antic and in the seas of the southern hemisphere they do not
exist. They are, however, represented in these regions by very closely
allied, but distinct forms--the _Homarus Americanus_ and the _Homarus
Capensis_: so that we may say that the European has one species of
_Homarus_; the American, another; the African, another; and thus the
remarkable facts of geographical distribution begin to dawn upon us.

Again, if we examine the contents of the earth's crust, we shall find in
the latter of those deposits, which have served as the great burying
grounds of past ages, numberless lobster-like animals, but none so
similar to our living lobster as to make zoologists sure that they
belonged even to the same genus. If we go still further back in time,
we discover, in the oldest rocks of all, the remains of animals,
constructed on the same general plan as the lobster, and belonging to
the same great group of _Crustacea_; but for the most part totally
different from the lobster, and indeed from any other living form of
crustacean; and thus we gain a notion of that successive change of the
animal population of the globe, in past ages, which is the most striking
fact revealed by geology.

Consider, now, where our inquiries have led us. We studied our type
morphologically, when we determined its anatomy and its development, and
when comparing it, in these respects, with other animals, we made out
its place in a system of classification. If we were to examine every
animal in a similar manner, we should establish a complete body of
zoological morphology.

Again, we investigated the distribution of our type in space and in
time, and, if the like had been done with every animal, the sciences of
geographical and geological distribution would have attained their
limit.

But you will observe one remarkable circumstance, that, up to this
point, the question of the life of these organisms has not come under
consideration. Morphology and distribution might be studied almost as
well, if animals and plants were a peculiar kind of crystals, and
possessed none of those functions which distinguish living beings so
remarkably. But the facts of morphology and distribution have to be
accounted for, and the science, whose aim it is to account for them, is
Physiology.

Let us return to our lobster once more. If we watched the creature in
its native element, we should see it climbing actively the submerged
rocks, among which it delights to live, b