becomes the retina. The cells of
this layer which lie farthest from the vitreous humour, or, in other
words, bound the original cavity of the sac, are metamorphosed into
the rods and cones. Suppose now that the sac of the brain could be
brought back to its original form; then the rods and cones would form
part of the lining of a side pouch of the brain. But one of the most
wonderful revelations of embryology is the proof of the fact that the
brain itself is, at its first beginning, merely an infolding of the
epidermic layer of the general integument. Hence it follows that the
rods and cones of the vertebrate eye are modified epidermic cells, as
much as the crystalline cones of the insect or crustacean eye are; and
that the inversion of the position of the former in relation to light
arises simply from the roundabout way in which the vertebrate retina is
developed.
Thus all the higher sense organs start from one foundation, and the
receptive epithelium of the eye, or of the ear, is as much modified
epidermis as is that of the nose. The structural unity of the sense
organs is the morphological parallel to their identity of physiological
function, which, as we have seen, is to be impressed by certain modes
of motion; and they are fine or coarse, in proportion to the delicacy
or the strength of the impulses by which they are to be affected.
In ultimate analysis, then, it appears that a sensation is the
equivalent in terms of consciousness for a mode of motion of the matter
of the sensorium. But, if inquiry is pushed a stage farther, and the
question is asked, What then do we know about matter and motion? there
is but one reply possible. All that we know about motion is that it is
a name for certain changes in the relations of our visual, tactile, and
muscular sensations; and all that we know about matter is that it is
the hypothetical substance of physical phenomena—the assumption of the
existence of which is as pure a piece of metaphysical speculation as is
that of the existence of the substance of mind.
Our sensations, our pleasures, our pains, and the relations of these,
make up the sum total of the elements of positive, unquestionable
knowledge. We call a large section of these sensations and their
relations matter and motion; the rest we term mind and thinking; and
experience shows that there is a certain constant order of succession
between some of the former and some of the latter.
This is all that just metaphysical criticism leaves of the idols
set up by the spurious metaphysics of vulgar common sense. It is
consistent either with pure Materialism, or with pure Idealism, but
it is neither. For the Idealist, not content with declaring the truth
that our knowledge is limited to facts of consciousness, affirms the
wholly unprovable proposition that nothing exists beyond these and the
substance of mind. And, on the other hand, the Materialist, holding by
the truth that, for anything that appears to the contrary, material
phenomena are the causes of mental phenomena, asserts his unprovable
dogma, that material phenomena and the substance of matter are the sole
primary existences.
Strike out the propositions about which neither controversialist does
or can know anything, and there is nothing left for them to quarrel
about. Make a desert of the Unknowable, and the divine Astræa of
philosophic peace will commence her blessed reign.
XI.
EVOLUTION IN BIOLOGY.
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 generation of living things took place; in opposition to the
hypothesis advocated, in the preceding century, by Harvey in that
remarkable work[63] 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 seorsumque
collocatus, et principium per se vivens; unde postea
membrorum ordo describitur; et quæcunque ad absolvendum
animal pertinent, disponuntur._[64] 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)_,[65] 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 its 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,[66] who found in them a support to his hypothesis
of monads, and by Malebranche;[67] 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,[68] 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.”[69]
“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.[70]
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.[71]
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 Laurillards’
“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.”[72]
Bonnet’s eminent contemporary, Buffon, held nearly the same views
with respect to the nature of the germ, and expresses them even more
confidently.[73]
“Ceux qui ont cru que le cœur 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’œuf
avant qui’il ait été couvé.”
“J’ai ouvert une grande quantité d’œufs 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.”[74]
The “moule intérieur” of Buffon is the aggregate of elementary
parts which constitute the individual, and is thus the equivalent
of Bonnet’s germ,[75] 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
Caspar 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[76] 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.[77]
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,
Cœlenterates, 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 the 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.”[78]
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.