484 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



can be taken as characteristic of a particular race of Paramecia; 

 certainly not without very precise definition of the conditions. It 

 appears probable that if all conditions of environment, growth, food 

 taken, etc., could be made absolutely the same for individuals derived 

 from the same ancestor, the coefficient of correlation would be close 

 to 1. 000.^^ By varying these conditions any degree of positive cor- 

 relation, down to zero, and many degrees of negative correlation 

 can be attained. 



V. INHERITANCE OF SIZE. 



Having examined the effects of growth and of environment on 

 size and form, we are now prepared to investigate how far these are 

 determined by internal factors, handed on from parent to progeny. 

 Without such a preliminary study of growth and environmental 

 action it would be impossible to investigate successfully the heredity 

 of size and form. 



We have already seen that not all differences in size are due to 

 growth and environment; in the first culture examined (Table I., 

 page 398) there were at least two sets of individuals of characteris- 

 tic different sizes, and these differences in size are lasting. Progeny 

 of the two typical individuals D and c, from these two sets, still 

 retain their characteristic relative sizes after more than a year of 

 culture under all sorts of conditions. 



The dift'erences between these two sets are about the same as 

 those which have been described as distinguishing two species, D 

 corresponding to the accounts of Paramecium caudatum, c to Para- 

 mecium aurelia. The next problem is to determine whether there 

 are still other races of Paramecium, distinguishable on the basis of 

 dift'erences in size, independently of the environment. Can we by 

 selecting individuals of differing sizes isolate races of corresponding 

 sizes? Can we find races of all sorts of sizes intermediate between 

 the largest and smallest adult representatives of such a heterogeneous 

 culture as is shown in Table I. ? 



The clear grouping of the culture of Table I. into two sets seems 

 to indicate that we have present simply two races or species. My 



"" Of course if all variation disappeared, as would perhaps be the case, 

 then the concept of correlation would have no further application. 



