282 THE AMERICAN NATURALIST [Vol. LI1 



further, since even though there does not seem to be a 

 sufficient difference between sexual and asexual reproduc- 

 tion in the matter of variation frequency to make it a 

 subject of experimental proof, certain theoretical points 

 raise the suspicion that there is such a difference. ' All we 

 would maintain is that to account for the general persist- 

 ence of sexual reproduction by such a cause, the differ- 

 ence in its favor should be so great that it could easily be 

 determined experimentally. Since this is not true, we 

 believe the hypothesis should be discarded. 



The points of theory referred to are these. It will be 

 allowed by all that there is some considerable evidence of 

 the chromosomes being the most important conservators 

 of hereditary factors— the physical bases of heredity in 

 whatever form they may be. If it is assumed then that 

 changes in constitution in these cell organoids are fol- 

 lowed by changes in type, and that such changes in con- 

 stitution are equally probable in all chromosomes, it 

 follows that parthenogenetic individuals having the hap- 

 loid number of chromosomes should show a larger propor- ' 

 tion of germinal variations than members of the same 

 species having the diploid number of chromosomes, be- 

 cause variations of all kinds should be recognizable in the 

 former case, while in the latter, recessive variations could 

 not be detected until the first or second filial generation, 

 and then only when the proper mating was made. There 

 is some evidence that this reasoning is not wholly improb- 

 able. But variations occur much more frequently in 

 heterozygotes than in homozygotes. To me this simply 

 means that bud-variations are detected more frequently 

 in heterozygotes than in homozygotes : and an interpreta- 

 tion is not hard to find. Retrogressive variations are 

 much more frequent than progressive variations, and a 

 retrogressive variation in a particular character shows 

 only when the organism Is heterozygous for that character. 

 If a retrogressive bud-variation arises in a homozygote 

 and gametes are afterwards developed from the sporting 

 branch it is not at all unlikely that the variation may show 

 in the next generation, but it will be attributed then to 



