fin--carrying your mind's eye onward
from the flapper of the Porpoise. And here you have the hinder limbs
restored in the shape of these ventral fins. If I were to make a
transverse section of this, I should find just the same organs that
we have before noticed. So that, you see, there comes out this strange
conclusion as the result of our investigations, that the Horse, when
examined and compared with other animals, is found by no means to
stand alone in nature; but that there are an enormous number of other
creatures which have backbones, ribs, and legs, and other parts arranged
in the same general manner, and in all their formation exhibiting the
same broad peculiarities.

I am sure that you cannot have followed me even in this extremely
elementary exposition of the structural relations of animals, without
seeing what I have been driving at all through, which is, to show you
that, step by step, naturalists have come to the idea of a unity of
plan, or conformity of construction, among animals which appeared at
first sight to be extremely dissimilar.

And here you have evidence of such a unity of plan among all the animals
which have backbones, and which we technically call "Vertebrata". But
there are multitudes of other animals, such as crabs, lobsters, spiders,
and so on, which we term "Annulosa". In these I could not point out to
you the parts that correspond with those of the Horse,--the backbone,
for instance,--as they are constructed upon a very different principle,
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 m