CHAPTER XXIII 



THE EVOLUTIONARY CONCEPT 



In Part I of this book we began our survey of the world of life by examin- 

 ing individual organisms at close hand to see how they are built and how 

 they work. Then, in Part II, we stepped back a little so as to see them, 

 with their parents and progeny, as links in a continuous chain of heredity. 

 Now we change our viewpoint once again. We climb to a hilltop, so to 

 speak, whence we may see the whole panorama of life in broad prospect 

 and trace the paths it has traveled since it emerged from the mists of 

 primordial time. We shall see from the fossils of the dead how the pro- 

 cessions divided and wandered, how their courses were guided by en- 

 vironment, what multitudes of them followed routes that led to extinc- 

 tion, and how the changed descendants of ancient stocks form the living 

 populations of the present. We shall note how the generations changed by 

 almost imperceptible degrees, through slow modification of the germ 

 plasm over the eons. We shall consider especially the ancestral stocks from 

 which the human species has emerged, and we shall examine the causes 

 and conditions responsible for the changes, and the consequences of the 

 blood relationship that exists among organisms by virtue of their common 

 ancestry. All these aspects of life are summed up in the phrase organic 

 evolution, which is the theme of this third section of the book. 



The variety and multiplicity of living things. One of the most striking 

 phenomena of life is the tremendous number of different kinds of or- 

 ganisms that exists today. Just how many kinds of animals and plants 

 there are we do not know. More than one million species 1 have already 

 been described and named, however, and it is not improbable that an 

 equal number remains to be discovered. Within this vast array we en- 

 counter the widest variations in size, form, degree of complexity, methods 

 of self-maintenance and reproduction, and relation to other organisms. 

 The size range extends from the ultramicroscopic to such enormous 

 living things as whales and sequoia trees. At one end of the scale of com- 

 plexity are the single-celled bacteria, in which not even a nucleus is 

 visible ; at the other are animals and plants made up of billions of special- 



1 According to a careful recent estimate, there are at least 750,000 described species 

 of animals and more than 600,000 described species of plants. 



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