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THE AMERICAN NATURALIST [Vol. LII 



The main argument in favor of Weismann's viewpoint 

 does not take long to state. It is this : Mendelian heredity 

 is a manifestation of sexual reproduction. Wherever 

 sexual reproduction occurs, there Mendelian heredity will 

 be found. The very fact that it describes the sexual 

 heredity of both animals and plants is sufficient proof of 

 its generality in this regard. Now if N variations occur 

 in the germplasm of an asexually reproducing organism, 

 only N types can be formed to offer raw material to selec- 

 tive agencies. But if N variations occur in the germ- 

 plasm of a sexually reproducing organism 2" types can be 

 formed. The advantage is almost incalculable. Ten 

 variations in an asexual species mean simply 10 types, 10 

 variations in a sexual species mean the possibility of 1,024 

 types. Twenty variations in the one case is again only 20 

 types to survive or perish in the struggle for existence ; 20 

 variations, in the other case, may present 1,032,576 types 

 to compete in the struggle. It is necessary to hedge the 

 argument by pointing out that these figures are the maxi- 

 mum possibilities in favor of sexual reproduction. It is 

 improbable that they ever actually occur in nature, for 2 20 

 types really to be found in the wild competing for place 

 after only 20 germinal variations would mean an enor- 

 mous number of individuals even if the 20 changes had 

 taken place in different chromosomes, and if the varia- 

 tions were linked at all closely in inheritance the number 

 required would be staggering. But there are breaks in 

 linked inheritance, and the possibility is as stated. 



These advantages remain even though it should be shown 

 later that the more fundamental and generalized char- 

 acters of an organism are not distributed by Mendelian 

 heredity. Loeb (1916) believes that the cytoplasm of the 

 egg is roughly the potential embryo and that the chromo- 

 somes, distributed as required by the breeding facts of 

 Mendelian heredity, are the machinery for impressing the 

 finer details. There is something to be said for this point 

 of view, though at present it is but a working hypothesis. 

 But granting its truth it does not detract from the ad- 

 vantages gained by sexual reproduction. Even the most 



