Strange, is it not, that the Porpoise should have in this queer-looking
affair--its flapper (as it is called), the same fundamental elements as
the fore-leg of the Horse or the Dog, or the Ape or Man; and here you
will notice a very curious thing,--the hinder limbs are absent. Now, let
us make another jump. Let us go to the Codfish: here you see is the
forearm, in this large pectoral 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,