INSECTS 257 



of variation supported the conclusion that the horns of 

 Dynastides had been acquired as ornaments, because, I 

 suppose, in that case the imperfect development would be of 

 little importance. But whether the structures are ornaments 

 or weapons, if only the best developed are successful, their 

 perfection ought to be transmitted to their offspring, so that 

 on this reasoning either sexual selection does not occur, or 

 it fails of its object. 



Now it seems to me that the views I have put forward 

 in the present work are capable of explaining, at least to a 

 certain extent, the extraordinary variability of the sexual 

 attributes of these Coleoptera. The chief principle of my 

 theory is that sexual characters have been developed by 

 stimulations associated with the functional activity of the 

 male sexual organs, and with the nervous excitement corre- 

 sponding to that functional activity. The sexual instinct 

 gives rise to certain habits of combat or courtship, these 

 habits cause special mechanical irritations of particular parts 

 of the body, and the consequent irritation and hypertrophy 

 take place during the time when the activity of all the organs 

 and tissues is intensified by sexual excitement. Heredity, 

 following the course of the original development, causes the 

 structural modifications only to develop in the individual 

 when the male organs are active and the sexual instincts are 

 excited, or when these conditions are approaching. Now in 

 insects we have evidence that the development of the sexual 

 organs depends largely on the nutrition of the larva. The 

 longer the larval state the more room there is for varia- 

 tions in the nutrition of the larva. Again, when the 

 food of the larva is relatively innutritious, as it is in the case 

 of the Lucanidse and Dynastides which live on decaying 

 vegetable matter, especially rotten wood, there is a still 

 greater probability that some of the larvae will be imperfectly 

 fed. It is therefore possible that some of the male individuals 



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