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THE AMERICAN NATURALIST [Vol. XL VIII 



ing this, the two species have clearcut differences in leaves (ever- 

 green or deciduous, unifoliolate or compound), in resistance to 

 cold (difference in ability to withstand certain degrees of tem- 

 perature) and in numerous fruit characters (presence or absence 

 of pubescence, quality of juice, quantity of seed, size of fruit, 



From the standpoint of modern theories of heredity as regards 

 variation in F x hybrid generations, it matters little whether so- 

 called species intergrade or whether their differences are clear-cut 

 and all variation is intraspecific. In either case, if crosses were 

 made, variation among the F x individuals from a single family 

 might or might not occur. In either case, no violence to modern 

 theories of heredity would result and no new problems would 

 arise. But if two species that differ from each other in part or 

 all of their characters, but breed true intra-specifically (geno- 

 typically homozygous) are crossed, and F 1 variation results, then 

 modern theories of heredity would be compelled to change front 

 and invoke the aid of new hypotheses. Swingle's data, assuming 

 that intraspecific variation in Citrus species occurs, does not 

 present a problem of this kind at all. C. aurantium and C. tri- 

 foliata each possess distinctive characters, but convincing data are 

 not at hand to warrant any belief in the homozygosity of these 

 differential characters or of even those the two species may have 

 in common. The evidence directly, and one might almost say 

 conclusively, opposes such a conclusion. If these species are not 

 homozygous in all of their characters, then one can not affirm, in 

 the light of modern theories, that all the gametes produced by a 

 particular group of individuals called a species are identical in 

 hereditary composition, nor even that the gametes of one indi- 

 vidual of such a species are identical as to hereditary potenti- 

 alities. At the risk of wasting valuable space by repeating what 

 is extremely common knowledge to genetic students, let us assume, 

 for the purpose of argument, that C. aurantium and C. trifoliata 

 are homozygous in all their respective characters except one. In 

 the former, the character A is heterozygous and peculiar to this 

 species. Likewise, in C. trifoliata, B is heterozygous and differ- 

 ential. All the remaining characters of the two species may be 

 symbolized, respectively, by the formulae XX and YY. When 

 XXAabb (C. aurantium) is crossed with YYaaBd (C. trifoliata), 

 the resulting progeny would appear in the approximate propor- 

 tion of 1 XYAaBb : 1 XYAabb : 1 XYaaBb : 1 XYaabb, providing 



