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