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THE AMERICAN NATURALIST [Vol. XLVIII 



taken under observation at least two were found to be 

 breeding true for all practical purposes in the F 5 and F 6 

 generations. We were able to reproduce the "Havana" 

 type by continued selection in Family 77 and were able 

 to produce strains breeding approximately true to 30 

 leaves or so by the selection of mother plants in several 

 families. But can we say that any of our families are 

 now fixed so that no progress can be made by selection? 

 We can not. But we can say that some of them are so 

 constant that it would be a loss of time for selection to be 

 continued for economic results. It is important to know 

 whether plant or animal populations can reach such a 

 state of constancy by inbreeding that no profitable results 

 can afterwards be obtained by the practical breeder. We 

 believe it demonstrated by even these few data that such 

 a state, a homozygous condition, occurs in a definite pro- 

 portion of F 2 offspring, and can be propagated commer- 

 cially at once if a sufficient number of families are grown 

 to be relatively certain of including the desired com- 

 bination. 



As to the problem of theoretical importance, the ques- 

 tion of the true constancy of homozygotes generation 

 after generation, we believe it to be fair to conclude that 

 a state so constant is reached, that even for the theoret- 

 ical purposes of experimental genetics it may be assumed 

 as actually constant. Further experiment and larger 

 numbers may show that selection can always cause a shift 

 in the mean, but will necessarily be a shift so slight that 

 it can be detected only by a long-continued experiment 

 and enormous numbers. Assuming for the purpose of 

 argument that this is the case, the matter would affect 

 only the question of the trend of evolution. It may come 

 to be believed, from evidence now unknown, that evolu- 

 tion may progress slowly in this manner, but if it does, 

 its course can hardly be demonstrated experimentally be- 

 yond a reasonable doubt. The problems of experimental 

 genetics can be attacked, however, from the standpoint 

 that experimental evidence of the shifting of the mean of 

 a homozygous population by selection is negligible. 



