ADAPTIVE VARIATIONS. 



for instance, the evolution of a typical aquatic plant 

 from a typical terrestrial one. Supposing it were pos- 

 sible to estimate the extent to which characters useful 

 to aquatic life were present in a group of terrestrial 

 plants, and supposing we were to plot out the fre- 

 quency of their distribution, then this might take the 

 form of the curve given in the extreme left of the 

 upper portion of the accompanying diagram. Here 



Distribution of characters in plants. 



alter exposure for after i 

 typical one. generation to for many 



terrestial plant: *quoous environment: generations. 



aquatic pU 



plant. 



Distributior 



of eha -acters 



animals. 



10 



20 



80 



90 



1.08 



80 40 50 60 7.0 



Percentage of aquatic characters. 



FIG. 30. Evolution of the aquatic plant and the aquatic animal. 



we see that the most frequently occurring plant had 

 10 per cent, of " aquatic " characters, the extremes 

 ranging from to 20 per cent. Supposing now this 

 group of plants were exposed for one generation to an 

 aqueous environment. It would be found at the end of 

 that time that the proportion of aquatic characters had 

 considerably increased, say to 26 per cent., but the fre- 

 quency of distribution of the characters about the 

 mean would still be symmetrical as it was before, the 

 extremes now varying from 14 to 38 per cent. Some of 

 the plants, therefore, would still possess fewer aquatic 



