426 Evolution and Adaptation 



the species in which parthenogenesis with the production of 

 males occurs — Nematus ribesii — is perhaps the most abun- 

 dant of saw-flies." 



It has been pointed out that in a number of species of 

 animals and plants only parthenogenetic females are present 

 at certain times. In a sense this means a preponderance of 

 one sex, but since the eggs are adapted only to this kind of 

 development, it may be claimed that the conditions in such 

 cases are somewhat different from those in which eggs that 

 would be normally fertilized may develop in the absence of 

 fertilization. Nevertheless, it is generally supposed that the 

 actual state of affairs is about the same. It is usually as- 

 sumed, and no doubt with much probability, that these 

 parthenogenetic forms have evolved from a group which 

 originally had both male and female forms. One of the most 

 striking facts in this connection is that in the groups to which 

 these parthenogenetic species belong there are, as a rule, 

 other species with occasional parthenogenesis, and in some of 

 these the males are also fewer in number than the females. 



In the aphids, the parthenogenetic eggs give rise during 

 the summer to parthenogenetic females, but in the autumn 

 the parthenogenetic eggs give rise without fertilization both 

 to males and to females. It appears, therefore, that we can 

 form no general rule as to a relation between fertilization 

 and the determination of sex. While in certain cases, as in 

 the bees, there appears to be a direct connection between 

 these two, in other cases, as in that of the aphids just men- 

 tioned, there is no such relation apparent. 



Geddes and Thompson have advocated a view in regard to 

 sex which at best can only serve as a sort of analogy under 

 which the two forms of sex may be considered, rather than 

 as a legitimate explanation of the phenomenon of sex. They 

 rest their view on the idea that living material is continually 

 breaking down and building up. An animal in which there 

 is an excess of the breaking-down process is a male, and 



