172 THE DOCTRINE OF DESCENT. 



probability they have been perfected by sexual selec- 

 tion, the best and loudest musicians being the most 

 favoured wooers. With one single exception, the fe- 

 males of the Orthoptera are dumb, but many possess 

 traces of the stridulating apparatus peculiar to the males. 

 Contrary 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 nervures 

 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 in- 

 heritance 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 

 more detail, processes are frequently abridged or totallv 

 omitted which once, while they were being acquired and 



