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THE AMERICAN NATURALIST [Vol. XLIV 



on this view; third, the mate of the accessory when it 

 exists, if it is not an X but a Y element, is ignored ; fourth, 

 in Acholla the Y element is larger than the combined X 

 elements all taken together ; fifth, it may seem to reduce 

 the male to a less highly developed form than the female 

 in the sense that it lacks a quantitative factor and leaves 

 unexplained the characters peculiar to the male. 



(B) If the three X's are not identical, but consist of 

 two female and one male element, and if they are undi- 

 rected on the spindle, the results can be explained only 

 by assuming selective fertilization. The assumption 

 meets with a flat contradiction in that it must assume 

 +hat the only X in the male is a female X. "Were it as- 

 sumed to be a male X, the scheme will not work out. 

 Moreover, there is no evidence for selective fertilization. 



(C) If the three X's are not identical, but male and 

 female, and are directed on the maturation spindles, 

 equality of males and females will result only on the as- 

 sumption of selective fertilization. Three assumptions, 

 all unknown, are necessary to work out this scheme. 



Let us turn our attention to the class with three X's 

 and one Y. I take the simplest case for analysis in 

 which all three X's are assumed to be alike. The sperm, 

 bearing the X, fertilizing any egg (for all eggs have an 

 X), produces a female, the sperm bearing the Y fertiliz- 

 ing any egg produces a male. "We know this, in fact, to 

 happen whenever we can identify Y. This scheme meets 

 with no difficulties on its own account, and appeals (with 

 certain modifications) more directly to my mind than 

 any other; but it meets with a difficulty when no Y is 

 present ; and also when Y is the same size as X, if this 

 size relation identifies Y with X. Unless these points 

 can be met the hypothesis is insufficient to meet the situa- 

 tion. I shall return to these difficulties in a moment and 

 try to meet them. This is the extreme application of the 

 particulate theory, and has the advantages and disad- 

 vantages of its kind. 



On the other hand, there is no need to assume that X 

 is the sex chromosome in the sense of carrying sex. The 



