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THE AMERICAN NATURALIST [Vol. XLIV 



is no evidence that they Mendelize in groups commen- 

 surate with the number of chromosomes. In two cases, 

 in fact, viz., in Pisum and in Antirrhinum it appears that 

 the number of the characters that have been shown to 

 Mendelize separately is greater than the number of their 

 chromosomes. 



This has seemed a fatal objection to the chromosome 

 view, but it may not be so, as Spillman has argued, so 

 long as it has not yet been shown that all of the domi- 

 nant characters may be present at the same time. But 

 even admitting this possible way of eluding the objec- 

 tion, the other point raised above concerning the ab- 

 sence of groupings of characters in Mendelian inheri- 

 tance seems a fatal objection to the chromosome theory, 

 so long as that theory attempts to locate each character 

 in a special chromosome. We shall have occasion to re- 

 turn to this point later. 



In recent years most workers in Mendelian inheri- 

 tance have adopted a new method of formulating their 

 theory. Characters that Mendelize are no longer allelo- 

 morphic to each other, but each character has for its 

 pair the absence of that character. This is the presence 

 and absence theory. We can apply this hypothesis to 

 the chromosome theory. For examples, let us assume a 

 new variety or race arises by the loss of a character 

 from that chromosome that has heretofore carried it. The 

 chromosome still remains in existence, since it may carry 

 many other characters besides the one that was lost, 

 and it becomes in the hybrid the mate of the one still re- 

 taining that character. If now separation occurs, two 

 classes of germ-cells result, one with and the other with- 

 out the character; and the observed numerical propor- 

 tions follow. There is nothing in this assumption that 

 meets with any greater difficulty on the chromosome 



as an assumption is neither more nor less in accord with 

 the postulated mechanism. 



More recently, still another interpretation has been 



