136 Messrs. Prout and Bacot. On Cross-breeding of [Jan. 8, 



form by Mr. J. E. Gardner, of Clapton, N.E. The former race is distin- 

 guished by its white or cream-coloured ground, almost devoid of grey 

 dusting, and scarcely variable except in the intensity of the transverse black 

 lines, which may be strong (the tendency in the particular strain with which 

 we worked), or broken up into dots, or obsolescent ; the latter is variable 

 within limits, but always with the pale (not white) ground-colour profusely 

 dusted with dark grey atoms, whether these be uniform throughout or more 

 concentrated in certain areas. The Hyeres form is therefore described in our 

 experiments as " light " (L), the London form as " dark " (D). It is necessary 

 to add that more or less intermediate phases of variation occur in some 

 parts of Germany, Italy, etc., so that we have been dealing with local races 

 rather than fixed recurrent " aberrations " or with incipient species. 



The first cross-pairing was obtained on June 21, 1906, and filial generations 

 I-X (Fx to F ]( ) on the well-known Bateson method) appeared from August, 

 1906, to November, 1908. Altogether, 5531 specimens have been analysed 

 in preparing these notes, so that they may be regarded as fairly compre- 

 hensive as an indication of the behaviour of this particular cross-pairing. 



B. Statistical Eesults. 



General Remarks. — Altogether our breeding of Acidalia virgularia has been 

 carried out to the tenth filial generation, and 5531 specimens have been 

 subjected to careful analysis, exclusive of a few which have been more 

 indefinitely summarised but which are confirmatory of the general results. 

 We feel that we may, therefore, speak authoritatively on the general course 

 of inheritance in the cross-breeding of these races, and that the imperfection 

 of our statistical analysis is not due to ignorance of the forms with which we 

 are dealing, but to the fact that their hybridisation really gives no segrega- 

 tion capable of analysis by the human eye. It is necessary to dwell somewhat 

 on this point. At first sight it might appear a confession of incompetence to 

 have to state — as we do quite frankly — that our figures are only approxi- 

 mations, and that in many cases a re-count (either by another entomologist or 

 even by ourselves) might easily result in a slight modification of them ; but 

 when it is understood that there is, in the cross-breeds, every conceivable 

 intergrade, it must be manifest that the distinction between " dark " and 

 " intermediate " on the one hand, and " light " and " intermediate " on the 

 other, becomes purely one of degree, and it is absolutely impossible to draw 

 a perfectly consistent line throughout. Having made a special study of the 

 family Geometridce for nearly twenty years, one of us (L. B. P.) can at least 

 claim to have acquired that eye for slight differences in them that will have 



