in] THE FACTORS OF DISTRIBUTION 75 



solution, and these liquids are separated from the 

 surrounding sea-water only by a membrane which is 

 very thin in certain places, over the gills for instance. 

 Now the sea also contains these same salts in greater 

 concentration than they are contained in the blood ; 

 and if the membrane covering the gills were a dead 

 one, then water would diffuse out from the blood and 

 lymph of the fish into the surrounding sea-water. 

 It does so diffuse but only to a very slight extent, and 

 not nearly so much as would be expected if the 

 process of diffusion in the case of a living membrane 

 were the same as that which holds good for an 

 inorganic one. Diffusion does not take place to the 

 full extent possible, for among all the chemical and 

 physical reactions that may occur in the tissues of 

 an animal some do not happen because of the power 

 of regulation possessed by the living organism. But 

 if the concentration of the sea- water is higher than 

 that of the blood, and if diffusion does not take place, 

 then work must be done to prevent it. If the fish 

 moves from a region of high to one of low concen- 

 tration less work must be done, and this is a reaction 

 on the part of the organism towards the change of 

 salinity. 



It is easy to show that changes in the intensity 

 of sunlight must directly affect a marine organism. 

 They do, of course, profoundly affect a green plant, 

 as we shall see later ; but animals which do not 



