430 



EVOLUTION AND ANIMAL LIFE 



which we find in tlie beha^dor of higher animals, yet with the simplest 

 possible basis in ways of action; a great portion of the behavior con- 

 sisting often of but one or two definite movements, movements that are 

 stereotyped in their relation to the environment. This method leads 

 upward, offering at every point opportunity for development, and 

 showing even in the unicellular organisms what must be considered the 



beginnings of intelligence and of many 

 other qualities found in liigher animals. 

 Tropic action doubtless occurs, but the 

 main basis of behavior is in these or- 

 ganisms the method of trial and error." 



Fig. 265. — Diagram showing how 

 the motile Protozoan, Stcntor, 

 reacts to light: A circular space 

 half in light and half in dark; 

 the animalcules collect in dark 

 area; 1, 2, and 3 shovr the 

 reaction of a specimen which 

 came to the light line. (After 

 Jennings.) 



Different one-celled animals show 

 differences in method or degree of 

 response to external influences. 

 Most protozoa wall discard grains 

 of sand, crystals of acid, or other 

 indigestible objects. Such peculi- 

 arities of different forms of life 

 constitute the basis of instinct. 



Instinct is automatic obedience 

 to the demands of external condi- 

 tions. As these conditions vary 

 mth each kind of animal, so must 

 the demand vary, and from this arises the great variety actu- 

 ally seen in the instincts of different animals. As the de- 

 mands of life become complex, so may the instincts become so. 

 The greater the stress of environment, the more perfect the 

 automatism, for impulses to safe action are necessarily adequate 

 to the duty they have to perform. If the instinct were inade- 

 quate, the species ^vould have become extinct. The fact that 

 its individuals persist shows that they are provided \^dth the 

 instincts necessary to that end. Instinct differs from other 

 allied forms of response to external conditions in being hereditar}^ 

 and continuous from generation to generation, and in being 

 common to the species and not characteristic of the individual. 

 This sufficiently distinguishes it from reason, but the line be- 

 tween instinct and reason and various forms of reflex action 

 cannot be sharply drawn. 



Some WTiters regard instincts as "inherited habit," while 

 others, with apparent justice, doubt if mere habits or voluntary 



