SINCE THE TIME OF CHARLES DARWIN, 



the concept of evolutionary change has permeated 

 the entire field of biology. It is accepted with- 

 out question by all working biologists, because of the immense 

 body of facts which now demonstrates its existence. Further- 

 more, change through time is now recognized as one of the 

 guiding principles of the universe, from stellar galaxies to hu- 

 man societies. Biologists of modern times are relatively little 

 concerned with showing that change has taken place. Even the 

 directions which evolutionary changes have taken in particular 

 groups of plants and animals have proven to be a less reward- 

 ing field of study than the mechanism of evolutionary change. 

 The challenging question which most modern evolutionists are 

 trying to answer is: "What forces or processes can bring about 

 the evolution of a population of organisms into a new type, 

 with a different appearance and a changed way of life?" 



This was, of course, the main question which Darwin him- 

 self was trying to answer. To do this, he developed the theory 

 of natural selection, for which he was able to gather together 

 a body of factual evidence, much of it experimental, which in 

 his day appeared impressive enough to convince most biologists 

 that his views were correct. 



But in the period after Darwin's death, and particularly in 

 the early part of the twentieth century, experimental biology 

 made great strides, both in the facts about living things which 

 it uncovered and the techniques of investigation which it de- 

 veloped. The biologists of this time, moreover, were concerned 

 chiefly with the mechanism of life. This mechanism can be 

 understood best by intensive studies of individual organisms, 

 while evolution is best understood by means of the comparative 

 study of populations. Consequently, biological techniques and 

 standards of experimentation far outstripped those which were 

 used by Darwin and which were adequate in his day, without 

 contributing much to our understanding of evolution. 



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