418 The American Naturalist. [May, 
THE HEREDITARY MECHANISM AND THE SEARCH 
FOR THE UNKNOWN FACTORS OF 
EVOLUTION. 
By 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.”? 
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 his- 
tory 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 of science would open if we should do so and should 
accept as established, laws which rest so largely upon negative 
reasoning. 
The growing sentiment of the necessity of induction and of 
inductive evidence is the least conspicuous, but really the most 
1 From the ‘‘ Biological Lectures” of Marine Biological Laboratory, Woods 
Holl. This lecture is mainly from an article published by the author, in Merkel 
u. Bonnet: Ergebnisse fur Anatomie und ara p Freiburg, 
1894, and partly from a paper before the Biological Section of the British Asso- 
ciation for the Advancement of Science: Certain Principles of Progressively 
meth Variation observed in Fossil Series. Nature, August 30, 
? Osborn: Are Acquired Variations Inherited? Address before the ares 
Society of EN aturalists. Amer. Naturalist, February, 1891. 
