( 32) 
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 conspicuous- 
ness brought about by increased height. 
The further growth of a class already proved to be large, 
would play havoe with a definition of species rigidly based 
upon discriminating structural characters alone. 
IV. Geographical Races or Sub-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 difficulty 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  difli- 
culties encountered by a mechanical adherence to defini- 
tion by diagnosis naturally lead to the consideration of 
the further difficulties 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 
