FIGURE 4. Buccal region of the same from below; lambda, labrum.

FIGURE 5. Head of the rarer form of the male, magnified 25 times.

FIGURE 6. Flagellum of the same, with olfactory filaments, magnified 90
times.)

In the first place, and before inquiring into its significance, I will
say a word upon this fact itself. It was natural to consider whether two
different species with very similar females and very different males
might not perhaps live together, or whether the males, instead of
occurring in two sharply defined forms, might not be only variable
within very wide limits. I can admit neither of these suppositions. Our
Tanais lives among densely interwoven Confervae, which form a coat of
about an inch in thickness upon stones in the neighbourhood of the
shore. If a handful of this green felt is put into a large glass with
clear sea-water, the walls of the glass are soon seen covered with
hundreds, nay with thousands, of these little, plump, whitish Isopods.
In this way I have examined thousands of them with the simple lens, and
I have also examined many hundreds with the microscope, without finding
any differences among the females, or any intermediate forms between the
two kinds of males.

To the old school this occurrence of two kinds of males will appear to
be merely a matter of curiosity. To those who regard the "plan of
creation" as the "free conception of an Almighty intellect, matured in
the thoughts of the latter before it is manifested in palpable, external
forms," it will appear to be a mere caprice of the Creator, as it is
inexplicable either from the point of view of practical adaptation, or
from the "typical plan of structure." From the side of Darwin's theory,