en 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, 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
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,