CH. IX] THE PRODUCTIVITY OF THE SEA 201 



become diminished ; if we take less then the area is not exploited 

 to the greatest advantage. But this limit of exploitation is 

 probably recognised by the fishermen, for when their operations 

 become less profitable they desert the partially impoverished 

 grounds for others. 



The productivity of the sea in different latitudes. In 

 comparing the pictures of life presented by land and sea I 

 supposed that we were exploring a part of the land in a temperate 

 zone. It is well known that the density of life, particularly plant 

 life, on the land is greatest in the tropics and least in the polar 

 regions. The intensity and duration of sunlight is at a maximum 

 in equatorial lands ; the temperature, too, is highest. Whether 

 we proceed from the torrid to the frigid zones, or ascend from the 

 sea-level into mountainous regions, we find that both the nature 

 and density of plant and animal life change regularly with the 

 isotherms. Everywhere (except in desert regions) the hotter 

 lands have the most luxuriant vegetation and the colder lands the 

 least. " He who," says Brandt, " has to force a way through the 

 dense plant life of a primeval tropical forest, and then sees the 

 stunted vegetation of Spitzbergen just emerging from the soil, is 

 easily convinced of this contrast." 



Naturally one supposes that this is also true of the sea. 

 In tropical regions Ave find a greater variety of marine animals. 

 The colouring is more vivid and the form is more varied and 

 ornamental. The massive coral and other lime formations due 

 entirely to the activity of coelenterates impress one with a sense 

 of the luxuriance of life, and we are apt to forget the factor of 

 time in the building up of these monuments of animal life. 

 Unconsciously one confuses variety of forms with richness of 

 individuals. The usual picture we obtain from records of voyages 

 in tropical seas is that of the wealth of life, and we are naturally 

 surprised to find that this impression is a false one and that the 

 tropical seas are not more abundant in plant and animal life than 

 the temperate and polar waters ; and indeed that the reverse is 

 really the case. 



There is little doubt that the distribution of life in the sea is 

 exactly opposite to that on the land. The greatest fisheries are 



