in] THE FACTORS OF DISTRIBUTION 73 



difference as we find between tropical and polar seas, 

 the duration of life is increased about a thousand-fold, 

 but the rate of development is reduced to only a third. 

 This means that a marine organism will reproduce 

 about three times less actively in cold seas than it 

 will in warm seas, but it will live a thousand times 

 longer ; and therefore it must be much more prolific 

 and there will be many more overlapping generations 

 in the polar than in the tropical seas. If the relation 

 holds good we must have a greater density of life 

 in the former waters than in the latter. We should 

 not, indeed, expect to find such a difference as would 

 be expected from the experiment described, for the 

 abundance of food would limit the development, and 

 so also would other factors the less intensity of 

 sunlight, for instance, in the case of a marine plant, 

 such as a diatom. But it seems clear that the 

 difference between rate of development and duration 

 of life is a factor. How exactly the temperature 

 affects the duration of life we do not know : it 

 may be that some substances are produced, and are 

 accumulated in the tissues, and that these substances 

 are detrimental to life. If these toxic substances 

 are produced at a greater rate the higher is the 

 temperature, then the duration of life will be 

 shorter at high than at low temperatures. 



How does a change in the salinity of the sea- water 

 affect a marine animal ? If a fish, say, moves from 



