in] THE FACTORS OF DISTRIBUTION 69 



' historical basis of acting' being the determining 

 factor in this choice. 



Thus the evolutionary history of a species has 

 determined to some degree its mode and extent of 

 distribution ; for the development of definite struc- 

 tures and habits restricts the animal to a particular 

 range of habitats. Nevertheless it is easy to see that 

 most species of marine animals would be distributed 

 over a much wider area than they actually are if 

 there were not some factors which limit this distri- 

 bution. These factors are those of temperature, 

 salinity, pressure of water, and intensity of sunlight. 

 We have next to consider in what way these affect 

 marine organisms. 



The temperature is probably the most important 

 of all these factors, and it is easy to see that there are 

 many ways in which a marine animal, such as a fish, 

 can be affected by changes in it. There is no reason 

 to suppose that a fish cannot feel changes of temp- 

 erature in the way we do, that is by its sense-organs. 

 It appears that sudden changes can produce patho- 

 logical effects, for fresh-water fish are known to be 

 affected by a form of dermal catarrh when they are 

 suddenly moved from water of high to water of 

 much lower temperature ; and there is a record of 

 the destruction of millions of tile-fish off the coast 

 of North America by a rapid and extensive chilling 

 of the water stratum in which they were living. 



