ADAPTATIONS IN ANIMALS AND PLANTS 53 



in June. From these individuals are produced the sixth genera- 

 tion. The sexes now come together for the first time and after 

 union lay their eggs on the witch-hazel. These eggs in turn 

 hatch into the stem mother first described. The sixth genera- 

 tion are wingless forms and reach maturity in two or three 

 weeks. Their eggs are laid about the middle or end of June 

 and it is the young from these eggs that hatch about the time 

 the flower buds are developing on the witch-hazel. 



In this cycle of life is exemplified a remarkable case of par- 

 thenogenesis, or the giving forth by birth of individuals from 

 an unimpregnated female. The virgin female here has the 

 latent power to give birth to live larvae, and each generation, 

 from the first to the sixth, has similar power. All the adult 

 females have some easily recognizable differences in structure, 

 and the whole cycle starts over again each alternating year, 

 commencing with the egg. 



Cockerell has advanced the idea of accounting for the evolu- 

 tion of galls on the theory that the secretions of certain earlier 

 mining insects caused a swelling to appear, where the larvae 

 lived, on which excrescence they fed. " It is easy to see that the 

 greater the excrescence, and the greater the tendency of the 

 larvae to feed upon it instead of destroying the vital tissues, 

 the smaller is the amount of harm to the plant. Now the 

 continued life and vitality of the plant is beneficial to the larvae, 

 and the larger or more perfect the gall, the greater the amount 

 of available food. Hence natural selection will have preserved 

 and accumulated the gall-forming tendencies as not only bene- 

 ficial to the larvae, but as a means whereby the larvae can feed 

 with least harm to the plant. So far from being developed 

 for the exclusive benefit of the larvae, it is easy to see that 

 allowing a tendency to gall-formation, natural selection would 

 have developed galls exclusively for the benefit of the plants 

 so that they might suffer a minimum of harm from the unavoid- 

 able attacks of insects." ^ 



The great number and variety of galls agree in presenting 

 a more or less elaborate structure, says Romanes, ^ which is 

 not only foreign to any of the uses of plant life, but singularly 



1 Entomologist, March, 1890. 



2 "Darwin and After Darwin," Part I, p. 293. 



