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 
