18 INTRODUCTION TO EVOLUTION 



ment that evolution cannot be a fact, for if it were there would be no 

 "lower" animals left — they would have all become men long since! 

 Thoughtful consideration of the foregoing discussion will demonstrate 

 the fallacy of such an idea. We have seen that animals are constantly 

 tending to become adapted to the environment in which they live. Hence 

 most fishes, either today or in Devonian times, may be thought of 

 as tending to become "better" fishes; only a few, and that in one stage 

 of the earth's history, became amphibians. Among the latter, in turn, only 

 a few members of one group had the potentialities, and the opportunity, 

 to become reptiles; the rest remained amphibians, becoming adapted to 

 a variety of habitats. So it must have been always, and with all groups 

 of animals. To only a minority of any group befell at once the potentiality 

 and the opportunity for radical change into something different. 



As we look about us today we see animals, each the product of long 

 evolution, occupying each its own environmental niche in the world. The 

 modern amoeba in its drop of water is admirably adapted to the condi- 

 tions of life as it finds them. It is not tending to become a "higher ani- 

 mal." There already are higher animals filling the available niches. But 

 hundreds of millions of years ago there were no higher animals; then 

 some one-celled animals having the necessary potentialities were pre- 

 sented with the opportunity to enter the vacant "higher-animal niches" 

 and did so. But still the "one-celled-animal niches" remained, and con- 

 tinued to be occupied by amoeba and its relatives to this day. Is not 

 the modern amoeba as successful in being an amoeba as we are in being 

 men? 



In the following chapters we shall note many instances of preadapta- 

 tion, as well as of the perfecting of adaptation of new structures once 

 they have appeared (postadaptation). In later chapters will be found 

 more complete discussions of the principles of evolutionary change 

 sketched above with the broadest possible strokes. The details of theory 

 can best be understood and appreciated after we have acquired a back- 

 ground of fact. 



References and Suggested Readings 



Darwin, C. The Origin of Species by Means of Natural Selection. 1859. Modern 

 Library series, Random House, New York; or Mentor Book MT294, New 

 American Library, New York. 



Goldschmidt, R. The Material Basis of Evolution. New Haven: Yale University 

 Press, 1940. 



Romer, A. S. The Vertebrate Story. Chicago: University of Chicago Press, 1959. 



