Chap. VIII. Sexual Selection. 209 



that the male uses his strong beak in chiselling the larvae of 

 insects out of decayed wood, whilst the female probes the softer 

 parts with her far longer, much curved and pliant beak : and 

 thus they mutually aid each other. In most cases, differences of 

 structure between the sexes are more or less directly connected 

 with the propagation of the species : thus a female, which has to 

 nourish a multitude of ova, requires more food than the male, 

 and consequently requires special means for procuring it. A male 

 animal, which lives for a very short time, might lose its organs 

 for procuring food through disuse, without detriment ; but he 

 would retain his locomotive organs in a perfect state, so that 

 he might reach the female. The female, on the other hand, 

 might safely lose her organs for flying, swimming, or walking, 

 if she gradually acquired habits which rendered such powers 

 useless. 



We are, however, here concerned only with sexual selection. 

 This depends on the advantage which certain individuals have 

 over others of the same sex and species solely in respect of 

 reproduction. When, as in the cases above mentioned, the two 

 sexes differ in structure in relation to different habits of life, 

 they have no doubt been modified through natural selection, and 

 by inheritaL oe lim ited to one and the same sex. So again the 

 primary sexual organs, and those for nourishing or protecting the 

 young, come under the same influence ; for those individuals which 

 generated or nourished their offspring best, would leave, cceteris 

 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 

 find the female, he requires 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, when adult, have their 

 legs and antennae modified in an extraordinary manner for the 

 prehension of the female ; hence we may suspect that it is be- 

 cause these animals are washed about by the waves of the 

 open sea, that they require these organs in order to propagate 

 their kind, and if so, their development has been the result of 

 ordinary or natural selection. Some animals extremely low in 

 the scale have been modified for this same purpose ; thus the 

 males of certain parasitic worms, when fully grown, have the 

 lower surface of the terminal part of their bodies roughened 



