which is also common to all of them; that is to say, the Lobster, the
Spider, and the Centipede, have a common plan running through their
whole arrangement, in just the same way that the Horse, the Dog, and the
Porpoise assimilate to each other.

Yet other creatures--whelks, cuttlefishes, oysters, snails, and all
their tribe ("Mollusca")--resemble one another in the same way, but
differ from both "Vertebrata" and "Annulosa"; and the like is true of
the animals called "Coelenterata" (Polypes) and "Protozoa" (animalcules
and sponges).

Now, by pursuing this sort of comparison, naturalists have arrived at
the conviction that there are,--some think five, and some seven,--but
certainly not more than the latter number--and perhaps it is simpler to
assume five--distinct plans or constructions in the whole of the animal
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