FIFTH LECTURE. 



THE HEREDITARY MECHANISM AND THE 



SEARCH FOR THE UNKNOWN FACTORS 



OF EVOLUTION. 1 



HENRY FAIRFIELD OSBORN. 



"Disprove Lamarck's principle and we must assume that there is some third 

 factor in Evolution of which we are now ignorant." 2 



Chief among the unknown factors of evolution are the relations which subsist 

 between the various stages of development and the environment. 



A STUDY of the recent discussion in the Contemporary Review 

 between Spencer and Weismann leads to the conclusion that 

 neither of these acknowledged leaders of biological thought 

 supports his position upon inductive evidence. Each displays 

 his main force in destructive criticism of his opponent; neither 

 presents his case constructively in such a manner as to carry 

 conviction either to his opponent or to others. In short, be- 

 neath the surface of fine controversial style we discern these 

 leaders respectively maintaining as finally established, theories 

 which are less grounded upon fact than upon the logical im- 

 probabilities of rival theories. Such a conclusion is deeply 

 significant ; to my mind it marks a turning point in the history 

 of speculation, for certainly we shall not arrest research with 

 any evolution factor grounded upon logic rather than upon 

 inductive demonstration. A retrograde chapter in the history 



1 This lecture is mainly from an article published by the author, in Merkel u. 

 Bonnet: Ergebnisse fur Anatomic und Entwickdungsgeschichte, Freiburg, 1894, 

 and partly from a paper before the Biological Section of the British Association 

 for the Advancement of Science : Certain Principles of Progressively Adaptive 

 Variation observed in Fossil Series. Nature, August 30, 1894. 



2 Osborn : Are Acquired Variations Inherited ? Address before the American 

 Society of Naturalists. Amer. Naturalist, February, 1891. 



