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 diiTerence 

 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 



^ Richardson's work on strawberries suggests that this plant may come 

 under the Abraxas type 



