192 THE PROBLEMS OF EVOLUTION 



regarded as the natural tendency of the Uving 

 substance by some writers. Natural tendency it is, 

 but in the complex of heritage and environment, as 

 indicated in Osborn's discussions of speciation, and 

 not of heritage itself. It seems that organic evolu- 

 tion is universal because it is unavoidable, not 

 because it is an inherent tendency, and that the 

 natural tendency of the organism is to maintain 

 its normal state. Examine a culture of Paramecium 

 in which bacteria are thickly clustered. It is un- 

 necessary to pursue the animals under these condi- 

 tions. They may be found nosing up against the 

 masses of food, almost motionless, as they eat; 

 even locomotion is at a minimum when it is not 

 necessary to supply the ordinary needs of the ani- 

 mal, and in the intestinal parasites a similar co- 

 ordination of surroundings with locomotion is even 

 more extremely developed. So it is with all organ- 

 isms in the presence of favorable conditions; they 

 do not seek a change when they have found the 

 optimum. Even intelligent man is less likely to 

 attempt new ventures when he has found his desire 

 than to settle down to enjoy his contentment. Most 

 human activity is the result of need, not neces- 

 sarily for the biological fundamentals but at least 

 for the gratification of some higher desire. The 

 most intellectual and altruistic workers would 

 probably not carry on their activities but for the 

 fact that they gain personal satisfaction in doing 



