24 DARWINISM AND THE PROBLEMS OF LIFE 



But Nature may go yet further. It may make the 

 habitation of the young animals fatal to them as adults. 



Many insects pass their youth in the water in the 

 form of larvae, but after their last cast of skin they 

 unfold pairs of wings, and become inhabitants of the air 

 and land. And if any mischance brings them back into 

 their former element, they are doomed, unless a friendly 

 grass -stalk provides the means of safety. May-flies, 

 dragon-flies, gnats, and many others are thus adapted 

 to two regions. 



Each region, therefore, is filled with a number of 

 forms of life. These do not live independently, 

 however, but are adjusted to each other. We know, 

 in fact, that if water-fleas are plentiful in a piece of 

 water, the condition of the fishes will be so much better, 

 because they form almost the entire food of the young 

 fishes. We must not forget, moreover, that the plants 

 in any region are important to the animals contained 

 in it. This becomes clear at once when we reflect that 

 it is they that form the nourishment of the plant-eating 

 animals ; and that they also provide more or less shelter 

 for the animals, and especially their young, and so 

 cannot be dispensed with. 



Thus each region is a self-contained whole, in which 

 plants and animals live in mutual relationship, and the 

 diminishing of one species always reacts on another. 

 But the foundation of all animal-life is found in the soil, 

 the distribution of water and land, light and air, and 

 the climate and other factors that we may call the 

 physical conditions of the place. The totality of animals 

 and plants that live under these conditions and are 



