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THE AMERICAN NATURALIST [Vol. LI 



the normal process of heredity in evolution as exhibited 

 by the bones of defunct mammals.^ It has been said that 

 the hypothesis rests on discontinuous variation of char- 

 acters, which does not exist. It is objected that the 

 hypothesis assumes that genetic factors are fixed and 

 stable in the same sense that atoms are stable, and that 

 even a slight familiarity with living things shows that no 

 such hard and fast lines exist in the organic world. These 

 and other things have been said about the attempts that 

 the students of Mendel's law have made to work out their 

 problems. 



I think, however, that while a few of these charges may 

 appear to be serious, some of them rest on a misunder- 

 standing of what numerical treatment of any problem in 

 science means, and others are due to differences of def- 

 inition. But the most common misunderstanding arises, 

 I venture to think, from a confusion of the problem con- 

 cerned with the sorting out of the hereditary materials 

 (the genes) to the eggs and sperms, with the problems 

 concerning the subsequent action of these genes in the 

 development of the embryo. 



What genes stand for can be most easily shown by 

 means of a few familiar illustrations. Mendel's cross 

 with yellow and green peas (or any similar case in which 

 two characters are contrasted with each other as a pair) 

 will serve as an example. In the second generation from 

 such a cross the numerical results, viz., three yellow to 

 one green, find their explanation on the assumption that 

 the two original germ plasms (briefly the yellow and the 

 green) or some element or elements in them separate 

 cleanly in the germ cells of the hybrid of the first gen- 

 eration. This cross does not tell us whether the two 

 germ plasms separate as wholes — one from the other— 

 or whether only some part or parts behave in this way. 



But the situation changes when two oi- more pairs of 

 contrasted characters are invoKcd in tlic same cross. 



iThis objection is not further coiisi,!,.,, ,! l,r-,v it has been dealt 



with elsewhere ("A Critigue of the Thooiy of Kvolutiou, ' ' 1916, p. 84). 



