2l6 UNIVERSITY OF MISSOURI BULLETIN 



transformation hypothesis, which was based both on observa- 

 tional data and logical inference, was in existence and acces- 

 sible to all during the two or three decades preceding the 

 "Origin of Species," yet the prevailing belief at the time was 

 that no serious proof of evolution had as yet been advanced. 

 These proofs of organic evolution may be briefly set forth. 



1. Both Chambers in his "Vestiges" and Spencer in his 

 essay of 1852 had urged with much force the argument from 

 the general presumption of science against supernatural ex- 

 planations, as well as the special presumption from geological 

 uniformitarianism, and, although it had failed to impress nat- 

 uralists at the time, yet a few years later we find Huxley, 

 Tyndall, Romanes and others advancing precisely the same 

 grounds for the acceptance of evolution. 



2. Homologous Structures. Researches in the field of 

 comparative anatomy of vertebrates had long since led to the 

 formulation of the principle of the Homology of Parts, which 

 was the recognition of the fact that corresponding organs of 

 different animals or plants exhibit a fundamental resemblance 

 in their plan of structure, even though the functions of the 

 parts in question may be quite different (e. g., the wing of a 

 bird and the foreleg of a mammal). This principle had been 

 generally regarded by the eighteenth century evolutionists as 

 furnishing indisputable proof of community of descent, for 

 the reason why the wing of a bird is structurally similar to 

 the foreleg of a mammal is because both have been inherited 

 from an ancestor possessing a forelimb constructed on the same 

 fundamental plan. 



3. Variability of Animals and Plants. It has already 

 been mentioned that the phenomena of variability in domesti- 

 cated animals and plants and the production of new varieties 

 by the selection of variations had appealed with force to a 

 number of pre-Darwinian evolutionists, and that this group of 

 facts had been urged, especially by Maupertuis, Erasmus Dar- 



