512 JENNINGS— HEREDITY IN PROTOZOA. [April 24. 



these differences were not inherited. Large and small specimens of 

 a single pure line produced progeny of the same mean size. 



2. The next question then was : What are the causes and the 

 nature of the variations in size among the different individuals 

 of a culture of Paramecium F Even in a pure line the indi- 

 viduals differ greatly. The " polygon of variation " of a given cul- 

 ture was looked upon as a mass of problems for analysis. What 

 determines the position which any given individual holds in such a 

 polygon, or in a correlation table? And why do different lots of 

 Paramccia difl'er in mean dimensions; in the amount of variability; 

 in proportions, and in the correlation between length and breadth ? 



The analysis of the factors in variation led to a detailed study 

 of (i) growth, (2) the effect of the environment; (3) inherited 

 differences in size. To these three matters the three main divisions 

 of the paper are devoted. To one or the other of these three cate- 

 gories most of the variations in size were found to belong. A 

 fourth category, consisting of variations connected with conjugation, 

 is reserved for consideration in a later paper. 



3. A large share of the differences in size to be observed in a 

 given culture are differences in growth. In study of variation in 

 protozoa it is as necessary to take growth into consideration as it is 

 in the study of higher animals; the part played by it is fully as great 

 in the protozoa as elsewhere. The paper gives a detailed study of 

 growth, based on the measurements of 1,500 specimens of various 

 known ages, in comparison with large numbers of " random sam- 

 ples." In this way a curve of growth was plotted (Diagram 5, page 

 449) ; this curve resembles essentially the curves of growth of higher 

 animals, as the rat, or man. In different parts of this curve of 

 growth individuals show different lengths, different breadths, and, of 

 course, different proportions of breadth to length. A flourishing 

 culture contains individuals in all stages of growth; so that this 

 affects largely the mean dimensions, the observed variations, and 

 the correlations between length and breadth. The precise effects of 

 growth on each of these matters are dealt with in detail in the 

 paper ; they will be summarized in later paragraphs. A summarized 

 account of growth and its effects is found in the body of the paper, 

 pages 447 to 458; the constants for dimensions and variation in dif- 



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