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THE AMERICAN NATURALIST [Vol.XLVI 



" unit-characters"— that is to say, that they may be inde- 

 pendently combined, disassociated and re-combined in 

 many different ways. The independence of these char- 

 acters often seems to be complete; more rarely it is 

 limited by definite phenomena of " coupling" or "repul- 

 sion." The interesting facts recently brought to light by 

 Bateson and Punnett in case of certain unit-characters 

 in plants, and by Morgan in case of sex-limited char- 

 acters in flies, demonstrate that coupling or repulsion, 

 as exhibited in the F 2 generation, are a consequence of 

 an original association or separation in the grandparental 

 gametes. In the cases referred to, characters that enter 

 the Fj zygote in the same gamete tend to "couple" (re- 

 main in association) in the gametes produced by this gen- 

 eration; while if these same characters enter the F, 

 zygote in different gametes they tend to "repel" each 

 other (remain separate) in the ensuing gamete-forma- 

 tion. This is almost a proof that the factors for coupled 

 characters are borne by a common vehicle or substratum 

 in the germ-cell, while in repulsion they are borne by 

 separate ones. Xot alone such facts, but the whole 

 history of unit-characters points unmistakably to the 

 conclusion that they are in some way connected with 

 material substances or bodies; and that it is the com- 

 binations, disassociations and recombinations of the latter 

 that explain the corresponding behavior of the former. 

 For example, in sex-limited heredity the peculiar linkage 

 of certain unit-characters with sex becomes readily intel- 

 ligible, as several wiiters have recently pointed out, if 

 factors necessary for the production of these characters 

 are associated in the same material body with a factor 

 that plays a certain necessary role in the production of 

 sex. In this particular case, as it happens, we are actu- 

 ally able to see a material body (the "X-chromosome") 

 which undergoes precisely such a mode of distribution 

 with respect to sex and sex-limited characters as is de- 

 manded by the hypothesis. The question must here be 

 squarely faced, in a very real and concrete form, whether 



