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SCIENCE. 



[N. S. Vol,. XIX. No. 472. 



sume its universal validity, and to overlook 

 apparent exceptions. Those who insist upon a 

 critical examination of all evidence against as 

 well as for the theory run the risk of being 

 regarded as lukewarm and as clogs on the 

 wheels of progress ; but they should not be de- 

 terred on that account from a full examination 

 of all the facts in the interest of truth. 



Mendel's Law is an hypothesis of great 

 value both because it fits so many cases of in- 

 heritance and because it is stimulating to ex- 

 perimental investigations which will determine 

 the limits within which it is valid. 



The first limitation of Mendel's Law is 

 ■ stated by De Vries (1902, p. 141), who cer- 

 tainly can not be accused of lukewarmness 

 toward the theory, since he was its redis- 

 coverer. He says in effect : Mendel's Law of 

 dichotomy holds in general only for phylogen- 

 etically recent characters, the so-called racial 

 characters ; and for only a part of those — what 

 part we do not know. Even Mendel recognized 

 that his rule was not generally applicable. 



The second limitation of Mendelism con- 

 cerns his theory of dominance. Sometimes 

 when dissimilar racial characters are crossed 

 one of them is dominant, and sometimes not. 

 For example, gray is (within limits) dominant 

 over white or black in mice, but when black 

 and white or black and yellow are crossed there 

 is no dominance of one color over the other; 

 both are recessive and a reversion to the primi- 

 tive gray occurs. The whole hypothesis of the 

 purity of the germ cells will bear careful 

 scrutiny. The best example of a recessive 

 character in animals is albinism, but even in 

 this case, as Darbishire has pointed out, the 

 recessives can not be perfectly pure and inde- 

 pendent of ancestry, for the mongrel mice 

 coming from a recessive crossed with a gray 

 differ according to the ancestry of the reces- 

 sive. The result of this study is, I think, to 

 add evidence that Mendel did not discover all 

 the important laws of inheritance, and that 

 further investigation will unquestionably re- 

 veaL other and still broader principles of 

 heredity. C. B. Davenport. 



Hull Zoological Laboratory, 

 University op Chicago, 

 December 13, 1903. 



literature cited. 



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Bateson, W., 1903. ' The Present State of ICnowl- 

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Castle, W. E., 1903. ' Mendel's Law of Heredity.' 

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Castle, W. L., and Allen, G. M., 1903. 'The 

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Crampe [H.], 1877. ' Kreuzungen zwischen Wan- 

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