On the Study of Zoology | Page 6

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, 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, 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 sexes, or
various breeds, are called, in technical language, species. The English
lobster is a species, our cray fish is another, our prawn is another. In
other countries, however, there are lobsters, cray fish, and prawns, very
like ours, and yet presenting sufficient differences to deserve
distinction. Naturalists, therefore, express this resemblance and this
diversity by grouping them as distinct species of the same "genus." But
the lobster and the cray fish, though belonging to distinct genera, have
many features in common, and hence are grouped together in an
assemblage which is called a family. More distant resemblances
connect the lobster with the prawn and the crab, which are expressed by
putting all these into the same order. Again, more remote, but still very
definite, resemblances unite the lobster with the woodlouse, the king