How is this Generation-Cycle to he explained? 155 



climate can give the first impulse towards a separation 

 between the two generations. As for the degree of 

 difference, that is determined by a factor which we 

 cannot make use of with exactness. It is essentially the 

 individual organization of each species, which at one time 

 inclines it to vary, and at another to remain constant. 

 Thus we find that, in spite of the most opposite outward 

 circumstances, almost no difference has taken place in 

 the generation of one species of gall-fly {aptcra — 

 terminalis)) while on the contrary we are struck by the 

 extraordinary differences which have arisen in those 

 of another {renum — crustalis). 



If therefore the two generations resembled each other 

 to begin with, which I believe cannot be doubted, the 

 inquiry which of the two generations is the original, or 

 which to-day most closely resembles it, becomes of the 

 greatest interest. With regard to this there are two 

 important points to be considered : — 



(i) Some species are only propagated parthenogene- 

 tically. (In Chapter II four species are described.) 



(2) No species of oak gall-fly is known to propagate 

 itself exclusively in a sexual manner. They are only 

 known to do so alternately with an agamous generation 

 in a generation-cycle. 



Therefore it seems to me reasonable to infer that the 

 present agamous form is either itself the original form, 

 or if not exactly identical with it, it is at least very 

 nearly related to it. 



There are wider questions still which must remain 

 unanswered. It cannot be doubted after all that has 

 been put forward in these pages that parthenogenesis 



