2 Roberts, On two caess of Parallelism in tlie Aphides. 



to what may be the true interpretation in these cases of 

 monoecious races, I am not at present prepared to express 

 an opinion ; though one cannot but be impressed by 

 analogous cases such as Pineus (Chermes) pini, which 

 appears to be capable of continuing to exist for an 

 indefinite time on trees of the genus Pinus, without any 

 sexual reproduction, while the race which Cholodkovsky 

 has named Chermes orientalis has a biennial life cycle 

 migrating from the Pinus to Picea orientalis, a spruce of 

 Caucasian origin, as its primary host and there reproducing 

 sexually. 



Before leaving the Chermesince, I may mention certain 

 other cases of parallelism to which Professor Paul Mar- 

 chal (4), of Paris, has drawn attention. These are taken 

 from the progeny of what he calls the " hiemosistens," the 

 wintering form in CnapJialodes and Dreyfusia (two of the 

 genera into which Borner has divided the old genus 

 Chermes). The wintering forms live on the secondary 

 host, a larch or silver fir (A dies), as the case may be 



The hiemosistens develops in the spring and lays eggs 

 which may produce no less that four different forms, 

 three of them at least morphologically distinct: (i.) the 

 sistens, which may not develop before the spring following ; 

 (ii.) the progrediens, another apterous form, which speedily 

 reproduces parthenogenetically ; (iii.) the exsul aiata, a 

 winged form, which remains upon the secondary host and 

 there lays eggs, and (iv.) the sexupara, the winged form, 

 which flies back to the primary host and there lays eggs, 

 which hatch into the only sexual generation of the life 

 cycle. Marchal states definitely that the germ plasm of 

 the progrediens and the exsul alata are different from 

 that of the sistens. 



It will be apparent that this phenomenon may be of 

 considerable interest from the evolutionary standpoint, 



