PREFACE 



Many of the problems of biology result from the fact 

 that changeful organisms live in changeful environments. 

 Life always involves continual adjustments to environ- 

 ment. Animals which invade new types of habitats must 

 be able to change their inherited adjustment patterns or 

 they must possess inherent qualities which make them 

 more or less immune to the effects of environmental 

 changes. 



The animals which dominate the earth today origi- 

 nated in the stable, saline, food-laden ocean. Primitive 

 animals for the most part remained in the sea, but as 

 the ages went by several types migrated into fresh 

 water and a few of the more progressive types even took 

 up life on land. Migrants into fresh water and land 

 habitats left the stability of the old marine haunts of 

 life, where they lived in a medium which resembled the 

 fluids such as are found in all protoplasms, where there 

 was no dearth of oxygen and very little change in tem- 

 perature. In fresh water they received little compensa- 

 tion, for they were obliged to live where there was often 

 great variation in salinity, oxygen, and temperature. But 

 those that attained to the land, although they were 

 obliged to endure extreme variations in temperature, had 

 to secure all their body salts from their food, and were 

 in continual danger of death from desiccation, were also 

 able to live in a rarer medium which permitted rapid 

 locomotion and by supplying an abundance of oxygen 

 made rapid metabolism possible. 



vii 



