172 THE DOCTRINE OF DESCENT. 



these animals are merely modified hairs ; that their con- 

 struction may be explained by their use ; and that in all 

 probability they have been perfected by sexual selec- 

 tion, the best and loudest musicians being the most 

 favoured wooers. With one single exception, the females 

 of the Orthoptera are dumb, but many possess traces of 

 the stridulating apparatus peculiar to the males. Con- 

 trary to the older opinion, that it was merely a case 

 of transmission emanating from the males, Graber has 

 made it " more than probable that the resonant ner- 

 vures of the females — of the stridulating Ephippigera 

 vitium — have been gradually developed independently 

 of the males, but in the same manner." In other cases, 

 on the contrary, the feebly developed nervures of the 

 females, unfit to produce audible stridulations, seem to 

 be an inheritance from the males. 



Heredity at corresponding periods of life is a well- 

 known phenomenon. The tendency to disease is trans- 

 mitted from the father or the mother to the child to 

 break out at the age at which they suffered. Generation 

 after generation, the milk teeth make room for the per- 

 manent teeth at a corresponding time. But all special 

 cases are mere results of the general law of develop- 

 ment, by which in the individual characters appear in 

 the sequence in which they were historically acquired 

 and became susceptible of transmission. Heredity, at a 

 definite age after the period at which we consider actual 

 development to be complete, is after all only a continu- 

 ation of the embryonic development, beginning with 

 fission, germ and ovum, of which the ninth chapter will 

 teach us the signification. In this development of the 

 individual, or ontogenesis, as will be shown below in 



