ost,
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 processes of growth, and many of the
subsequent modifications, are essentially the same in principle in
almost all.

In conclusion, let me, in a few words, recapitulate the positions which
I have laid down. And you must understand that I have not been talking
mere theory; I have been speaking of matters which are as plainly
demonstrable as the commonest propositions of Euclid--of facts that must
form the basis of all speculations and beliefs in Biological science.
We have gradually traced down all organic forms, or, in other words, we
have analyzed the present condition of animated nature, until we found
that each species took its origin in a