8 Heredity and Eugenics 



one that prevailed for nearly one hundred years, was obser- 

 vational. Series of intergrading plants and animals were 

 observed, and by comparing them it was inferred that they 

 represented the series of transformations that had occurred 

 actually in nature. This has remained the only possible 

 method for the paleontologist, and he also has the advantage 

 of dealing with series of enormous length. It is to be 

 expected, therefore, that the paleontologist will be impressed 

 most strongly by those explanations of evolution which 

 have been derived from observation and comparison. 



The later method of attacking the problem, a method 

 that has developed with great rapidity during the last 

 decade, is experimental. Plants and animals are taken in 

 hand and are made to show their possibilities. 



It should be kept in mind that the problem is to explain 

 how one species can produce another. The study of organic 

 evolution deals only with the succession of forms, with the 

 production of new forms by previously existing ones. It 

 has nothing to say concerning origins. How the numerous 

 series of living forms may have originated is certainly 

 beyond the reach of biological science as yet. When one 

 goes beyond the observed changes, and tries to trace the 

 successions back to their source, he is in a region of specu- 

 lation, and outside the boundaries of science. One may 

 stand beside a great stream and discover that its waters 

 are moving; he also recognizes the direction of the move- 

 ment; but he can know nothing of the distant sources 

 of the stream, for he sees only a very small section. So 

 the scientific recognition of organic evolution simply 

 observes the movement and its direction. The sources are 

 far too distant for observation, and the possibilities are 

 too numerous for profitable speculation. It is evident that 



