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: 1 — "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 

 larvae 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 Tenthredinidae 

 are of the male sex, and sometimes the abnormal progeny is 

 of both sexes. In the case of one species — the common 

 currant-fly, Nematus 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. 



