84 



THE AMEBIC AN NATURALIST [Vol. XLV 



cultures of this genotype. A third lives along in a sickly 

 way, barely maintaining its existence. 



Thus we get in our laboratory striking cases of nat 

 ural selection between genotypes. To recall our com 

 parison with human beings, if we could mix an entire 

 community composed homogeneously of, let us say, 

 Koosevelts, with another of copies of your ash man — 

 which would be likely to survive? If we place together 

 in the same culture two genotypes of Paramecium, as I 

 have many times done, almost invariably one nourishes 

 while the other dies out. This ruins many a carefully 

 planned experiment ; it must take place on a tremendous 

 scale in nature. 



What distinguishes the different genotypes then is, 

 mainly, a different method of responding to the environ- 

 ment. And this is a type of what heredity is; an organ- 

 ism's heredity is its method of responding to the envir- 

 onmental conditions. Under a given environment the 

 genotype A is large, while the genotype B is small. 

 Under a given environment the strain C conjugates, 

 while D does not. Under a given environment the strain 

 E divides rapidly, F slowly or not at all. The various 

 strains thus differ hereditarily in these respects, and we 

 may say that the differences are matters of heredity. 

 And yet we can get these same contrasts within any 

 genotype (as our diagram illustrates), by varying the 

 environment. The genotype A under one environment 

 is large ; under another it is small. Under one environ- 

 ment the type C conjugates ; under another it does not. 

 Under one environment E divides rapidly ; under another, 

 slowly. Are then size, conjugation and rate of fission 

 after all determined by heredity or by environment? 

 Such a question, when thus put in general terms, is 

 everywhere an idle and unanswerable one. All environ- 

 mental effects are matters of heredity when we compare 

 types differing in their reaction to the environment; all 

 hereditary characters are matters of environmental ac- 

 tion when we compare individuals of the same heredity 

 under effectively different environmental conditions. 



