world; and that the hundreds of thousands of species of creatures on
the surface of the earth, are all reducible to those five, or, at most,
seven, plans of organization.

But can we go no further than that? When one has got so far, one is
tempted to go on a step and inquire whether we cannot go back yet
further and bring down the whole to modifications of one primordial
unit. The anatomist cannot do this; but if he call to his aid the
study of development, he can do it. For we shall find that, distinct
as those plans are, whether it be a porpoise or man, or lobster, or any
of those other kinds I have mentioned, every one begins its existence
with one and the same primitive form,--that of the egg, consisting, as
we have seen, of a nitrogenous substance, having a small particle or
nucleus in the centre of it. Furthermore, the earlier changes of each
are substantially the same. And it is in this that lies that true
"unity of organization" of the animal kingdom which has been guessed at
and fancied for many years; but which it has been left to the present
time to be demonstrated by the careful study of development. But is it
possible to go another step further still, and to show that in the same
way the whole of the organic world is reducible to one primitive
condition of form? Is there among the plants the same primitive form
of organization, and is that identical with that of the animal
kingdom? The reply to that question, too, is not uncertain or
doubtful. It is now proved that every plant begins its existence under
the same form; that is to say, in that of a cell--a particle of
nitrogenous matter having substantially the same conditions. So that
if you trace back the oak to its first germ, or a man, or a horse, or
lobster, or oyster, or any other animal you choose to name, you shall
find each and all of these commencing their existence in forms
essentially similar to each other: and, furthermore, that the first