336 
SEXUAL SELECTION. 
Part II. 
minute shields or cell-like bodies, which exhibit beau- 
tiful changing colours ; these being absent in the 
females, and in the case of one species in both sexes . 11 
It would, however, be extremely rash to conclude that 
these curious organs serve merely to attract the females. 
In the female of a Brazilian species of Gelasimus, the 
whole body, as I am informed by Fritz Muller, is of a 
nearly uniform greyish-brown. In the male the posterior 
part of the cephalo-thorax is pure white, with the 
anterior part of a rich green, shading into dark brown ; 
and it is remarkable that these colours are liable to 
change in the course of a few minutes — the white 
becoming dirty grey or even black, the green “ losing 
much of its brilliancy.” The males apparently are 
much more numerous than the females. It deserves 
especial notice that they do not acquire their bright 
colours until they become mature. They differ also 
from the females in the larger size of their chelm. 
In some species of the genus, probably in all, the 
sexes pair and inhabit the same burrow. They are 
also, as we have seen, highly intelligent animals. 
From these various considerations it seems highly 
probable that the male in this species has become 
gaily ornamented in order to attract or excite the 
female. 
It has just been stated that the male Gelasimus does 
not acquire his conspicuous colours until mature and 
nearly ready to breed. This seems the general rule in 
the whole class with the many remarkable differences 
in structure between the two sexes. We shall here- 
after find the same law prevailing throughout the great 
sub-kingdom of the Yertebrata, and in all cases it is 
eminently distinctive of characters which have been 
11 Clans, 4 Lie freilebenden Copepoden,’ 1863, s. 35. 
