268 THE APODID^ part ii 



In the first case, if this arrangement proved of any 

 real advantage to the young, it would certainly bring 

 about such modification in the parent animal as would 

 lead to the formation of new species, differing from 

 those which did not so shelter their young. This 

 may seem a small point around which to mould a 

 new species, but not if we give to the reproductive 

 function its true value in the economy of life. Every 

 other function is in fact subordinate to it, and it is 

 therefore capable of modifying every part of the body 

 in order to ensure its own efficiency. Hence, given a 

 certain number of Apodidae which have inherited a 

 tendency to drop their eggs under the dorsal shell, 

 because in this way a greater number are able to 

 develop and survive in the struggle for existence, these 

 animals would, in course of time, be modified so as 

 to perfect this arrangement. The shield would grow 

 further down at the sides so as to press more closely 

 against the body, and the hinder part of the body 

 would come into closer contact with the hinder edge 

 of the shield, both alterations serving to prevent 

 the eggs or embryos from slipping out from under 

 their cover. It is also probable that processes of 

 the terga might grow up so as to close the 

 posterior opening (see Fig. 60). 



On the other hand, again, these very alterations, 

 which make the falling out of the eggs more difficult, 

 at the same time make the falling in of the eggs 

 more difficult ; hence the gradual movement of the 

 genital aperture up the sides under the shell so as to 

 ensure the egg finding its way into the cavity under 



