The Descent of Man

certainly suffice to hold the female,—for both
are now used for this purpose,—the two male
forms probably originated by some having varied
in one manner and some in another; both
forms having derived certain special, but nearly
equal advantages, from their differently shaped
organs.
It is not known that male crustaceans fight together
for the possession of the females, but it
is probably the case; for with most animals
when the male is larger than the female, he
seems to owe his greater size to his ancestors
having fought with other males during many
generations. In most of the orders, especially in
the highest or the Brachyura, the male is larger
than the female; the parasitic genera, however,
in which the sexes follow different habits of life,
and most of the Entomostraca must be excepted.
The chelae of many crustaceans are weapons
well adapted for fighting. Thus when a
Devil-crab (Portunus puber) was seen by a son