specific Relations of Fitness 147 



sided matter. A thing is fit or it is not. A little reflection, 

 however, will show us that from the very nature of the idea 

 fitness must be a matter of mutuality. We need constantly 

 to be reminded that so far as concerns " life," the physical 

 environment also shows a remarkable fitness. The physical 

 and chemical properties of water, both in its gaseous and in its 

 liquid state, the characteristics of carbon and of nitrogen 

 and of the other chemical elements that enter into the com- 

 position of protoplasm are admirably adapted to bring about 

 between them all the distinctive characteristics of living 

 matter. If there were no water, or if its freezing and boiling 

 points were considerably different from what they are, there 

 could be no life, such as we know. We may say the same of 

 each of the other important physical factors, such as the light 

 and heat from the sun. Life being what it is, it can arise 

 and perpetuate itself only in our kind of material world. 



On the other hand, it is just as true to say that the 

 world being what it is, it can maintain only the kinds of life 

 which we find in the actually living plants and animals. The 

 environment is fit for life — this kind of life. Life is adapted 

 to the environment of this kind of world. But this world is 

 itself constantly changing, within certain limits, in all de- 

 tails. Being alive means maintaining a constant change in 

 response to these external changes and in some special re- 

 lationship to them. 



Specific Relations of Fitness 



From what we know of living processes, we can see 

 some of the directions in which organisms must be adapted. 

 The getting of food, of water and of air, are primary con- 

 ditions of living for practically all organisms. We are im- 

 pressed with the fact that the one-celled plants making their 

 food out of water, carbon dioxid and salts, live on exposed 

 surfaces in the air, in fresh water, and in salt water, whereas 

 there are complex plants sometimes of enormous size also 

 living on land, in fresh water and in salt water. There are 

 one-celled animals that feed on bacteria, ferocious beasts of 



