346 ANIMAL BIOLOGY 



that the environment rather than the organism itself is the domi- 

 nant factor, and this is true to a considerable degree. But Parame- 

 cium is not merely an automaton. Its behavior is modifiable and, 

 in the long run, is adapted to the usual changes of its surroundings. 

 It forms immense aggregations where food and other conditions 

 are favorable. That the reactions are adequate for the simple life 

 and methods of reproduction of Paramecium is attested by its 

 success — it is one of the most common and widely distributed 

 animals. (Fig. 27.) 



In such simple beginnings, then, must be sought the largely 

 automatic responses of animals to the changes in external con- 

 ditions, known as reflexes and instincts. Both apparently are 

 chiefly the result of inherited nervous structure and therefore may 

 be regarded as inherited behavior. And increase in the complexity 

 of life processes has involved at the same time an increase in the 

 number and complexity of reflexes and instincts. The primitive 

 reflexes and instincts of Hydra lead it to seize small organisms 

 within reach of its tentacles and pass them to its mouth; the 

 Earthworm, to swallow decaying leaves as it burrows through the 

 soil; the Crayfish, to grasp its prey with its large claws, tear it 

 into pieces by means of certain appendages about the mouth 

 which are adapted just for the purpose — and so on to the higher 

 Vertebrates where the feeding instincts reach their maximum of 

 complexity. The remarkable behavior of Ants and Bees is essen- 

 tially a complex of instincts. Moreover, instincts of fear, self- 

 defense, play, care of the young, etc., render a considerable part 

 of the behavior of even the higher organisms more automatic than 

 is perhaps, at first thought, apparent. (Figs. 139, 143.) 



But just as the behavior of Paramecium and its allies is modi- 

 fiable, so reflexes and instincts which seem the most fixed show at 

 least a slight degree of adaptability to unusual conditions. Indeed 

 new reflexes, known as conditioned reflexes, may be established 

 as the result of experience during the life of the individual. And 

 it is this ever-present power of modifiability, which is in man called 

 'choice,' that leavens the whole and becomes the dominant factor 

 in the behavior of the highest animals; while reflex action and in- 

 stinct are relegated to a subsidiary though by no means unimpor- 

 tant role. A large part of human education consists in establishing 

 other fixed adaptive responses called habits which join the earlier 

 reflexes and instincts in relieving the conscious life of innumerable 



