MODIFICATION AND DIAGNOSIS 75 



Darwin considered that ' physical conditions have a more 

 direct effect on plants than on animals '. 1 Undoubtedly 

 the view at the time was that of Buffon, the idea of an 

 operation of the environing forces almost as direct as 

 those which produce the weathering of rocks or the 

 whitening of an exposed flint. But it is probable that 

 the more intimately we know the conditions of plant- 

 life, the more fully it will be recognized that all such 

 changes are adaptive. I will mention merely by way of 

 illustration that my attention has been called in recent 

 years to the dwarfing effect of the prevalent south- 

 western winds on the vegetation of the exposed chalk 

 downs of the Isle of Wight. It has occurred to me 

 as a mere suggestion, but one worth investigating, that 

 the effect of wind upon a tall flower-head might be such 

 as to render less easy and less frequent the visits of 

 insects. If this were so, it would perhaps explain why 

 certain species of entomophilous plants liable to grow in 

 such situations have gained a special susceptibility to 

 the stimulus provided by constant winds during some 

 particular period of growth. The absence of this stimulus 

 would also correspond to a condition in which the plants 

 would gain in the conspicuousness brought about by 

 increased height. 



The further growth of a class already proved to be 

 large would play havoc with a definition of species rigidly 

 based upon discriminating structural characters alone. 



d. Geographical Races or Sub- Species : — If we depend 

 upon unaided Diagnosis there is no means of discriminat- 

 ing between species and those sub-species of which the 

 whole mass of individuals are distinguished by recogniz- 

 able characters. Here again the mere beginning of the 

 difficulty is in sight ; for as museums recognize more and 

 more the necessity for long series of specimens with exact 

 geographical data, so will the comparatively simple con- 

 ception of the single species be replaced again and again 

 by the far more complex but much truer idea of sub- 

 specific groups still fused by Syngamy into a single 



1 See the letter from Darwin to Lyell [June 14, i860], Life and Letters, 

 vol. ii, p. 319. 



