16 THE PROBLEMS OF EVOLUTION 



in a previous state, and changes forced upon living 

 things have met with a response in ever-increasing 

 diversity of life. It is primarily a tale of gradually 

 increasing independence of environment; a change 

 of acidity in the surrounding medium which would 

 be fatal to many Protozoa is incidental to others, 

 and a change of temperature which stiffens the 

 most active insect into immobility merely stimu- 

 lates homoiothermal man. As fixed conditions 

 have persisted, many organisms have developed a 

 narrowly limited dependence upon them, but the 

 major tendency in evolution has been toward 

 greater and greater freedom. This does not mean 

 that true independence is even a possibility. Ter- 

 restrial organisms are as dependent upon adequate 

 water as aquatic creatures, but they have devel- 

 oped the ability to conserve their more limited 

 supply enough to invade the driest parts of the 

 earth. They encounter a much greater fluctuation 

 of temperature than aquatic organisms, and in 

 their highest groups, the mammals and birds, they 

 have met this condition by developing the ability 

 to maintain a constant temperature regardless of 

 that of their surroundings. 



The organism has gained complexity. If we 

 could have been on hand to see its entire history 

 the later steps might be intelligible in the light of 

 the earlier, but since we are a late product of the 

 entire chain of events, and a very complex one, we 



