CH. XL. NATURAL SELECTION. 429 



attends to one peculiarity, which interests him ; but nature 

 does it very slowly, because no animal can live unless every 

 part of it is fitted for its life better than in those which are 

 killed off. 



Selection by Natural Causes. — Now Mr. Wallace has 

 calculated that one pair of birds having four young ones 

 a year, would, if all their children, grandchildren, and great- 

 grandchildren, lived and paired, produce about two thou- 

 sand million descendants in fifteen years. And Mr. Huxley 

 tells us that a single plant producing fifty seeds a year would 

 if unchecked cover the whole globe in nine years, and leave 

 no room for other plants. 



It is clear, therefore, that out of these numbers millions 

 must die young, and it is only the most fitted in every way 

 that can live and multiply. One example from Mr. Darwin's 

 book will show you how complicated the causes are, which 

 determine what particular kinds shall flourish. He tells us 

 that the heartsease and the Dutch clover, two common 

 plants, can only form their seeds when the pollen is carried 

 from flower to flower by insects. Humble-bees are the only 

 insects which visit these flowers, therefore if the humble-bees 

 were destroyed in England there would be no heartsease or 

 Dutch clover. 



Now the common field-mouse destroys the nests of the 

 humble-bee, so that if there are many field-mice the bees 

 will be rare, and therefore the heartsease and clover will not 

 flourish. But again, near the villages there are very few field- 

 mice, and this is because the cats come out into the fields 

 and eat them ; so that where there are many cats there are 

 i^^ii mice and many bees, and plenty of heartsease and Dutch 

 clover. Where there are few cats, on the contrary, the mice 

 flourish, the bees are destroyed, and the plants cease to bear 



