394 ADAPTIVE VARIATIONS. 



characters than were possessed by a small number of 

 the original group of plants, in accordance with Dar- 

 win's dictum concerning plants that " whether the 

 station (they inhabited) was unusually dry or humid, 

 variations adapting them in a slight degree for directly 

 opposite habits would occasionally arise." It will be 

 noticed that in the diagram the curve of distribution of 

 the characters is made slightly more flat topped than the 

 other curves, indicating that the variability of a group 

 of plants suddenly exposed to a changed environment 

 is increased. Supposing that this group of plants is 

 exposed to the aqueous environment for a number of 

 generations, then, through the cumulative action of con- 

 ditions of life, the adaptation will become considerably 

 increased, and the plants will now show, on an average, 

 say 40 per cent, of the aquatic characters of a typical 

 aquatic plant. This increase of adaptation from the 

 stage reached after one generation is supposed to be 

 more or less permanent and hereditary, or would still 

 be present if the plants were returned to their original 

 dry land environment. But, however many genera- 

 tions the plants be kept in their watery surroundings, 

 it is supposed that they will never become adapted to 

 it like typical aquatic plants. In order to evolve such 

 plants, Natural Selection must be present in addition, 

 and in this case the distribution of the plants, in respect 

 of aquatic characters, will ultimately arrive at that in- 

 dicated in the curve on the extreme right of the 

 diagram. 



The lower half of the diagram is meant to represent 

 the evolution of an aquatic animal, such as a mammal, 

 from a land animal. Such an animal would in the first 



