Advantages of Parthenogefiesis. xix 



form ; or that perfectly formed sexual organs could 

 have been evolved unless the sexual had been the 

 earlier generation. 



The power of producing parthenogenetic eggs is 

 widely distributed among the arthropods, and appears 

 to come into operation whenever it secures the existence 

 of the species more effectually than sexual reproduction. 

 When at one season of the year sexual, and at another 

 agamous, reproduction is the more beneficial, then 

 heterogeny will be found to prevail. In one group 

 amphimixis has been wholly abandoned, but its members 

 are enormously prolific, and their eggs have the power 

 of resting over more than one year. By means of 

 partial parthenogenesis, a much more rapid increase 

 is ensured than could have been possible, in the same 

 time, by sexual reproduction only. Every individual 

 of the winter generation, unhindered by the require- 

 ments of fertilization, is engaged in laying eggs ; the 

 number of the sexual individuals hatched from these 

 eggs is consequently enormously greater than it could 

 have been, had only half the winter generation been 

 of the female sex, and had that half, in order to be 

 fruitful, been dependent on the chances of fecundation. 

 When generations alternate, there are alternating 

 advantages to the species. The winter generations 

 emerge from the gall at a time when pairing is not 

 easy, and it is a distinct gain to the race when every 

 individual has the power of reproducing itself inde- 

 pendently. The summer generations, on the contrary, 

 appear in halcyon days, when there is nothing to mar 

 their nuptial flight, and then the species obtains greater 

 variation, and those physiological advantages of amphi- 

 mixis which parthenogenesis cannot afford. 



But amphimixis is in no way essential to heredity. 



b2 



