THE MODERN IDEA OF EVOLUTION 



Darwin's Work Corrected and Amplified, But Its Essential Parts Not Supplanted- 

 Cause of Variations Still a Mystery But Manner of 

 Their Inheritance Made Clear 



IOOK at the surface of a pond 

 ruffled by the wind, and you see 

 that it is constantly changing. 

 Its aspect is different in each two 

 successive seconds, and yet after an 

 hour of this perpetual change, the pond 

 looks just as it did when you began to 

 observe it. Ceaseless change here pro- 

 duces nothing new ; when the wind dies 

 down in the evening the pond is no dif- 

 ferent than it was in the calm before 

 dawn. 



But ever since men began to leave 

 their thoughts on record, they have 

 made note of the fact that this is not 

 the most usual sort of change. Think- 

 ers have recognized that everything is 

 changing, and that when a living thing 

 changes, it usually becomes something 

 different from what it was before. They 

 have watched the progressive change of 

 a seed to a full-grown plant, of an egg 

 to an adult bird; and men with a his- 

 torical viewpoint have noted that sim- 

 ilar changes occur in institutions and 

 in nations, a small beginning leading 

 through a long series of changes to a 

 new condition, where differentiation 

 took the place of the original simplicity. 

 Greek and Hindu speculators surmised 

 that this condition of progressive change 

 applied to species of animals and 

 plants, one following another, so that 

 from simple beginnings complex crea- 

 tures finally developed. 



This idea of progressive change, of de- 

 velopment and differentiation, is the 

 idea of evolution. There is nothing new 

 about the idea — it is not at all a product 

 of modern science. As a philosophical 

 speculation it has existed certainly for 

 several thousands of years, and many 

 attempts have been made to convert it 

 from a pure speculation into a demon- 

 strated theory. 



During the past century, the attempt 



to find an explanation of the way in 

 which evolution might take place has 

 been more persistent than ever before. 

 Four main lines of speculation may be 

 distinguished. 



FOUR HISTORIC EXPLANATIONS 



1. It has been suggested that the en- 

 vironment acts directly on living crea- 

 tures and causes changes in them. This 

 \dew was elaborated about the begin- 

 ning of the last century by Geoffroy 

 St. Hilaire. He assimied that as the sur- 

 roundings of a plant or animal change, 

 the plant or animal itself must of 

 necessity respond by a change. He did 

 not assiime that the response to the new 

 environment was always a favorable one 

 or, as we say, an adaptation. If it was 

 unfavorable, the individual or the race 

 died out. If it was favorable, the 

 individual or race was able to meet the 

 requirements of its changed surround- 

 ings, and survived. 



St. Hilaire was unable to secure gen- 

 eral acceptance of his theory, because he 

 offered no adequate proof that things 

 ever happened as he described them; 

 yet his conception of evolution contains 

 elements that form the background of 

 our thinking today, and within the 

 last few years his explanation has been 

 revived in a mystical form by the 

 French philosopher Bergson, who has 

 secured for it a certain popularity among 

 laymen — ^not among biologists. 



2. The second of the four great his- 

 torical explanations appeals to a change 

 not immediately connected with the 

 outer world, but to one within the or- 

 ganism itself. It suggests that any 

 organ or structure of an individual that 

 is much used will increase in size and 

 strength, and this increase will be trans- 

 mitted to the individual's descendants. 

 Similarly, it is supposed that disuse will 



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