PROTOZOA AND HEREDITY 195 



the upper layers. By degrees, just like men, they be- 

 come regular inhabitants of the slums, and show the 

 effects of this in their appearance. Consequently Jen- 

 nings found that even within a pure line the individuals 

 differed in size, the largest being very much larger than 

 the smallest. Yet if he selected one of the largest and . 

 one of the smallest to start new lines, their progeny 

 varied over the same average under similar conditions. 

 It made no difference whether the ancestor of the new 

 group was large or small, because these differences were 

 not inherited. Similarly, among people, the descend- 

 ants of an ignorant man, who had never been educated, 

 would not necessarily show any inferiority to those of 

 one who had had every advantage. 



2. Nevertheless, when various "wild" Paramecia, of Races of 

 different sizes, were selected to start pure lines, it was 

 found that there were races differing in average size. 

 Jennings isolated eight such races. There were also 

 races differing in various other characters. Each one of 

 these races varied, owing to environmental effects, but 

 the ranges of variation were not the same. . Thus one 

 race might vary from A to D, another from E to E, a 

 third from C to F. Now the smallest member of one 

 race might be much smaller than the largest of the next, 

 yet if the first race averaged largest, its small represen- 

 tative would give rise to animals averaging larger than 

 the progeny of the large member of the other race. 

 Thus the result depends upon the hereditary composi- 

 tion, of the race, and not upon the appearance of the 

 individuals. It is often impossible to determine, on mere Effects of 

 inspection, whether a character is due primarily to heredity JUenf 

 or environment. Of course all characters are actually vironment 

 due to the combination of both factors, but one or other 

 may be responsible for the conspicuous deviation from 



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