CH. XLI. NATURAL SELECTION. 467 



two thousand 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 ycais, 

 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 

 v/ill 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 few mice and many bees, and plenty of heartsease 

 and Dutch clover. Where there are few cats, on the con- 

 trary, the mice flourish, the bees are destroyed, and the plants 

 cease to bear seed and to multiply. And so you see that it 

 actually depends upon the number of cats in the neighbour- 

 hood how many of these flowers there are growing in our 

 fields. 



But now let us suppose for a moment that among the 

 field-mice there are some whose skin has a slightly peculiar 

 smell, so that the cats do not eat them when they can find 

 others. Clearly these mice would live longest and have 

 most offspring; and of these again, those with strong smell' 



