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 organisation 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 movable, 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. Our lobster has not always been what we see it;
it was once an egg, a semifluid mass of yolk, not so big as a pin's head,
contained in a transparent membrane, and exhibiting not the least trace
of any one of those organs, the multiplicity and complexity of which, in
the adult, are so surprising. After a time, a delicate patch of cellular
membrane appeared upon one face of this yolk, and that patch was the
foundation of the whole creature, the clay out of which it would be
moulded. Gradually investing the yolk, it became subdivided by transverse
constrictions into segments, the forerunners of the rings of the body.
Upon the ventral surface of each of the rings thus sketched out, a pair
of bud-like prominences made their appearance--the rudiments of the
appendages of the ring. At first, all the appendages were alike, but, as
they