DEVELOPMENT OF CHALCIDS 885 



Wheeler (15) lias put forward a theory of the origin of 

 parasitism in the Aculeata. He supposes that parasitism arose 

 within the species, when certain individuals acquired the habit 

 of laying their eggs in the brood cells of their neighbours, 

 instead of working for themselves ; and he supports his sugges- 

 tion by the significant fact that the existing parasites are 

 frequently generically allied to the host species. 



But this theory can hardly be extended to the Parasitica, 

 even if we regard them as a heterogeneous group, derived from 

 different ancestral stocks, and classified together in virtue of 

 characters acquired independently by members of different 

 families in adaptation to parasitic life. The existing Parasitica 

 are a vast class, of infinite variety of size, structure, and 

 habit ; and^ with the exception of most of the Cynipids and 

 a few Chalcids, which are gall-formers on plants, all are parasitic 

 upon insects, frequently upon families distantly related to 

 them. 



To suppose that the parasitic habit arose spontaneously 

 in a common ancestor, and was perpetuated by natural selec- 

 tion, involves the assumption of a considerable initial mutation. 

 If, as among the bees and wasps, we found that phylogenetic 

 relationship between host and parasite was the usual rule, we 

 might suppose that parasitism arose within the species in the 

 Parasitica, as Wheeler suggests for the Aculeata ; but there is 

 as much to be said against as for this view, since the modern 

 Parasitica include, not only their own allies, but almost every 

 stage of almost every family of insects among their hosts. 



Nevertheless, parasitism must have had a beginning, and 

 the suggestion may be put forward that the parasitic habit 

 arose among these Hymenoptera from the inquiline habit. In 

 other words, the proto-Parasitica were phytophagous, and 

 oviposited on plants. A further stage was reached, when, for 

 the better protection of the eggs, they resorted to the shelter 

 of galls and other deformities produced by members of their 

 own tribe, and by other insects. Here they became established 

 as commensals or inquilines, and from the inquiline habit to 

 the parasitic habit is possibly not a great step. The Chalcid, 



