being
apparently indifferent. Perhaps the most striking of all this series
of observations is that lately made by T.H. Morgan (Morgan, "Proc. Soc.
Exp. Biol. Med." V. 1908, and von Baehr, "Zool. Anz." XXXII. page 507,
1908.), 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 degenerating.
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 grossulariata). The accessory chromosome has been
now observed in most of the great divisions of insects (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.), 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