rpoise, 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
forms, the diversity of the physiological functions which are exerted by
each.

If I were to take an oak tree as a specimen of the plant world, I should
find that it originated in an acorn, which, too, commenced in a cell;
the acorn is placed in the ground, and it very speedily begins to absorb
the inorganic matters I have named, adds enormously to its bulk, and
we can see it, year after year, extending itself upward and downward,
attracting and appropriating to itself inorganic materials, which it
vivifies, and eventually, as it ripens, gives off its own proper acorns,
which again run the same course. But I need not multiply examples,--from
the highest to the lowest the essential features of life are the same,
as I have described in each of these cases.

So much, then, for these particular features of the organic world, which
you can understand and comprehend, so long as you confine yourself to
one sort of living being, and study that only.

But, as you know, horses are not the only living creatures in the world;
and again, horses, like all other animals, have certain limits--are
confined to a certain