THE AHEBICAN NATURALIST [Vol. XLIII 



the same conditions. We find that many of these differ- 

 ences are inherited; from large individuals we get large 

 races ; from small individuals small ones. We find, then, 

 that Paramecium consists of many races, differing from 

 each other in mean size slightly but constantly. Eight of 

 these different races were isolated and propagated for 

 hundreds of generations ; some were carried through sev- 

 eral complete "life cycles." Each such race consisted 

 of specimens all derived from a single parent individual. 

 Unquestionably many other races exist, that could be 

 isolated by proper means. 



Fig. 3 is a diagram showing the relative mean lengths 

 of the eight races isolated, as determined by measuring 

 at intervals lots of 100 or more individuals of each race. 

 The mean length for any race is constant under given 

 conditions. The differences between adjacent races are 

 very slight ; thus, between the races c and i of the diagram 

 the difference in mean length was but five to seven 

 microns or .00028 inch. For measuring such constant 

 differences between races even the "fourth decimal place 

 of the biometrician," so heavily contemned of late, would 

 seem to be required. This gives us something of a 

 measure of the minuteness of the steps bv which evolu- 



