XVIII INSTINCTS AND TROPISMS 345 



the brachial region of the spinal cord. It is a tendency in- 

 dependent of the higher nervous centers, and it is brought 

 about by certain changes that take place in the organism 

 during the spring, probably as an indirect result of the 

 ripening of the sexual products. 



Most of the things that the frog does fall into a compara- 

 tively few categories. Its actions do not show an indefinite 

 diversity like those of an ape or a human being. It uses its 

 hind limbs, for instance, in leaping and swimming, and 

 occasionally for pushing against some object or removing an 

 irritating substance from the skin. The two former move- 

 ments are much alike and the two latter both consist in 

 bringing the legs forward to the point of stimulation. The 

 legs of a frog are not moved about in a great variety of ways 

 for different purposes like the arms of a man. They have a 

 few simple tricks which they are admirably adapted to per- 

 form, but beyond these the range of their powers is very 

 limited. The same is true of the fore legs, the head, and the 

 body as a whole. Movements of the fore and hind limbs 

 may be combined in various ways, as in leaping, diving, 

 crawling, burrowing, righting movements, but the number of 

 combined actions employed is much less than in higher 

 forms. The frog is a sort of mechanism beautifully adapted 

 to the performance of a number of actions, but its repertoire 

 is not at all extensive, and it has little power of improvising 

 new roles. Its modes of response are dependent upon the 

 way it is organized, for as a machine is constructed so will 

 it work. A frog is as incapable of leading a life like that of 

 a cat as a machine made for grinding corn is incapable of 

 weaving cloth or printing books. Its kind of 'life is laid out 

 for it by the forces that have shaped its bodily structure. 



Reactions to Light. — In addition to responding to vari- 

 ous objects of sight, frogs react to light in a very peculiar 



