(JOG 



THE AMERICAN NATURALIST [Vol. XL VII 



zygous strains can be quite different from each other in 

 actual inherent vigor. . . . Thus we see the true explana- 

 tion of the apparent degeneration that so many observers 

 have attributed to inbreeding per se" (p. 37). 



As has been said the experimental data of East and 

 Hayes are derived solely from the results of self-ferti- 

 lizing plants. Self-fertilization is in one sense the closest 

 of all possible forms of inbreeding (since q„ +x = l), but it 

 involves at least one difference in principle from the 

 closest inbreeding which it is possible to accomplish in 

 obligate bisexual forms. This difference is in the fact 

 that while, on the one hand, if a population is subjected to 

 self-fertilization generation after generation the propor- 

 tionate number of pure homozygotes in the population 

 automatically increases, 14 there is, on the other hand, 

 absolutely no such automatic increase in the proportion 

 of homozygotes necessarily following any other sort of 

 inbreeding except self-fertilization. 



The proportion of homozygotes can only be increased 

 during continued inbreeding other than by self-fertiliza- 

 tion, if there is at the same time a continued selection 

 (associative mating) of gametically like individuals. 



While this point seems to have been quite generally 

 overlooked the proof of these above statements is very 

 simple, and anyone can work it out for himself. It follows, 

 indeed, directly from Pearson's 15 demonstration that the 

 individuals of the segregating generation, if they breed 

 at random inter se, will " continue to reproduce them- 

 selves in the same proportion as a stable population." 16 

 Pearson, at the conclusion of his analytical proof, says: 

 "It is thus clear that the apparent want of stability in a 

 Mendelian population, the continued segregation and 

 ultimate disappearance of the heterozygotes, is solely a 



very clear and detailed maMertiy Joinings ^(Amek. Nat., Vol. XLVI, pp. 

 487-491, 1912). 



"Pearson, K., Phil. Trans. Soy. Soc. (A), Vol. 203, pp. 59 and 60, 1904. 



M G. H. Hardy (Science, X. 8., Vol. XXVIIT, pp. 49-50, 1908) has given 

 a proof of this same point. Cf. also Spillman (ibid., Vol. XXVIII, pp. 

 252-254, 1908). 



