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.