16 THE LIFE-STORY OF INSECTS [CH. 



recognition of a life-story, not merely in the indi- 

 vidual but in the race. We cannot doubt that the 

 ancestors of these wingless insects possessed wings, 

 which in the course of time have been lost by the 

 whole species or by the members of the female sex. 

 It is generally assumed that this loss has been 

 gradual, and so in many cases it probably may have 

 been. But there are species of insects in which 

 some "generations are winged and others wingless; 

 a winged mother gives birth to wingless offspring, 

 and a wingless parent to young with well-developed 

 wings. Such discontinuity in the life-story of ;i 

 single generation forces us to recognise the possi- 

 bility of similar sudden mutations in the course of 

 that age-long process of evolution to which the facts 

 of insect growth, and indeed of all animal develop- 

 ment, bear striking testimony. 



CHAPTER III 



THE LIFE-STORIES OF SOME SUCKING INSECTS 



WE may now turn our attention to some examples 

 of the remarkable alternation of winged and wing- 

 less generations in the yearly life-cycle of the same 

 species, mentioned at the end of the last chapter. 



