e outermost jaw I find it
consists of three distinct portions, an inner, a middle, and an outer,
mounted upon a common stem; and if I compare this jaw with the legs
behind it, or the jaws in front of it, I find it quite easy to see,
that, in the legs, it is the part of the appendage which corresponds
with the inner division, which becomes modified into what we know
familiarly as the "leg," while the middle division, disappears, and the
outer division is hidden under the carapace. Nor is it more difficult to
discern that, in the appendages of the tail, the middle division appears
again and the outer vanishes; while, on the other hand, in the foremost
jaw, the so-called mandible, the inner division only is left; and, in
the same way, the parts of the feelers and of the eye-stalks can be
identified with those of the legs and jaws.

But whither does all this tend? To the very remarkable conclusion that a
unity of plan, of the same kind as that discoverable in the tail or
abdomen of the lobster, pervades the whole organization of its skeleton,
so that I can return to the diagram representing any one of the rings of
the tail, which I drew upon the board, and by adding a third division to
each appendage, I can use it as a sort of scheme or plan of any ring of
the body. I can give names to all the parts of that figure, and then if
I take any segment of the body of the lobster, I can point out to you
exactly, what modification the general plan has undergone in that
particular segment; what part has remained moveable, and what has become
fixed to another; what has been excessively developed and metamorphosed,
and what has been suppressed.

But I imagine I hear the question, How is all this to be tested? No
doubt it is a pretty and ingenious way of looking at the structure of
any animal, but is it anything more? Does Nature acknowledge, in any
deeper way, this unity of plan we seem to trace?

The objection suggested by these questions is a very valid and important
one, and morphology was in an unsound state, so long as it rested upon
the mere perception of the analogies which obtain between fully formed
parts. The unchecked ingenuity of speculative anatomists proved itself
fully competent to spin any number of contradictory hypotheses out of
the same facts, and endless morphological dreams threatened to supplant
scientific theory.

Happily, however, there is a criterion of morphological truth, and a
sure test of all homologies