78 LIFE IN THE SEA [CH. 



affected favourably or unfavourably according to 

 the direction of the variation. If the latter is great 

 enough the species will not reproduce at all even 

 if it should still be able to live. At all times the 

 organisms of a species are subject to destruction by 

 their natural enemies, and the species is only able 

 to maintain itself if its powers of reproduction are 

 at least equal to the rate at which it is being 

 destroyed. It will not usually be the case that a 

 species lives under optimal conditions : generally it 

 will just be able to hold its own against its enemies 

 but at certain times and places the conditions become 

 optimal and then the species attains a temporary 

 maximum of abundance. Such a maximum occurs 

 during the spring in the case of the diatoms, for the 

 temperature, the food, and the intensity of sunlight 

 have then become optimal. 



Where in the sea the conditions are optimal there 

 the species has its centre of distribution. From this 

 centre it spreads out in all directions that are possible, 

 for the tendency is for it to enlarge its area of distri- 

 bution, and the further we go from the centre the less 

 abundant it will be, and at a certain distance it will 

 disappear entirely. Near an imaginary contour line 

 drawn round the centre of distribution the rate of 

 reproduction is just equal to the rate of destruction ; 

 beyond this line the latter becomes the more powerful 

 of the two, and the species becomes rarer and rarer 



