OF NUTRITION AND GROWTH.
The body of an animal is a kind of internal mould, in which the
nutritive matter assimilates itself with the whole in such a manner
that, without changing the order and proportion of the parts, each
receives an augmentation, and it is this augmentation of bulk which
some have called _expansion_, because they imagined every difficulty
would be removed by the supposition that the animal was completely
formed in the embryo, and that it would be easy to conceive that its
parts would expand, or unfold in proportion as it would increase by the
addition of accessory matter.
But if we would have a clear idea of this augmentation and expansion,
how can it be done otherwise than by considering the animal body,
and each of its parts, as so many internal moulds which receive the
accessory matter in the order that results from the position of all
their parts? This expansion cannot be made by the addition to the
surfaces alone, but, on the contrary, by an intimate susception which
penetrates the mass, and thus increases the size of the parts, without
changing the form, from whence it is necessary that the matter which
serves for this expansion should penetrate the internal part in all
its dimensions; it is also as necessary that this penetration be made
in a certain order and proportion, so that no one point can receive
more than another, without which some parts would expand quicker than
others, and the form be entirely changed. Now what can prescribe this
rule to accessory matter, and constrain it to arrive perpetually and
proportionally to every point of the internal parts, except we conceive
an internal mould?
It therefore appears certain that the body of an animal or vegetable
is an internal mould of a constant form, but where their masses may
augment proportionably, by the extension of this mould in all its
external and internal dimensions. That this extension also is made
by the intus-susception of any accessory or foreign matter which
penetrates the internal part, and becomes similar to the form and
identical substance with the matter of the moulds themselves.
But of what nature is this matter which the animal or vegetable
assimilates with its own substance? what can be the nature of that
power which gives it the activity and necessary motion to penetrate the
internal mould? and if such a power does exist, must it not be similar
to that by which the internal mould itself would be produced?
These three questions include all that can be desired on this subject,
and seem to depend on each other so much, that I am persuaded the
reproduction of an animal or vegetable cannot be explained in a
satisfactory manner, if a clear idea of the mode of the operation of
nutrition is not obtained; we must, therefore, examine these three
questions separately, in order to compare the consequences resulting
therefrom.
The first, which relates to the nutritive nature of this matter, is
in part resolved by the reasons we have already given, and will be
fully demonstrated in the succeeding chapter. We will shew that there
exists an infinity of living organic particles in Nature; that their
production is of little expence to Nature, since their existence is
constant and invariable, and that the causes of death only separate
without destroying them. Therefore the matter which the animal or
vegetable assimilates is an organic matter of the same nature as the
animal or vegetable itself, and which consequently can augment the
size without changing the form or quality of the matter of the mould,
since it is in fact of the same form and quality as that which it is
constituted with. Thus, in the quantity of aliments which the animal
takes to support life, and to keep its organs in play, and in the sap,
which the vegetable takes up by its roots and leaves, there is a great
part thrown off by transpiration, secretion, and other excretory modes,
and only a small portion retained for the nourishment of the parts and
their expansion. It is very probable, that in the body of an animal
or vegetable there is formed a separation of the brute particles of
the aliments and the organic; that the first are carried off by the
causes just mentioned; that only organic particles remain, and that
the distribution of them is made by means of some active power which
conducts them to every part in an exact proportion, insomuch that
neither receive more or less than is needful for its equal nutrition,
growth, or expansion.
The second question, What can be the active power which causes this
organic matter to penetrate and incorporate itself with this internal
mould? By the preceding chapter it appears, that there exists in
Nature powers relative to the internal part of matter, and which have
no relation with its external qualities. These powers, as already
observed, will never come under our cognizance, because their action is
made on the internal part of the body, whereas our senses cannot reach
beyond what is external; it is therefore evident, that we shall never
have a clear idea of the penetrating powers, nor of the manner by which
they act; but it is not less certain that they exist, than that by
their means most effects of Nature are produced; we must attribute to
them the effects of nutrition and expansion, which cannot be effected
by any other means than the penetration of the most intimate recesses
of the original mould: in the same mode as gravity penetrates all parts
of matter, so the power which impels or attracts the organic particles
of food, penetrates into the internal parts of organized bodies, and
as those bodies have a certain form, which we call the internal mould,
the organic particles, impelled by the action of the penetrating force,
cannot enter therein but in a certain order relative to this form,
which consequently it cannot change, but only augment its dimensions,
and thus produce the growth of organized bodies; and if in the
organized body, expanded by this means, there are some particles whose
external and internal forms are like that of the whole body, from those
reproduction will proceed.
The third question, Is it not by a similar power the internal mould
itself is reproduced? It appears, that it is not only a similar but
the same power which causes expansion and reproduction, for in an
organized body which expands, if there is some particle like the whole,
it is sufficient for that particle to become one day an organized
body itself, perfectly similar to that of which it made a part. This
particle will not at first present a figure striking enough for us to
compare with the whole body; but when separated from that body, and
receiving proper nourishment, it will begin to expand, and in a short
time present a similar being, both externally and internally, as the
body from which it had been separated: thus a willow or polypus, which
contain more organic particles similar to the whole than most other
substances, if cut into ever such a number of pieces, from each piece
will spring a body similar to that from whence it was divided.
