doctrine of the functions or
actions of animals. It regards animal bodies as machines impelled by
certain forces, and performing an amount of work, which can be
expressed in terms of the ordinary forces of nature. The final object of
physiology is to deduce the facts of morphology, on the one hand, and
those of distribution on the other, from the laws of the molecular
forces of matter.

Such is the scope of zoology. But if I were to content myself with the
enunciation of these dry definitions, I should ill exemplify that method
of teaching this branch of physical science, which it is my chief
business to-night to recommend. Let us turn away then from abstract
definitions. Let us take some concrete living thing, some animal, the
commoner the better, and let us see how the application of common sense
and common logic to the obvious facts it presents, inevitably leads us
into all these branches of zoological science.

I have before me a lobster. When I examine it, what appears to be the
most striking character it presents? Why, I observe that this part which
we call the tail of the lobster, is made up of six distinct hard rings
and a seventh terminal piece. If I separate one of the middle rings, say
the third, I find it carries upon its under surface a pair of limbs or
appendages, each of which consists of a stalk and two terminal pieces.
So that I can represent a transverse section of the ring and its
appendages upon the diagram board in this way.

If I now take the fourth ring I find it has the same structure, and so
have the fifth and the second; so that, in each of these divisions of
the tail, I find parts which correspond with one another, a ring and two
appendages; and in each appendage a stalk and two end pieces. These
corresponding parts are called, in the technical language of anatomy,
"homologous parts." The ring of the third division is the "homologue"
of the ring of the fifth, the appendage of the former is the homologue
of the appendage of the latter. And, as each division exhibits
corresponding parts in corresponding places, we say that all the
divisions are constructed upon the same plan. But now let us consider
the sixth division. It is similar to, and yet different from, the
others. The ring is essentially the same as in the other divisions; but
the appendages look at first as if they were very different; and yet
when we regard them closely, what do we find? A stalk and two terminal
divisions, exactly a