248 THE PRINCIPLES OF [Part II. 



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 ex- 

 clusive 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 inher- 

 itance limited to one and the same sex. So, again, the 

 primary sexual organs, and those for nourishing or pro- 

 tecting the young, come under this same head ; for those 

 individuals which generated or nourished their offspring 

 best, would leave, cceteris paribus, the greatest number to 

 inherit their superiority ; while 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 certain moths cannot unite 

 with the females if their tarsi or feet are broken. The 

 males of many oceanic crustaceans have their legs and 

 antennse modified in an extraordinary manner for the pre- 

 hension 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 

 of life, and the male has more highly-developed sense or 

 locomotive organs than the female, it may be that these 

 in their perfected state are indispensable to the male for 



