Chap. Y. 
LAWS OF YAEIATIOK. 
157 
is less rigid in its action than ordinary selection, as it 
does not entail death, but only gives fewer offspring to 
the less favoured males. Whatever the cause may be 
of the variability of secondary sexual characters, as 
they are highly variable, sexual selection will have had 
a wide scope for action, and may thus readily have suc¬ 
ceeded in giving to the species of the same group a 
greater amount of difference in their sexual characters, 
than in other parts of their structure. 
It is a remarkable fact, that the secondary sexual 
differences between the two sexes of the same species 
are generally displayed in the very same parts of the 
organisation in which the different species of the same 
genus differ from each other. Of this fact I will give 
in illustration two instances, the first which happen to 
stand on my list; and as the differences in these cases 
are of a very unusual nature, the relation can hardly be 
accidental. The same number of joints in the tarsi is a 
character generally common to very large groups of 
beetles, but in the Engidae, as Westwood has remarked, 
the number varies greatly; and the number likewise 
differs in the two sexes of the same species: again in 
fossorial hymenoptera, the manner of neuration of the 
wings is a character of the highest importance, because 
common to large groups; but in certain genera the 
neuration differs in the different species, and likewise 
in the two sexes of the same species. This relation has 
a clear meaning on my view of the subject: I look at 
all the species of the same genus as having as certainly 
descended from the same progenitor, as have the two 
sexes of any one of the species. Consequently, what¬ 
ever part of the structure of the common progenitor, or 
of its early descendants, became variable; variations of 
this part would, it is highly probable, be taken advan¬ 
tage of by natural and sexual selection, in order to fit 
