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 each of these complex structures that we have mentioned. This
embryo, as it is called, then passes into other conditions. I should
tell you that there is a time when the embryos of neither dog, nor
horse, nor porpoise, nor monkey, nor man, can be distinguished by any
essential feature one from the other; there is a time when they each and
all of them resemble this one of the Dog. But as development advances,
all the parts acquire their speciality, till at length you have the
embryo converted into the form of the parent from which it started. So
that you see, this living animal, this horse, begins its existence as a
minute particle of nitrogenous matter, which, being supplied with
nutriment (derived, as I have shown, from the inorganic world), grows up
according to the special type and construction of its parents, works and
undergoes a constant waste, and that waste is made good by nutriment
derived from the inorganic world; the waste given off in this way being
directly added to the inorganic world; and eventually the animal itself
dies, and, by the process of decomposition, its whole body is returned
to those conditions of inorganic matter in which its substance
originated.

This, then, is that which is true of every living form, from the lowest
plant to the highest animal--to man himself. You might define the life
of every one in exactly the same terms as those which I have now used;
the difference between the highest and the lowest being simply in the
complexity of the developmental changes, the variety of the structural