426 Evolution and Adaptation 
the species in which parthenogenesis with the production of 
males occurs — Vematus 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 
