184 THE SCIENCE OF BIOLOGY 



evidence. Palaeontology shows a progressive appearance of 

 higher and higher forms of life in geologic time. There are 

 also special cases, such as the remains of the horse, elephant, 

 and the camel, which are very convincing (Figs. 24 and 25). 

 The present geographical distribution of animal and plant 

 life is, in many instances, explicable by reference to their 

 distribution at an earlier period. Innumerable facts receive a 

 rational explanation in terms of evolution. Physiological 

 study shows that the living substance of all animals and 

 plants exhibits fundamental resemblances in waste, repair 

 and growth, irritability, reproduction, and the like. Hence, 

 all protoplasm is perhaps genetically akin. Recent work on 

 the chemico-physical properties of blood is very impressive, 

 because it confirms conclusions regarding relationship that 

 have been independently derived from anatomy and embry- 

 ology. In all of the foregoing, the facts are as they should be 

 if evolution has occurred. The first three groupings are thus 

 circumstantial evidence. But all evidence for the larger 

 evolutionary changes must be of this nature. The strength 

 of circumstantial evidence lies in its amount and variety, and 

 in these respects this evidence is very strong. 24 



Fragmentary records of the changes in animals and plants 

 under domestication together with scientific investigations 

 in breeding, heredity, and variation constitute the evidence 

 of an experimental nature. The modifications observed are 

 small, as compared with those postulated by evolution. But 

 the time has been short, and little more than evidence for the 

 mutability of natural species or of domestic breeds could be 

 expected. The significant fact is that the experimental 

 results tell a similar story in favor of evolution. There 

 seems to have been a certain amount of evolutionary change 

 in our domesticated species during the past ten or fifteen 



24 These conventional lines of evidence are presented at length in many texts 

 which deal with evolution. G. J. Romanes, "Darwin and after Darwin," 

 Vol. I; S. Herbert, "First Principles of Evolution"; W. B. Scott, "Theory of 

 Evolution," all contain excellent statements. 



