Recent Developments in Heredity and Evolution 5 



2. THE. FACT OF EVOLUTION 



It may be helpful to indicate some of the things that 

 began to open the eyes of thinking men and finally compelled 

 them to accept organic evolution as a fact. 



1. The growing proof that the inorganic world had been 

 formed by a process of slow evolution rather than by a 

 series of miraculous catastrophes, compelled the suggestion 

 that the organic world may have developed in the same 

 gradual way by natural processes. 



2. The observed intergrading of species, frequently so 

 complete as to make distinct boundaries for species impos- 

 sible, strongly suggested the passing of one into the other. 

 Dr. Asa Gray once remarked that he did not believe there 

 are any species of North American asters, although he had 

 been studying them for twenty years. Of course this was 

 an expression of despair rather than of belief, but it illus- 

 trates the situation. Botanists for a long time emphasized 

 the boundaries of species by preserving in their herbaria 

 what they called "typical specimens" and discarding the 

 intergrades, so that in turning over the sheets of a herbarium 

 the species looked quite distinct; but any excursion into 

 the field brought trouble. 



3. Observations began to multiply showing that plants 

 and animals are often able to respond to changed con- 

 ditions and change their own form or structure. This 

 was called the power of "adaptation," and it has been 

 a most persistent idea. The fact of change was evident, 

 but its explanation has been outgrown. But taking it 

 as a fact, it was evident that the small changes observed 

 would suggest the possibility of indefinitely extended 

 changes. 



