334 



THE THEORIES OF EVOLUTION 



tive explanation of heredity would apply most fit- 

 tingly. This disposes in a relatively satisfactory 

 manner of the question as to how those variations ap- 

 pear, acquire constancy and become the distinctive 

 characters of species. 



The next question which the theories of orthogene- 

 sis have endeavoured to solve is the following: why 

 do certain forms, certain characters succeed one an- 

 other, in the history of life, along a determined direc- 

 tion, without reiteration or retrogression. No satis- 

 factory answer has ever been offered and it is 

 doubtful whether our present state of knowledge can 

 enable us to find one. 



It may be also that the influence of certain factors 

 outlasts whatever direct action of those factors we 

 observe at the time and brings about changes whose 

 connection with those factors we fail to perceive. It 

 may be that the chemical constitution of the organism 

 limits in some unknown way the action of those fac- 

 tors to one single mode, excluding all other modes, 

 just as the eye reacts in always the same way upon all 

 stimuli whatsoever. In this connection we must con- 

 fine ourselves to mere hypotheses and as "internal 

 evolutionary tendencies" and other verbal explana- 

 tions of the same type explain nothing, we must con- 

 fess that here again the question is still open. 



The next problem to be solved is that of the gradual 

 differentiation of beings, of their organic advance. 



