in] THE FACTORS OF DISTRIBUTION 61 



falls to a much greater extent than it does as we 

 descend into the depths of the ocean abysses. If we 

 ascend to the summit of a mountain 24,000 feet high 

 the pressure does not decrease by nearly so much as one 

 atmosphere, but as we descend to a corresponding 

 depth in the sea the pressure rises to about 700 

 atmospheres. The extremes of salinity variation in 

 the sea are about 30 and 36 except in very exceptional 

 cases. The annual change of temperature is much 

 less on the land than in the sea. The annual change 

 of sunlight intensity due to the seasons is of course 

 the same in each case, and so also is -its variation 

 from place to place since this depends on the latitude. 

 The distribution of plants and animals on the land 

 is restricted by mechanical barriers, by mountain 

 chains, deserts, seas, great rivers, etc. In many cases 

 we find that these barriers are insurmountable so 

 that the great continents and the oceanic islands are 

 characterised by the possession of species peculiar to 

 themselves. Birds and insects may fly, or be blown, 

 for great distances, and plant-seeds may be transported 

 in several ways ; but we find that the barriers have 

 operated in producing the present distribution and 

 diversity of life ; and that apart from human agencies 

 very few species of plants or animals would be truly 

 cosmopolitan. In the sea, however, there are no such 

 mechanical barriers to distribution, for the watery 

 medium is everywhere continuous : on the other hand 



