Lectures and Essays | Page 6

force, and any one of them with the other, each being
measurable in terms of the other,--even so, I say, that great law is
applicable to the living world. Consider why is the skeleton of this
horse capable of supporting the masses of flesh and the various organs
forming the living body, unless it is because of the action of the same
forces of cohesion which combines together the particles of matter
composing this piece of chalk? What is there in the muscular
contractile power of the animal but the force which is expressible, and
which is in a certain sense convertible, into the force of gravity which it
overcomes? Or, if you go to more hidden processes, in what does the
process of digestion differ from those processes which are carried on in
the laboratory of the chemist? Even if we take the most recondite and
most complex operations of animal life--those of the nervous system,
these of late years have been shown to be--I do not say identical in any
sense with the electrical processes--but this has been shown, that they
are in some way or other associated with them; that is to say, that every
amount of nervous action is accompanied by a certain amount of
electrical disturbance in the particles of the nerves in which that
nervous action is carried on. In this way the nervous action is related to
electricity in the same way that heat is related to electricity; and the
same sort of argument which demonstrates the two latter to be related
to one another shows that the nervous forces are correlated to electricity;
for the experiments of M. Dubois Reymond and others have shown that
whenever a nerve is in a state of excitement, sending a message to the
muscles or conveying an impression to the brain, there is a disturbance
of the electrical condition of that nerve which does not exist at other
times; and there are a number of other facts and phenomena of that sort;
so that we come to the broad conclusion that not only as to living
matter itself, but as to the forces that matter exerts, there is a close
relationship between the organic and the inorganic world--the
difference between them arising from the diverse combination and
disposition of identical forces, and not from any primary diversity, so
far as we can see.

I said just now that the Horse eventually died and became converted
into the same inorganic substances from whence all but an
inappreciable fraction of its substance demonstrably originated, so that
the actual wanderings of matter are as remarkable as the
transmigrations of the soul fabled by Indian tradition. But before death
has occurred, in the one sex or the other, and in fact in both, certain
products or parts of the organism have been set free, certain parts of the
organisms of the two sexes have come into contact with one another,
and from that conjunction, from that union which then takes place,
there results the formation of a new being. At stated times the mare,
from a particular part of the interior of her body, called the ovary, gets
rid of a minute particle of matter comparable in all essential respects
with that which we called a cell a little while since, which cell contains
a kind of nucleus in its centre, surrounded by a clear space and by a
viscid mass of protein substance (Figure 2); and though it is different in
appearance from the eggs which we are mostly acquainted with, it is
really an egg. After a time this minute particle of matter, which may
only be a small fraction of a grain in weight, undergoes a series of
changes,--wonderful, complex changes. Finally, upon its surface there
is fashioned a little elevation, which afterwards becomes divided and
marked by a groove. The lateral boundaries of the groove extend
upwards and downwards, and at length give rise to a double tube. In the
upper smaller tube the spinal marrow and brain are fashioned; in the
lower, the alimentary canal and heart; and at length two pairs of buds
shoot out at the sides of the body, which are the rudiments of the limbs.
In fact a true drawing of a section of the embryo in this state would in
all essential respects resemble that diagram of a horse reduced to its
simplest expression, which I first placed before you (Figure 1).
Slowly and gradually these changes take place. The whole of the body,
at first, can be broken up into "cells," which become in one place
metamorphosed into muscle,--in another place into gristle and bone,--in
another place into fibrous tissue,--and in another into hair; every part
becoming gradually and slowly fashioned, as if there were an artificer
at work in