( c ) 



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 

 entomopliilous 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 conspicuous- 

 ness brought about by increased height. 



The fuither growth of a class already proved to be large, 

 would play havoc with a definition of species rigidly based 

 upon discriminating structural characters alone. 



IV. Geographical Races or Suh-Species : — If we depend upon 

 unaided diagnosis there is no means of discriminating between 

 species and those sub-species of which the whole mass of in- 

 dividuals are distinguished by recognisable characters. Here 

 again the mere beginning of the difliculty is in sight ; for as 

 museums recognise more and more the necessity for long series 

 of specimens with exact geographical data, so will the compara- 

 tively simple conception 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 species, 

 but as it were trembling on the edge of disruption, ever ready, 

 by the development of pronounced preferential mating or by 

 the accumulated incidental effects of isolation prolonged beyond 

 a certain point, to break up into distinct and separate species. 



V. Results of Artificial Selection : — These obvious diffi- 

 culties encountered by a mechanical adherence to defini- 

 tion by diagnosis naturally lead to the consideration of 

 the further difliculties presented by domestic races of 

 animals and plants. The wide structural differences be- 

 tween the forms accumulated by human selection greatly 

 impressed Darwin. Thus he wrote to Hooker, September 

 8, [1856]: — "By the way, I have been astonished at the 



