NATURAL SELECTION; SEXUAL SELECTION 63 



Natural selection does not create species, it enforces adaptation. 

 If a species or a group of individuals cannot fit itself to its 

 environment, it will be crowded out by others which can do so. 

 It will then either disappear entirely from the earth, or it will be 

 limited to that region or to those conditions to which it is 

 adapted. A partial adjustment tends to become more perfect, 

 for the individuals least fitted are first destroyed in the struggle 

 for existence. Very small variations may sometimes, therefore, 

 lead to great changes. A side issue apparently unimportant 

 may perhaps determine the fate of a species. Any advantage 

 however small may possibly turn the scale of life. "Battle 

 within battles must be continually recurring, with varying suc- 

 cess, yet in the long run the forces are so nicely balanced that 

 the face of nature remains for a long time uniform, though 

 assuredly the merest trifle would give the victory to one organic 

 being over another." 

 Darwin says: 



" I have found that the visits of bees are necessary for the fertili- 

 zation of some kinds of clover; for instance, twenty heads of white 

 clover (Trifolium repens) yielded two thousand two hundred and ninety 

 seeds, but twenty other heads protected from the bees produced not 

 one. Again, one hundred heads of red clover (Trifolium pmtense) 

 produced two thousand seven hundred seeds, but the same number of 

 protected heads produced not a single seed. Humble-bees alone visit 

 red clover, as other bees cannot reach the nectar. . . . Hence we may 

 infer as highly probable that, if the whole genus of humble-bees became 

 extinct or very rare in England, the heartsease and red clover would 

 become very rare or wholly disappear. The number of humble-bees 

 in any district depends in a great measure on the number of field mice, 

 which destroy their combs and nests; and Colonel Newman, who has 

 long attended to the habits of humble-bees, believes that more than 

 two-thirds of them are thus destroyed all over England. Now the 

 number of mice is largely dependent, as everyone knows, on the num- 

 ber of cats; and Colonel Newman says: 'Near villages and small towns 

 I have found the nests of humble- bees more numerous than else- 

 where, which I attribute to the number of cats that destroy the mice.' 

 Hence it is quite credible that the presence of feline animals in large 

 numbers in a district might determine, through the intervention 

 first of mice and then of bees, the frequency of certain flowers in 

 that district." 



