THE EVIDENCE FOR ORGANIC EVOLUTION 499 



evolutionary change can be ascertained apart from its causes. 

 Although this evidence for organic evolution as the fact is subject 

 to the limitations of all historic data, it has proved convincing to 

 biologists, because so many special cases tell the same story. 

 Perhaps no one of the principal lines of evidence would be suffi- 

 cient in itself, but taken together they are overwhelming. The 

 following outline indicates the relationship between these lines of 

 evidence and the order in which they will be presented: 



Evidence from Structure 

 Comparative Anatomy 

 Comparative Embryology 

 Classification 



Evidence from Distribution 

 Paleontology 

 Zoogeography 



Evidence from Physiology 

 General Physiological Resemblances 

 Specific Physiological Resemblances 



Evidence from Observation and Experimentation 



Unconscious Experiments upon Animals and Plants since their 



Domestication 

 Conscious Experiments by Breeders and Scientific Investigators. 



The first three groups include only indirect or circumstantial 

 evidence; that is, the facts are what one might expect if evolution has 

 occurred. The fourth may be called direct or observational evi- 

 dence, since it shows the evolutionary process at work. The lim- 

 itation of circumstantial evidence is that one does not observe the 

 occurrence; but such evidence may be convincing if extensive 

 and diversified. The limitation of observational and experimental 

 evidence is that so little evolutionary change can be observed in a 

 brief period of time. 



Evidence from Structure 



Comparative Anatomy. — Whether one examines the broader 

 features of Anatomy in the several phyla of animals or the lesser 

 structures in a single group, one finds everywhere facts that are 



