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, whose multiplicity and
complexity, 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 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