210 LECTUEES ON BIOLOGY 



to the formation of new ' elementary species.' Nevertheless, the 

 theory of mutation cannot satisfy us as being the sole factor of 

 development, for it leaves unexplained above all the origin of 

 numberless adaptations. As mutations do not tend in any direc- 

 tion it would mean confiding too much in chance if we were to 

 regard it as the creator of organic fitness. 



If, in conclusion, we review the results at which we have 

 slowly arrived we shall be compelled to confess that research 

 and speculation has hitherto not been able to formulate a 

 universal law of evolution. We are still at the beginning of 

 a long, tedious journey. The deeper we seek to penetrate into 

 the mysteries of life the greater become the difficulties. All 

 theories that have been promulgated are only able to explain a 

 strictly limited part of phenomena, leaving by far the greater 

 number unexplained. Darwinism yields no information con- 

 cerning the causes of variability ; the theory of Lamarck fails in 

 front of passive adaptations ; the theory of mutation plays with 

 chance even more than selection ; and vitalism finally brings 

 forces into play, of the nature of which we are not even able to 

 form an idea. 



It is not pleasing to admit this insecurity, but a hypothesis 

 is nevertheless much to be preferred to wearing dogmatic blinkers 

 and resting in delusive security. It is true that there are many 

 investigators who regard all questions as solved, and' with the 

 aid of a few well-sounding phrases furnish an explanation of 

 every phenomenon ; but truth is not found with the dogmatist. 



We have, however, attained one result : no longer will it be 

 possible to shout : " Hie theory of selection, hie Lamarckism, 

 hie hypothesis of mutation ! " The forces which these different 

 theories regarded as the foundation of organic genesis do not 

 exclude each other, but we must rather assume, on the grounds 

 of our experience, that selection and mutation, direct influence 

 of environment, and internal forces of the organism, as yet 

 unknown but for all that not unknowable, all participate in the 

 evolution of organic life. 



