56 Life and Death, Heredity and Evolution 



one set of conditions, others to other conditions. The exist- 

 ence of these strains presents an enormous diversity within 

 the species ; anything that we learn about one strain cannot 

 be transferred directly to another. A similar condition of 

 affairs has since been found to occur in other Rhizopods. 

 Root, 1 (1918) shows that many diverse stocks exist in 

 Centropyxis aculeata; and Hegner, 2 (1918) shows the same 

 to be true in Arcella dentata. 



In many other Protozoa most of the differences between 

 the stocks are in respect to the characters not readily de- 

 tectible by the eye. In Paramecium caudatum, or Para- 

 mecium aurelia, for example, the visible differences between 

 the strains are mainly in respect to size, and since the size 

 is changed greatly during growth, it requires thorough study 

 to detect the differences of strain. But such study shows 

 that strains of different size do exist (Figure 22). Within 

 each strain there is great variation of size among the dif- 

 ferent individuals, owing to differences of growth and of 

 environment. But each strain or family has its own char- 

 acteristic average size. If we pick out any individual of a 

 given strain and allow it to produce many offspring, we shall 

 find that their average size will correspond to that of the 

 family from which they came; an individual from a larger 

 strain will produce larger progeny ; one from a smaller strain 

 will produce smaller progeny. Thus a "population" of 

 Paramecium as we find it in nature is made up in the way 

 shown in Figure 22. There are many strains, diverse in 

 mean size, but each with many individuals of diverse size. 



In Paramecium, as in Difflugia, these strains differ in other 

 respects also. I found that some multiply rapidly, others 

 slowly; that some conjugate frequently, others rarely. 



'Root, F. M., Genetics, March, 1918. 



' Hegner, R. W., Proceedings of National Academy, September, 1918. 



