22 THE MAIN CURRENTS OF ZOOLOGY 



field of biology and classes them as outstanding 

 advances. 



The Doctrine of Organic Evolution. The doc- 

 trine of organic evolution is so comprehensive in its 

 range that it has entered into the whole frame-work 

 of human thinking and its acceptance has so pro- 

 foundly changed conceptions of man and nature that 

 it has produced a great mental revolution. There is a 

 natural cleavage between thought before 1859 and 

 thought after that date regarding the biological 

 interpretation of the universe. The doctrine of 

 organic evolution has done more to enrich zoology 

 than any other advance. In its full sweep it is one of 

 the greatest acquisitions of human knowledge. Al- 

 though utilized in different departments of learning 

 it is a biological doctrine and receives its fullest 

 illustration in zoology. It is the recognition that the 

 higher animals have been derived by modification 

 from the simpler ones and it cleared up many of their 

 heretofore perplexing relations. It is the discovery of 

 the lineage of animals and plants. 



Although it is commonly believed that Charles 

 Darwin (Fig. 5) was the founder of this doctrine it 

 was clearly expressed more than fifty years before 

 the publication, in 1859, of Darwin's Origin of 



