256 THE PKINCIPLES OF Part II. 



or walking, if she 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, ceteris 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. W T allace 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 antennae modified in an extra- 

 ordinary manner for the prehension of the female ; 

 h^nce 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. 



W 7 hen the two sexes follow exactly the same habits 



