grew, most of them became distinguished into a stem and two terminal
divisions, to which, in the middle part of the body, was added a third
outer division; and it was only at a later period, that by the
modification, or absorption, of certain of these primitive constituents,
the limbs acquired their perfect form.

Thus the study of development proves that the doctrine of unity of plan
is not merely a fancy, that it is not merely one way of looking at the
matter, but that it is the expression of deep-seated natural facts. The
legs and jaws of the lobster may not merely be regarded as modifications
of a common type,--in fact and in nature they are so,--the leg and the
jaw of the young animal being, at first, indistinguishable.

These are wonderful truths, the more so because the zoologist finds them
to be of universal application. The investigation of a polype, of a
snail, of a fish, of a horse, or of a man, would have led us, though by a
less easy path, perhaps, to exactly the same point. Unity of plan
everywhere lies hidden under the mask of diversity of structure--the
complex is everywhere evolved out of the simple. Every animal has at
first the form of an egg, and every animal and every organic part, in
reaching its adult state, passes through conditions common to other
animals and other adult parts; and this leads me to another point. I have
hitherto spoken as if the lobster were alone in the world, but, as I need
hardly remind you, there are myriads of other animal organisms. Of these,
some, such as men, horses, birds, fishes, snails, slugs, oysters, corals,
and sponges, are not in the least like the lobster. But other animals,
though they may differ a good deal from the lobster, are yet either very
like it, or are like something that is like it. The cray fish, the rock
lobster, and the prawn, and the shrimp, for example, however different,
are yet so like lobsters, that a child would group them as of the lobster
kind, in contradistinction to snails and slugs; and these last again
would form a kind by themselves, in contradistinction to cows, horses,
and sheep, the cattle kind.

But this spontaneous grouping into "kinds" is the first essay of the
human mind at classification, or the calling by a common name of those
things that are alike, and the arranging them in such a manner as best to
suggest the sum of their likenesses and unlikenesses to other things.

Those kinds which include no other subdivisions than the