CHAPTER XLVIII 

 INSTINCTS 



291. A chain of reflexes. When an infant of a certain age 

 sees a small object, there is at once started a reflex ending in 

 the touching of the object. When the palm of the hand comes 

 in contact with the object, there is started the reflex of 

 closing the fingers. The touch of the object against the palm 

 and fingers starts the reflex that carries the object to the 

 mouth, and at the same time it stimulates the muscles that 

 open the mouth. The contact of the object with the lips and 

 tongue may set up a reflex of closing down on the object, 

 or it may set up a reflex of rejection if the taste is what 

 we call disagreeable. 



Here we see movements in a connected series, but each part 

 of the series is a very definite reflex, depending upon the 

 association of afferent neurons connected with sensory organs, 

 efferent neurons connected with muscles, and associative neurons 

 in the spinal cord or the lower part of the brain. Such a 

 series we may call an instinct. 



292. Instincts not perfect adaptations. A frog would starve to 

 death with hundreds of dead worms and insects all about him, be- 

 cause eating movements of this animal can be started only by the 

 sight of a moving object. On the other hand, the frog will swallow 

 bits of doth that are dangled in front of him, and that have no 

 food value whatever. 



Again, the female fly that is about to lay her eggs is guided entirely 

 by odor. If a piece of paper that has been soaked in meat juice 

 is placed on a table, the flies will come and lay their eggs upon it, 

 although this is extremely wasteful of eggs, and suicidal for the 

 species if persisted in. Of course, in a state of nature the only things 



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