igoS.l JENNINGS- HEREDITY IN PROTOZOA. 479 



length likewise increased. This is due to the two facts mentioned 

 in the first sentence of this paragraph. 



Effect of Environment on Variation. — The amount of observed 

 variation, as measured by the coefficient of variation, depends largely 

 on environmental conditions ; this is true both for length and for 

 breadth. In the D line the coefficient of variation for length varies 

 in different cultures from 6.821 to 13.262;'^ for. breadth it varies 

 from 8.896 to 28.879. Ii^ the c line the coefficient varies for length 

 from 7.134 to 15.279; for breadth from 10.700 to 24.291. 



The effects on the coefficient of variation of changes in nutrition 

 vary much in dift'erent cases ; increased nutrition sometimes increases 

 the coefficient, sometimes decreases it, sometimes produces first one 

 effect, then the other. There are evident physiological reasons for 

 the dift'erent eft'ects. In a starving culture the first eft'ect of rich 

 nutrition is to cause many of the individuals to increase in size, 

 while those individuals in which the eft'ects of starvation had gone 

 far do not at first take food and change. Hence there is a great 

 increase in the coefficients of variation ; in changing from row^ 8 to 

 row 9 (Table XVIII.) both coefficients approximately doubled in 

 twenty-four hours. Later, though the animals w^ere kept in the 

 same fluid, the coefficients decreased again — all of the specimens 

 having reached more nearly a condition of equilibrium. If the 

 animals are fairly well fed before the additional nutrition is met, an 

 early effect is to cause rapid multiplication ; the consequent presence 

 of both young and old individuals in the culture increases the coeffi- 

 cients of variation, and particularly that for length. An example 

 of this is seen in the change from row 3 to row 4, Table X\TII. A 

 little later, when the multiplication has ceased, the coefficients of 

 variation become small again. The coefficients of variation are 

 likely to be small in starving cultures, owing to the fact that there is 

 little multiplication and the adults have reached a condition of rela- 

 tive equilibrium. By taking into consideration the immediate and 

 the remote effects of a given agent on growth and multiplication, its 

 effects on the coefficients of variation usually become intelligible. 



°0f course the cultures contain specimens in all stages of growth; as 

 we have previously seen, the coefficient of variation becomes much less when 

 the animals are selected with reference to age. 



