SEX INHERITANCE 79 
sophila, man, cat; and the plants; Lychnis and Bry- 
onia. The cytological evidence refers to the same 
type the insect groups of bugs, flies, beetles, grass- 
hoppers; the spiders, certain worms (Ascaris), echino- 
derms, amphibia and mammals (including man). 
The genetic evidence has placed in the Abraxas 
type several moths and butterflies, and several birds; 
viz., chickens, ducks, and canaries.! Favorable 
cytological evidence has been found only in the case 
of a few moths. 
In many cases of the Drosophila type, in which 
the history of the sex chromosomes has been worked 
out cytologically, it has been found that in the male 
there is a pair of chromosomes, the two members 
of which are different in size or shape. These are the 
“sex chromosomes” and are designated as X and Y. 
In many species of the Drosophila type the Y is 
slightly smaller than the X, and in the various other 
species of this type all gradations in the relative size 
of the Y are found, between this condition and the 
condition where Y is completely absent. In some 
related species, on the other hand, the chromosomes 
which obviously correspond to X and Y are alike in 
appearance. It is not, after all, the size difference 
usually visible in the male, between X and Y, which 
gives these two chromosomes their significance in sex 
determination, but rather a difference in the factors 
they contain. The size difference is an incidental 
concomitant, or, as it were, a token or label that is 
1 Richardson’s work on strawberries suggests that this plant may come 
under the Abraxas type. : 
