58 LIFE IN THE SEA [CH. 



to the terrestrial deserts is to be found in the great 

 abyssal plains of the ocean beds. Plant life is most 

 abundant in the shallow water near the shore, and 

 least abundant at the surface in the mid-oceanic areas. 

 It is most abundant just a little way beneath the 

 surface of the sea everywhere and then it decreases 

 as we descend into the depths. It is absent entirely 

 at the bottom of the deep sea. 



We have seen also that there are cyclical changes 

 in the abundance of life of all kinds ; changes which 

 are seasonal ones. Each species is normally restricted 

 to some particular area of sea or sea-bottom, but 

 this area expands or contracts with the seasons 

 and so the range of geographical distribution of the 

 organisms of the species also expands and contracts ; 

 that is, they are able to migrate. We have now to 

 consider in what way these general facts of distribution 

 and distributional changes are to be associated with 

 the physical conditions in the sea, and how fluctua- 

 tions of life are to be associated with fluctuations in 

 the physical conditions. 



We consider first of all the physical conditions of 

 temperature, salinity, and sunlight; and then the 

 nature and abundance of food in the sea. These are 

 the factors of distribution. The surface temperature 

 of the sea is highest a little way north from the equator, 

 and then it decreases towards the poles, falling at a 

 nearly equal rate as we pass north or south. This 



