Races of Paramecium. 523 



Furthermore, it is easy to find specimens of either group that do 

 not show the forms described as characteristic for either (see 

 fig. 20, 22) ; it is indeed perhaps hardly proper to try to discuss in 

 any precise way the forms of these animals without taking into 

 consideration the environmental conditions, since these change 

 the form enormously. After working with the animals continu- 

 ously for some years and drawing great numbers with the camera, 

 I find that I carry away the impression, however, that there is a 

 real difference between the two groups; that in the caudatum 

 races the posterior half of the body is as a rule relatively more 

 long and slender than in aurelia, so that the total breadth of the 

 animals is a little less in proportion to the length, and that the 

 posterior tip in caudatum is more commonly sharply pointed than 

 in aurelia. An idea of what is meant will be gained by comparing 

 fig. 15, / (caudatum) with fig. 15, a (aurelia). But it would be 

 easy to point out cases in which these differences do not hold, as a 

 glance at the figures accompanying this paper will show. The 

 differences if they are real are therefore of what might be called 

 a statistical character; they would come forth only on comparing 

 large numbers of individuals. In a former paper (.Jennings, 

 '08), it was shown that there is such a statistical difference in 

 regard to one of the points mentioned; from measurements of 

 very large numbers of indi vidua' s under all sorts of environments 

 it was shown that the aurelia races average a little broader in 

 porportion to the length than do the caudatum races (p. 501). 

 But individuals of any race of either group may under the in- 

 fluence of varied environments take on forms either much more 

 slender than that characteristic of caudatum (as in fig. 15, j), or 

 much plumper than that characteristic of aurelia (as in fig. 22). 



To further test these points, careful measurements were made 

 of representatives of the two groups that had been living for a 

 month together in the same culture. On October 8, 1908, a mixed 

 culture was made containing the race Lo (caudatum) and k 

 (aurelia.) On November 6, samples from this culture were killed, 

 and twenty-five representatives of each race were subjected to 

 very careful measurements, in order to determine (1) whether the 

 relative breadth differs even when the animals are living in the 



