174 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Dreyfus, on the other hand, takes a different view, and points 

 out with much probabihty that there are two parallel series of 

 life-cycles, a feature occurring perhaps with the majority of 

 insects with a cyclical life-history, and one which will serve to 

 explain many of the anomalies found, for example, in the 

 Phylloxera. By these " parallel series " he means that all the 

 members of one and the same generation do not migrate similarly, 

 but have somewhat different habits and offspring, and that thus 

 two different lines of descent are formed which converge later to 

 the same point, the generation from which the next cycle starts. 

 Thus, according to him, the whole of the winged forms of C.Abietis 

 do not migrate from the Spruce, but a certain percentage remain 

 to lay their eggs on it, and thus a series of generations is esta- 

 blished which live exclusively on the Spruce. In support of this 

 he mentions collections of young Spruces crippled with galls, in 

 whose neighbourhood there is only one solitary Larch some way 

 off, which is not very thickly infested with C. Laricis, He has also 

 never seen a specimen of C. Abietis fly away from these infected 

 trees, and he even thmks that C. Laricis remaining on Larches 

 may possibly achieve a bisexual generation. Low points out that in 

 Lapland, where the Larch does not grow, Chermes certainly exists 

 — perhaps only another species (C. strobilobius) — and its migra- 

 tion must therefore take place on to the Pine. Such migration 

 Cholodovsky claims to have observed. 



The necessity, therefore, of the conjunction of the two trees 

 for the support of these insects cannot be taken as proved, and 

 the questions proposed above have still to be answered. 



Here in Great Britain the fact of the two trees being intro- 

 duced, and not indigenous, has caused an irregular distribution of 

 them, which affords many opportunities for observations on this 

 subject. May we hope that they will be seized ? 



It does not follow because Spruce is not protected by isolation 

 from Larch that the converse is not true, because the Spruce is 

 undoubtedly the primary host of these insects, and there is less 

 probability of their being able to do entirely without it ; and in 

 any case young trees in a nursery must be infected from outside, 

 and preferably from a tree of the opposite species. It would 

 therefore answer to try the experiment of growing Larches and 

 Spruces in separate nurseries, away from communication with 

 each other or from older trees of the opposite species, and it 



