4 Introduction 



adult life his bifocal vision and prehensile limbs remind him of ages 

 long gone by when his ancestors lived in trees. 



The characteristics of fossils indicate the nature of the environ- 

 ment in which the animals that formed them lived (Hubbard 6C 

 Wilder, 1930). The great steps in evolutionary progress appear to 

 have occurred when plants or animals surmounted the greatest en- 

 vironmental obstacles to invade new types of habitats where new 

 earth resources could be utilized — when organisms left the ocean to 

 take up life in fresh water and when they left the water for the land. 

 This book is devoted chiefly to the consideration of such emigra- 

 tions. 



Since early geological eras the composition of the ocean has 

 changed. There was formerly less salt. At present sodium and 

 magnesium are slowly increasing, but calcium and potassium are 

 practically stationary. Calcium is continually iadded to and is con- 

 tinually precipitated out of sea water. The development of vege- 

 tation and soils on land has retarded the movement of potassium to 

 the sea. "Briefly animal as well as vegetable protoplasm owes its 

 relation to the elements sodium, potassium, calcium, and magne- 

 sium, to the composition of sea water which obtained when all 

 forms were unicellular, just as the blood plasma owes its relations 

 to the same four elements to the composition of sea water when 

 prevailing circulating fluids were established. In other words, the 

 relation of protoplasm to salts is due to the action of sea water, for 

 incalculably long periods of time on the living matter of unicellular 

 organisms" (Macallum, 1904) . In particular areas at various times 

 in the past there have been considerable changes in sea level. "Al- 

 though the slow widespread rise and fall of the sea surface have 

 produced important geological results, yet the existing raised beaches 

 and the upward growth of coral reefs into atolls are due to local 

 variations in level of land, and not to general rise and fall of the 

 sea" (Gregory, 1931), 



