THE SUN-PERCH AND THE CATFISH 



is no competition between them. All are fashioned for 

 the work they have to do, and their particular condition 

 is thus determined by that of its neighbours. To this 

 state of affairs we give the name ' organic correlation \ 

 There is equilibrium among all. If one were to increase 

 out of proportion, or were replaced or taken away, its 

 neighbours would feel the consequences, and a new 

 equilibrium would have to be established. So with 

 species living in a definite place; they are in mutual 

 relation as it were, as the organs are to the body. 



" Nature has a biological correlation as well as an 

 anatomical correlation. The species co-ordinate the 

 needs of life as between themselves. They have a 



Fig. 43. — The Wels, a giant Catfish. Length sometimes as much as 

 13 feet. 



toleration and a balance of their own. This equilibrium 

 varies in its details, but is maintained as a whole. There 

 is a mutual solidarity. If one attains to too great a 

 domination, or tends to weaken or disappear, its neigh- 

 bours are affected, and the equilibrium is destroyed. 

 Then Nature intervenes to produce a new equilibrium. 

 " It is clear that this equilibrium relates to all the 

 external circumstances which react on the creatures, 

 and upon which they depend, from the temperature of 

 the waters to their composition, the content of sub- 

 stances in solution. But the main consideration is that 

 of food supply. Every species, so far as this is con- 

 cerned, has its own peculiar characteristics and mode of 

 action ; it must have its particular kind of food, and this 

 the water must supply. When it finds this, it prospers, 

 or at any rate, manages to support itself; when it fails 

 204 



