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.