Lecture XXIV 

 the hypothesis of periodic mutations 



The prevailing belief tliat 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 aefinite results, the general 

 nature of which is already known. ^i^Jld, 



To my mind the real value of the discovery 



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