94 Heredity and Variation in Modern Lights 



proving that the sperms possessing this accessory body were destined 

 to form females on fertilisation, while sperms without it form males, 

 the eggs being apparently indifferent. Perhaps the most strildng of 

 all this series of observations is that lately made by T. H. Morgan ^ 

 since confirmed by von Baehr, that in a Phylloxeran two kinds of 

 spermatids are formed, respectively with and without an accessory 

 (in this case, double) chromosoma Of these, only those possessing the 

 accessory body become functional spei'matozoa, the others degene- 

 rating, "We have thus an elucidation of the puzzling fact that in 

 these forms fertilisation results in the formation of females only. 

 How the males are formed — for of course males are eventually 

 produced by the parthenogenetic females — we do not knoAv. 



If the accessory body is really to be regarded as bearing the factor 

 for femaleness, then in Mendelian terms female is DD and male is 

 DR.- The eggs are indifferent and the spermatozoa are each male, 

 or female. But according to the evidence derived from a study of 

 the sex-limited descent of certain features in other animals the 

 conclusion seems equally clear that in them female must be regarded 

 as DR and male as RR. The eggs are thus each either male or 

 female and the spermatozoa are indifferent. How this contradictory 

 evidence is to be reconciled we do not yet know. The breeding work 

 concerns fowls, canaries, and the Currant moth {Abraxas grossu- 

 lariata). The accessory chromosome has been now observed in most 

 of the great divisions of insects^ except, as it happens, Lepidoptera. 

 At first sight it seems difficult to suppose that a feature apparently 

 so fundamental as sex should be differently constituted in different 

 animals, but that seems at present the least improbable inference. 

 I mention these two groups of facts as illustrating the nature and 

 methods of modern genetic work. We must proceed by minute and 

 specific analytical investigation. Wherever we look we find traces 

 of the operation of precise and specific rules. 



In the light of present knowledge it is evident that before we can 

 attack the Species-problem with any hope of success there are vast 

 arrears to be made up. He would be a bold man who would now 

 assert that there was no sense in which the term Species might not 

 have a strict and concrete meaning in contradistinction to the term 

 Variety. We have been taught to regard the difference between 

 species and variety as one of degree. I think it unlikely that this 



^ Morgan, Proc. Soc. Exp. Biol. Med. v. 1908, and von Baehr, Zool. Am. xxxii. p. 507) 

 1908. 



^ As Wilson has proved, the unpaired body is not a universal feature even in those 

 orders in which it has been observed. Nearly allied types may differ. In some it is 

 altogether unpaired. In others it is paired with a body of much smaller size, and by 

 selection of various tjpts all gradations can be demonstrated ranging to the condition 

 in which the members of the pair are indistinguishable from each other. 



