58 Life and Death, Heredity and Evolution 



Jollos observed also that some strains are more resistant 

 to poisons than others. Hance (1917) found that a cer- 

 tain race of Paramecium caudatum has a tendency to pro- 

 duce one to three extra contractile vacuoles, so that its 

 members may have three to five of these, in place of the two 

 found in most races. Powers and Mitchell (1910) found 

 a race of Paramecium that had several micronuclei, in place 

 of the single one, or the pair, commonly found. 



All together, it is clear that the different races of Par- 

 amecium present the greatest diversities in all sorts of 

 structural, and particularly physiological, characters, so 

 that from our knowledge of the biology of one race we cannot 

 be certain as to that of the others. 



A similar condition of affairs is known to exist in many 

 species of bacteria. Diverse families exist, differing in their 

 nutritive peculiarities, in their resistance to chemical agents, 

 in their virulence as producers of disease; in the way they 

 grow on artificial media, and the like. 



The condition of affairs has been found to exist in all 

 the Protista that have been thoroughly studied from this 

 point of view. It is probable that it will be found in all 

 species ; certainly it would be of much interest to examine 

 thoroughly any species that seems to be uniform, in order 

 to discover if there is such a thing as a species that does 

 not consist of hereditarily diverse stocks. 



The same condition of things is likewise found in higher 

 organisms. It is worth while to recall the name and work 

 of the man who first recognized that the so-called species of 

 animals and plants are really made up of a great number of 

 hereditarily diverse stocks, which remain quite distinct so 

 long as they are reproduced without crossing; for this work 

 was done long ago, and, like the work of Mendel, remained 

 for many years quite without influence on the world. In 



