456 GENERAL BIOLOGY 



needful, are readily brought about. The nerve impulse 5 

 that will bring them to pass find established channels 

 through the well integrated mass of neurones comprising the 

 nervous system, and they run through these channels, 

 obliterating the effects of other unimportant stimuli. 

 Thus, we have a mechanism which furnishes the conditions 

 for a great variety of responses, and that may be organized 

 by experience for the ready performance of specific acts. 



Such a nervous equipment suffices for the carrying out of 

 reactions that are determined by sense perception automati- 

 cally, the right of way being given to the kind of stimuli 

 that in the past development of the system have determined 

 its paths. The paths are redeveloped in successive genera- 

 tions. The mechanism is perfected with the survival of the 

 fittest, at least, with the elimination of the unfit, or ill ad- 

 justed. Such a mechanism gives reactions that are, there- 

 fore, wonderfully adapted to the particular set of circum- 

 stances under which they have been developed; but they 

 are inexorably fixed in the structure of the nervous system. 



Such reactions characterize the habits of most of the lower 

 animals. The moth that flies to a candle flame is stimulated 

 irresistibly by the light. Its ancestors for ages have flown 

 to white objects (flowers) at night. In a proper environ- 

 ment there is nothing better for a moth to do than this. 

 Thus, it gets its living. But candles being introduced into 

 its environment, it flies to a candle, not being able to distin- 

 guish between candle light and the light reflected from a 

 flower, or, at least not being able to respond differently, or 

 even to withhold response. Hence, although it may be 

 merely singed with the first contact, it repeats the act so 

 long as it is able to respond to the light stimulus. Thus it 

 goes down before a peril, new to its racial experience, and 

 not provided for in its nervous organization. If candle 

 lights were to become universal, its race would be doomed to 



