140 ANIMAL LIFE 



distinguishing and preferring chemical substances, 

 which enables all living things to absorb nutritive 

 substances when surrounded by a mixture of hetero- 

 geneous bodies. 



It is only in some of the higher animals that the 

 organs of sense become so highly developed and the 

 perceptions arising through them so well defined 

 that sensations become phenomena apart from those of 

 motion, digestion, or of others immediately concerned in 

 the welfare of the body, so that we cannot yet resolve 

 their complex actions into the simpler responses from 

 which they have sprung. 



TheOriginof Sense-organs from the Skin. — The skin, 

 upon which the outer world impinges, contains the 

 sense-organs which receive and transmit its messages, 

 and the deeper parts of the skin develop points from 

 which their needs emerge. Thus, upon any organism 

 a double set of impulses falls, one from without, 

 another from within. Each of these is at first 

 intimately connected with the fundamental acts of 

 life, and in order to correlate the supply indicated 

 from without with the demand indicated from within 

 a nervous system has been gradually elaborated. In 

 the hrst instance it appears to be related rather to the 

 efficient response of the body to the external messages 

 — pressure, food, light, and so on — and in particular 

 to movement. But it is clear that, since these 

 responses are of vital importance, they must be cor 

 related with the inner needs of the body ; and so there 

 come to be not only a nervous regulation of move- 



