Rac^s of Paramecium. 549 



same conditions. A less extended examination of a number of 

 other local forms of Daphnia showed these likewise to be perma- 

 nent. By diverse alterations in the environmental conditions 

 it was easy to modify the various races in such a way as to cause 

 their characteristics to overlap, but resemblances so caused were 

 not hereditary, in the sense of being retained when the two races 

 were bred under the same conditions. These results of Wolte- 

 reck with Daphnia are thus quite parallel, with our own for Para- 

 mecium; it is probable that they are typical of what will generally 

 be found in organisms. 



These minutely differing "races," "pure lines" or "genotypes" 

 in lower organisms evidently correspond to "hereditary individ- 

 ual differences" in such an organism as man. Thus every indi- 

 vidual of man probably represents a different "genotype" from 

 every other, save in the case of identical twins, which apparently 

 belong to the same genotype. The application of the genotype 

 concept to man will best be realized by conceiving sexual repro- 

 duction to cease, and each individual to reproduce by budding 

 or fission, as in Hydra or Paramecium. We should then have as 

 many diverse pure lines as there are individuals with diverse hered- 

 itary characters. Thus there is no other organism in which we 

 have so extensive and minute a knowledge of "genotypic" dif- 

 ferentiation as in man. 



9. Origin of the diverse races 



It is to the origin of the diverse races that further investigations 

 will be directed; the present paper does not deal primarily with 

 this point. But it is needful to bring out certain points, particu- 

 larly in regard to some theories that have been proposed on this 

 matter. 



Popoff in his recent series of brilliant papers ('08-'09) has set 

 forth an explanation of how races of different size arise, particu- 

 larly in the Protozoa, though the same process applies less directly 

 also to the Metazoa. Popoff's view is, in a word, that cells of 

 different size arise as a result of inequalities in nuclear division. 

 There is conceived to be a definite proportionality, under given 



JOURNAL or MORPHOLOGY, VOL. 21, NO. 4. 



