86 THE AMERICAN NATURALIST [Vol.XLV 



istics are hereditary (as we heard maintained at Woods 

 Hole some summers ago) ; or, with Brooks, that there is 

 no such thing as heredity. If we always compare two 

 concrete cases, asking to what is due the difference be- 

 tween them, and remembering that a difference in hered- 

 ity means different response to the same environment, 

 we shall avoid these confusions, and shall find the con- 

 cept of heredity most useful. 



Do hereditary differentiations ever arise within our 

 genotypes, so that from one genotype we get two? In 

 other words, do we get from a single type strains that 

 differ in their behavior under the same environment — 

 the differences persisting from generation to generation! 

 This is of course one of the fundamental questions. The 

 genotypes of Paramecium, like those of most other or- 

 ganisms that have been carefully studied, are singularly 

 resistant, remaining quite constant in most respects, so 

 far as has been determined. This is an example of what 

 gives the genotype concept its practical and theoretical 

 importance. This is what is meant by saying that selec- 

 tion and environmental action are usually without in- 

 herited effect within the genotype. To find differentia- 

 tions within the genotypes of Paramecium, we must 

 examine certain characteristics that are most delicately 

 poised in their responses to all sorts of conditions; such 

 is the rate of multiplication. Studying carefully this 

 most sensitive character, we find that differences do arise 

 within the genotype. Under given conditions, certain 

 rare individuals are found that divide more slowly than 

 usual, others more rapidly, and these differences are 

 perpetuated from generation to generation indefinitely. 

 How are these hereditary differentiations produced? 



The origin of these differentiations is in Paramecium 

 as elusive as in most other cases where they have been 

 discovered. Apparently they arise in our organism as a 

 result of conjugation within the genotype. Certainly if 

 after an epidemic of conjugation within the genotype we 

 cultivate many isolated exconjugants, we find a certain 

 small number of strains that differ in their rate of fission 



