458 JENNINGS— HEREDITY IN PROTOZOA. [April 24. 



being prepared to understand the part played by this in the observed 

 variations and correlations, we may pass to other factors affecting 

 these. 



IV. THE EFFECTS OF ENVIRONMENTAL CONDITIONS 

 ON DIMENSIONS, VARIATION AND CORRELATION. 



The data for the study of growth, just concluded, show inci- 

 dentally that environmental conditions affect profoundly the dimen- 

 sions, variation and correlation in Paramecium. As we have seen, 

 samples taken from the same culture on two successive days are not 

 strictly comparable for determining matters relating to growth, 

 because of the environmental changes from day to day, inducing 

 marked changes in the organisms. Thus, in a given culture we found 

 that the mean length at the age of iHo li hours was 161.524 microns ; 

 three days later specimens more than twice as old, from the same 

 culture, were smaller, measuring but 149.636 microns. We wish 

 now to investigate the causes of such differences. 



We shall not attempt at present a systematic investigation of the 

 effects of dift'erent chemical and physical agents on size, form and 

 variation, though this is a matter which much needs study. Our 

 present object is rather to examine the effects of altered nutritional 

 conditions and of the commoner "favorable" and "unfavorable" 

 conditions. We shall study the variations from the standpoint of 

 interest in the organism rather than in the agents inducing them, the 

 purpose being to form a conception of the changes which may be 

 looked for in Paranicchim as a result of common alterations, mainly 

 nutritional, in its cultural conditions. One of the results of this study 

 will be to show that we cannot assign a definite effect to each agent 

 taken in any absolute way. What effect a given agent will have 

 depends on the previous condition of the organisms on which it acts. 

 The same agent produces at one time an increase in size, at another 

 a decrease; at one time it increases the variability; at another it 

 decreases it. A given agent may either increase the positive corre- 

 lation between length and breadth, or it may decrease it or convert 

 it into a negative correlation. In succeeding days the same agent 

 may produce these diverse effects on the same set of Parauiecia. 



