58 THE REALM OF ORGANISMS CONTRASTED 



6. A System of Inter-related Lives. 



The hosts of living organisms are not random creatures, 

 they can be classified in battalions and regiments. Neither 

 are they isolated creatures, for every thread of life is inter- 

 twined with others in a complex web. This is one of the 

 fundamental biological concepts the correlation of organ- 

 isms in the web of life and it is as characteristically Dar- 

 winian as the struggle for existence. No creature lives or 

 dies to itself; there is no insulation. Long nutritive chains 

 often bind a series of organisms together in the very funda- 

 mental relation that one kind eats the other. All things are 

 in flux, there is a ceaseless circulation of matter; all flesh 

 is grass and all fish is diatom ; and so the stuff of the world 

 goes round from one incarnation to another. One organism 

 gets linked on to others, and becomes dependent on them 

 for the continuance of its race. Flowers and insects are fit- 

 ted to one another as hand to glove. Cats have to do with 

 the plague in India as well as with the clover crop at home. 

 The young of the fresh-water mussels are carried about for 

 a time by minnows, and the young of the fish called 

 the bitterling are harboured within the fresh-water mussel. 

 Squirrels affect the cornfields and water wagtails the sheep- 

 folds. In short, we get a glimpse of Nature as a vast sys- 

 tem of inter-linked lives a Systema Naturae in a new sense 

 a web with a pattern (see Thomson, 1914, 1916). With- 

 out entering upon any discussion of the weaving or evolu- 

 tion of the web through untold ages, let us take the realm 

 of organisms as it is, and emphasise the fact that just as 

 there is a correlation of organs in the body, so there is a 

 correlation of organisms in the world of life. When we 

 learn something of the intricate give and take, supply and 



