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 striking 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) chromosome. Of these, only those possessing the 
accessory body become functional spermatozoa, 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 know. 
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. Anz. xxx. p. 507, 
1908. 
2 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 types all gradations can be demonstrated ranging to the condition 
in which the members of the pair are indistinguishable from each other, 
