in] THE FACTORS OF DISTRIBUTION 77 



inhabit a wide area of sea, but is restricted to only 

 a part of that area it is because it finds in the area 

 which it has come to inhabit its optimal conditions 

 of life. In the course of its evolution not only has 

 a definite form been developed, but also an adapta- 

 tion to certain conditions of temperature, depth of 

 water, salinity, and sunlight. It does not follow that 

 because the optimal temperature for a marine diatom, 

 say, is 10C. it cannot exist at 0C. or 15C., but 

 its vitality is greatest at 10 C., and it reproduces 

 most accurately at that temperature. If we take 

 some beef tea and inoculate this with some species 

 of bacillus we shall probably find that the most rapid 

 reproduction takes place at a certain temperature 

 37 C. may be a common optimum. Far below this 

 temperature the bacillus would still be able to live, 

 but it will not reproduce when the temperature 

 approaches the zero of the Centigrade scale, and at a 

 temperature not very far above the optimum it will die. 

 Most species of unicellular organisms are able to live 

 at any temperature found hi the sea and some are 

 said to be able to withstand even the cold of liquid 

 air ; almost any salinity found in the sea is fit for 

 marine life, and any degree of ordinary illumination. 

 But the rate of reproduction of any organism is 

 greatest for a certain combination of all the physical 

 factors, and if these are varied, or even if one of 

 them is varied greatly, the rate of reproduction is 



