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THE AMERICAN NATURALIST [Vol. XL V 



That this is also true of other plants is derivable from 

 Darwin's own work. 



This decrease in size and vigor is accompanied by the 

 gradual lessening of variability, and when that state is 

 finally reached in which there is no further decrease'in 

 size and vigor, it seems probable that there will be also 

 no further noticeable change in variability. This does 

 not mean, of course, that there will be no variability, for 

 even the most uniform group of plants or animals will 

 of necessity show slight variations produced by differ- 

 ent conditions of life, food supply and so forth. But 

 present evidence does not warrant the belief that such 

 fluctuations affect in the least the fundamental qualities 

 of the genotype. 



In 1908 I suggested a hypothesis to explain the appar- 

 ent deterioration attendant upon self-fertilization, by 

 pointing out that in plants, such as maize, which show 

 superiority as a result of cross-fertilization, this superi- 

 ority is of the same nature as that so generally met with 

 in F, hybrids. I assumed that the vigor in such cases is 

 due to the presence of heterozygous elements in the 

 hybrids, and that the degree of vigor is correlated with 

 the number of characters in respect to which the hybrids 

 are heterozygous. I do not believe that this correlation 

 is perfect, of course, but approximate, as it is readily 

 conceivable that even though the general principle should 

 be correct, heterozygosis in some elements may be with- 

 out effect upon vigor, or even depressing. The presence 

 of unpaired genes, or the presence of unlike or unequal 

 paired genes, was assumed to produce the greater func- 

 tional activity upon which larger size and greater effi- 

 eienry depend. This idea lias been elaborated by Dr. 

 East 6 and shown to agree with his own extensive experi- 



gests that this stimulation due to hybridity may be anal- 

 ogous to that of ionization. 



Mr. A. B. Bruce proposes a slightly different hypothe- 



