Recent Developments in Heredity and Evolution g 



people in general are more interested in speculation than 

 in plain facts ; and they are so interested in such speculations 

 as the origin of life and the origin of man that they come 

 to believe that these speculations belong to the scientific 

 study of organic evolution. But we are simply collecting 

 the facts of change and trying to discover the causes and 

 processes of change in the plants and animals that can come 

 under our observation. We can thus discover laws of 

 evolution, just as we discover the law of gravitation, by 

 observing them in operation. Of course the ultimate 

 questions continually suggest themselves, but it must not 

 be thought that any proposed answers to them are a part 

 of biological science. 



I. Environment. — The first attempt at what might be 

 called a scientific explanation of organic evolution, because 

 based upon observation, was that it is caused by changes in 

 environment. This explanation began to take definite form 

 during the last decade of the eighteenth century, in the 

 writings of such observers as Erasmus Darwin of England, 

 St. Hilaire of France, and Goethe of Germany. Environ- 

 ment is a term quite variable in its biological application, 

 but we do not need to discuss it in this connection. These 

 older observers saw changes occurring in plants and animals 

 (especially the latter), in response to changes in seasons, 

 in exposure, in climate, etc. ; and their picture of the process 

 of evolution was that plants and animals are plastic organ- 

 isms that are being molded by their environment. The 

 environment and the molding were not analyzed, but 

 thought of in a very superficial sense; so that it was not 

 long before it was recognized that the changes thus induced 

 are too superficial and ephemeral to furnish an adequate 

 explanation of evolution. But it must not be forgotten 



