Sex as an Adaptation 425 
females and the unfertilized eggs males. The same relation 
is probably true also in the case of ants and of wasps. 
In the saw-flies, the conditions are very remarkable. Sharp 
gives the following account of some of these forms:!— “It 
is a rule in this family that males are very much less nu- 
merous than females, and there are some species in which no 
males have been discovered. This would not be of itself 
evidence of the occurrence of parthenogenesis, but this has 
been placed beyond doubt by taking females bred in confine- 
ment, obtaining unfertilized eggs from them, and rearing the 
larvee produced from the eggs. This has been done by nu- 
merous observers with curious results. In many cases the 
parthenogenetic progeny, or a portion of it, dies without 
attaining full maturity. This may or may not be due to con- 
stitutional weakness, arising from the parthenogenetic state. 
Cameron, who has made extensive observations on this subject, 
thinks that the parthenogenesis does involve constitutional 
weakness, fewer of the parthenogenetic young reaching 
maturity. This, he suggests, may be compensated for — 
when the parthenogenetic progeny is all of the female sex — 
by the fact that all those that grow up are producers of eggs. 
In many cases the parthenogenetic young of Tenthredinidz 
are of the male sex, and sometimes the abnormal progeny is 
of both sexes. In the case of one species —the common 
‘currant-fly, Mematus ribesii—the parthenogenetic progeny 
is nearly, but not quite always, entirely of the male sex; this 
has been ascertained again and again, and it is impossible to 
suggest in these cases any advantage to the species to com- 
pensate for constitutional parthenogenetic weakness. On the 
whole, it appears most probable that the parthenogenesis, and 
the special sex produced by it, whether male or female, are 
due to physiological conditions of which we know little, and 
that the species continues in spite of the parthenogenesis 
rather than profits by it. It is worthy of remark that one of 
1 “The Cambridge Natural History,’ Vol. V, “Insects,” by David Sharp. 
