186 THE NATURE AND ORIGIN OF LIFE 



secretions and the rest, which result from reflexes, have a 

 precise utility in the struggle of the organism against the 

 environment, that is, are adapted to avoid causes of destruc- 

 tion and to profit by favourable events. A cat draws back 

 a burnt paw quickly (defensive reflex) ; the same cat, see- 

 ing and smelling a bowl of milk, approaches and laps it 

 up (series of offensive reflexes). Such adaptation is studied 

 by the science of the origin of species. 



Sense Organs and Sensorial Surfaces 

 In an animal as complicated as the cat or man, the 

 epithelial cells which line the surface of the body are not 

 all alike. They are susceptible to a rupture of their colloid 

 equilibrium from various causes. Causes of rupture from 

 the outside are infinite in number and variety ; it is probable 

 that many have no action on the epithelial surfaces of 

 animals. Among those whose activity is a matter of daily 

 observation are light, sonorous vibrations, mechanical 

 shock, heat, chemical or colloid substances. 



With the lower animals it is probable that all the surface 

 cells are more or less influenced in their equilibrium by these 

 various agents. With the higher animals the division of 

 labour which results, as we have seen, from functional 

 assimilation, develops in certain cells a sensitiveness to 

 certain particular agents. Thus light acts violently on the 

 visual cells and produces modifications which are scarcely 

 perceptible in the cells that clothe the hand or foot. In 

 the same way, the savour of food (a chemical and colloid 

 peculiarity) impresses itself on the cells of taste and has 

 no action on the surface of the eye and so on. 



The name of sensor ial surfaces is applied to those epithelial 

 surfaces which are adapted to receiving impressions from 

 this or that external agent. It is as if, in the palace just 



