Life and Instinct 201 



firmly drawn in by instinct, but altered and refined in many 

 places by intelligence, and paralleled and shaded at every point 

 by reflex action or the power of motor response. It does not 

 seem probable that such actions can be completely resolved into 

 a number of simple reflex components, but even should this be 

 done the historic and much-abused term instinct could still be 

 used to advantage. 



Many instincts mature at birth, while others are delayed 

 until needed; some are called forth but once only, others re- 

 peatedly. As we have seen (in Chapter I.), the parental in- 

 stincts are periodic in appearance and serial in character. 



The catalogue of instinctive acts which even the higher 

 animals perform is so great that one is not at a loss for illustra- 

 tions. To those already given I will now add the following, from 

 animals both high and low in the scale. 



When the spring comes, the young bird, who returns to the 

 place of its birth, is prompted to find a mate, and with her soon 

 begins to build a nest. Though unattended by instructors, and 

 unprepared by practice, it uses the inherited tools of its guild 

 bill, breast, and feet with nice precision, and be it Oriole, 

 Robin, Flycatcher, or Vireo, follows with wonderful closeness 

 the type of architecture which its ancestors have used for 

 ages. 



Why does the Robin, in its first attempt at nest-building, 

 begin by laying a foundation of dry grass or stubble, and add 

 to this mud softened with water, which it then heaps about 

 its breast, and with repeated turnings and shakings gradually 

 molds into a symmetrical cup, often selecting a rainy day for 

 the purpose, and why does it finally finish the whole with a 

 lining of fine grass? One might as well ask why the Robin lays 

 blue eggs, or why it utters its well-known call. It acts in these 

 ways because it must, because Robins have been doing these 

 things for hundreds of generations. It not only inherits tools, 

 but a certain aptitude for their use. Its organization compels 

 or determines its actions. 



If the young of a viviparous shark be taken from the body 

 of its mother and cast into the sea, they will swim off, without 



