330 



THE CHANGING GENERATIONS 



ized, highly integrated, cooperating cells. Inside the limitations imposed 

 by the two great nutritional methods — food manufacture in plants and 

 food capture in animals — there are great differences in the arrangements 

 for doing the tasks necessary for individual maintenance, accompanied 

 by striking modifications in body form and organization. As for environ- 

 ment, animals and plants live in almost every conceivable type of terres- 

 trial and aquatic situation. The seas, the streams and lakes, the soil, 

 the forests and grasslands — all are populated by myriads of living crea- 

 tures. Even the driest deserts, the arctic wastes, the lightless depths of the 

 ocean, and steaming hot springs are not without their inhabitants. 



Fig. 23.1. "One of the most striking phenomena of life is the tremendous number of kinds 

 of organisms." (Rearranged from a picture by Artzybasheff, jacket of Hegner, Parade of the 

 Animal Kingdom, by permission The Macmillan Company.) 



The existence of so many and such varied forms of life poses important 

 problems. How did all these kinds of animals and plants come into exist- 

 ence? What is the explanation of the orderliness that underlies their 

 diversity — of the graded degrees of resemblance between organisms that 

 permit us to classify them in a systematic hierarchy? How does it happen 

 that each species is adapted to exist in some particular sort of environ- 

 ment? And that while adaptation is sometimes extraordinarily perfect, it 

 is also sometimes faulty or incomplete? Why should each species inhabit 

 only some particular part of the earth's surface and be absent from other 

 places where it could equally well exist? 



Such questions as these are of major concern to the biologist. To find 

 their answers has been no easy task; many of the answers are still not 

 fully known. Our present understanding of them has been obtained by 



