EVOLUTION d 



10. Evolution. — This term is used by biologists to 

 describe the facts that the numerous varieties of both 

 animals and plants can be readily arranged into more or 

 less complete series gradually increasing in complexity 

 from simple one-celled forms at one end to the most 

 highly organized plants and animals at the other end; 

 and further, that these series closely parallel the arrange- 

 ment of the fossil remains of past geologic ages which 

 are found embedded in the rocks. Since it is obvious that 

 these series represent a gradual development of organ- 

 isms, many theories have been propounded to explain the 

 facts of evolution. Theory and fact must be carefully 

 distinguished by the student. The series of plants and 

 animals are unquestioned facts, whereas the explanations 

 are only more or less probable theories. The observations 

 and experimental data which have been collected within 

 the last twenty years have tended to eliminate many of 

 these theories because they do not agree with our new 

 knowledge, and to strengthen others because they are 

 more in accord with it. 



Evolution is linked in the minds of most people with 

 the name of Charles Darwin because he was the first to 

 put clearly before the world evidence in support of the 

 idea that all protoplasm is related; that the present 

 forms of life are descended from previously existing 

 forms ; and that in the course of descent variations have 

 occurred which were inherited; and finally, that those 

 inherited variations which were well fitted to their en- 

 vironment have survived. This idea has proved and 

 still is a most potent and fruitful stimulus to biological 

 investigation in all fields. 



11. The Distribution of Animals and Plants has 

 also long offered interesting problems both to the layman 

 and the scientist. The biologist must account for two 

 sorts of facts. In the first place he finds that widely 

 separated regions of the earth with similar climatic and 

 soil conditions have animals and vegetation of very simi- 