Now in a body, every particle of which is like itself, the organization
is the most simple, as we have observed in the first chapter; for it
is only the repetition of the same form, and a composition of similar
figures, all organized alike. It is for this reason that the most
simple bodies, or the most imperfect kinds, are reproduced with the
greatest ease, and in the greatest plenty; whereas, if an organized
body contains only some few particles like itself, then, as such alone
can arrive to the second expansion, consequently the reproduction will
be more difficult, and not so abundant in number; the organization
of these bodies will also be more compounded, because the more the
organized parts differ from the whole, the more the organization of
this body will be perfect, and the more difficult the production will
be.
Nourishment, expansion, and propagation, then, are the effects of one
and the same cause. The organized body is nourished by the particles
of aliments analogous to it; it expands by the intimate susception
of organical parts which agree with it, and it propagates because
it contains some original particles which resemble itself. It only
remains to examine, whether these similar organic particles come into
the organized body by nutriment, or whether they were there before,
and have an independent existence. If we suppose the latter, we shall
fall in with the doctrine of the infinity of parts, or similar germs
contained one in the other; the insufficiency and absurdity of which
hypothesis we have already shewn; we must therefore conclude that
similar parts are extracted from the food; and after what has been
said, we hope to explain the manner in which the organic molecules are
formed, and how the minute particles unite.
There is, as we have said, a separation of the parts in the nutriment;
the organic from those analogous to the animal or vegetable, by
transpiration and other excretory modes; the organical remain and serve
for the expansion and nutriment of the body. But these organic parts
must be of various kinds, and as each part of the body receives only
those similar to itself, and that in due proportion, it is very natural
to imagine, that the superfluity of this organic matter will be sent
back from every part of the body into one or more places, where all
these organical molecules uniting, form small organized bodies like
the first, and to which nothing is wanting but the mode of expansion
for them to become individuals of the same species; for every part
of the body sending back organized parts, like those of which they
themselves are composed, it is necessary, that from the union of all
these parts, there should result organized bodies like the first. This
being admitted, may we not conclude this is the reason why, during the
time of expansion and growth, organized bodies cannot produce, because
the parts which expand absorb the whole of the organic molecules which
belong to them, and not having any superfluous parts, consequently are
incapable of reproduction.
This explanation of nutrition and reproduction will not probably be
received by those who admit but of a certain number of mechanical
principles, and reject all which do not depend on them; and as what
has been said of nutrition and expansion comes under the latter
description, they will possibly treat it as unworthy dependance. But
I am quite of a different opinion from these philosophers; for it
appears to me that, by admitting only a certain number of mechanical
principles, they do not see how greatly they contract the bounds of
philosophy, and that for one phenomenon that can be explained by a
system so confined, a thousand would be found exceeding its limits.
The idea of explaining every phenomenon in nature by mechanical
principles was certainly a great and beautiful exertion, and which
Descartes first attempted. But this idea is only a project, and if
properly founded, have we the means of performing it? These mechanical
principles are the extent of matter, its impenetrability, its motion,
its external figure, its divisibility, and the communication of
movement by impulsion, by elasticity, &c. The particular ideas of each
of these qualities we have acquired by our senses, and regard them as
principles, because they are general and belong to all matter. But are
we certain these qualities are the only ones which matter possesses,
or rather, must we not think these qualities, which we take for
principles, are only modes of perception; and that if our senses were
differently formed, we should discover in matter, qualities different
from those which we have enumerated? To admit only those qualities
to matter which are known to us, seems to be a vain and unfounded
pretension. Matter may have many general qualities which we shall ever
be ignorant of; she may also have others that human assiduity may
discover, in the same manner as has recently been done with respect
to gravity, which alike exists in all matter. The cause of impulsion,
and such other mechanical principles, will always be as impossible to
find out as that of attraction, or such other general quality. From
hence is it not very reasonable to say, that mechanical principles are
nothing but general effects, which experience has pointed out to us in
matter, and that every time a new general effect is discovered, either
by reflection, comparison, measure, or experience, a new mechanical
principle will be gained, which may be used with as much certainty and
advantage as any we are now acquainted with?
The defect of Aristotle's philosophy was making use of particular
effects as common causes; and that of Descartes in making use of only
a few general effects as causes, and excluding all the rest. The
philosophy which appears to me would be the least deficient, is that
where general effects are only made use of for causes, and seeking to
augment the number of them, by endeavouring to generalize particular
effects.
In my explanation of expansion and reproduction, I admit the received
mechanical principles, the penetrating force of weight, and, by
analogy, I have strove to point out that there are other penetrating
powers existing in organized bodies, which experience has confirmed. I
have proved by facts, that matter inclines to organization, and that
there exists an infinite number of organic particles. I have therefore
only generalized some observations, without having advanced any thing
contrary to mechanical principles, when that term is used as it ought
to be understood, as denoting the general effects of Nature.