256 THE PRINCIPLES OF Part IL 
or walking, if slie gradually acquired habits which ren- 
dered such powers useless. 
We are, however, here concerned only with that kind 
of selection, which I have called sexual selection. This 
depends on the advantage which certain individuals have 
over other individuals of the same sex and species, in 
exclusive relation to reproduction. When the two sexes 
differ in structure in relation to different habits of life, 
as in the cases above mentioned, they have no doubt 
been modified through natural selection, accompanied 
by inheritance limited to one and the same sex. So 
again the primary sexual organs, and those for nourish- 
ing or protecting the young, come under this same head ; 
for those individuals which generated or nourished their 
offspring best, would leave, cseteris paribus , the greatest 
number to inherit their superiority ; whilst those which 
generated or nourished their offspring badly, would leave 
but few to inherit their weaker powers. As the male 
has to search for the female, he requires for this purpose 
organs of sense and locomotion, but if these organs are 
necessary for the other purposes of life, as is generally 
the case, they will have been developed through natural 
selection. When the male has found the female he 
sometimes absolutely requires prehensile organs to hold 
her ; thus Dr. Wallace informs me that the males of cer- 
tain moths cannot unite with the females if their tarsi 
or feet are broken. The males of many oceanic crusta- 
ceans have their legs and antennas modified in an extra- 
ordinary manner for the prehension of the female ; 
hence we may suspect that owing to these animals 
being washed about by the waves of the open sea, they 
absolutely require these organs in order to propagate 
their kind, and if so their development will have been 
the result of ordinary or natural selection. 
When the two sexes follow exactly the same habits 
