154 How is this Generation-Cycle to he explained? 



differentiated into the two sexes, and that these have 

 been inherited from the sexual generation. But with 

 regard to sexual generations which only produce 

 females, we may assume that every ^^^ without 

 exception has been fertilized, and that such eggs, as in 

 the case of bees, invariably produce females. 



If we look at these experiments aright, we perceive 

 that parthenogenesis has been evolved in different ways, 

 according to the requirements of each individual case. 

 It is essential therefore to inquire how each species 

 has developed its own mode of reproduction. 



I had started on my inquiry into parthenogenesis 

 with the theory that it was on exactly the same level as 

 sexual propagation, and that there could be no criteria 

 by which one generation could be subordinated to the 

 other. But another, and a very important fact, shows 

 that the two generations of gall-flies are co-ordinate the 

 one with the other. 



When we try to explain the occurrence, at the present 

 time, of two generations so utterly different from each 

 other as those we find in the oak gall-flies, we are 

 obliged to admit unconditionally, that originally this 

 difference did not exist, but that both generations were 

 similar. For it is a definite law that the offspring 

 inherits, with the greatest constancy, the organization 

 and physical form of its parents. If differences arise 

 between two originally identical generations, we must 

 refer them to an alteration in the external conditions of 

 life. Of these we place first change of climate, for we 

 know from Weismann's experiments on the seasonal 

 dimorphism of certain butterflies, that differences of 



