328 
SEXUAL SELECTION. 
Part IL 
Sub-Mngdom of the Arthrojooda : Class, Crustacea — 
In this great class we first meet with undoubted se- 
condary sexual characters, often developed in a remark- 
able manner. Unfortunately the habits of crustaceans 
are very imperfectly known, and we cannot explain the 
uses of many structures peculiar to one sex. With 
the lower parasitic species the males are of small size, 
and they alone are furnished with perfect swimming- 
legs, antennae and sense-organs ; the females being 
destitute of these organs, with their bodies often consist- 
ing of a mere distorted mass. But these extraordinary 
differences between the two sexes are no doubt related 
to their widely different habits of life, and consequently 
do not concern us. In various crustaceans, belonging 
to distinct families, the anterior antennae are furnished 
with peculiar thread-like bodies, which are believed to 
act as smelling-organs, and these are much more nume- 
rous in the males than in the females. As the males, 
without any unusual development of their olfactory 
organs, would almost certainly be able sooner or later 
to find the females, the increased number of the smell- 
ing- threads has probably been acquired through sexual 
selection, by the better provided males having been the 
most successful in finding partners and in leaving off- 
spring. Fritz Muller has described a remarkable dimor- 
phic species of Tanais, in which the male is represented 
by two distinct forms, never graduating into each other. 
In the one form the male is furnished with more 
numerous smelling-threads, and in the other form with 
more powerful and more elongated chelae or pincers 
which serve to hold the female. Fritz Muller suggests 
that these differences between the two male forms of the 
same species must have originated in certain individuals 
having varied in the number of the smelling-threads, 
whilst other individuals varied in the shape and size of 
