Lecture XXIV 



THE HYPOTHESIS OF PERIODIC MUTATIONS 



The prevailing belief that slow and gradual, 

 nearly invisible changes constitute the process 

 of evolution in the animal and vegetable king- 

 dom, did not offer a strong stimulus for experi- 

 mental research. No appreciable response to 

 any external agency was of course to be ex- 

 pected. Responses were supposed to be pro- 

 duced, but the corresponding outward changes 

 would be too small to betray themselves to the 

 investigator. 



The direct observation of the mutations of 

 the evening-primrose has changed the whole 

 aspect of the problem at once. It is no longer 

 a matter dealing with purely hypothetical con- 

 ditions. Instead of the vague notions, uncer- 

 tain hopes, and a priori conceptions, that have 

 hitherto confused the investigator, methods of 

 observation have been formulated, suitable for 

 the attainment of definite results, the general 

 nature of which is already known. 



To my mind the real value of the discovery 



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