CH. XLI. DARWIN'S THEORY. 465 



Secondly, no two living beings are ever exactly alike ; 

 but children always inherit some of the characters of their 

 parents, so that if any being has a peculiarity which makes 

 it better fitted for its life, and consequently lives long and 

 has a large family, some of its descendants will most likely 

 inherit that peculiarity. 



Now it is not difficult to understand that if useful 

 peculiarities of different kinds are handed down in this way 

 from parent to child, those who inherit them will in time 

 begin to be remarkable for different qualities. For example, 

 suppose that in a nest of young birds, one with strong wings 

 lives and has young because it can fly far and get food, 

 while another also lives and has young because its feathers 

 are dark, and the hawks cannot see it in the grass. Then 

 those descendants of the strong- winged bird which also 

 have strong wings, will be most likely to live on in each 

 generation, and will pass on this peculiarity to their children ; 

 while the descendants of the dark-coloured bird will also sur- 

 vive in each generation exactly in proportion as their plum- 

 age is adapted to hide them ; and thus the strong-winged 

 birds and the dark-winged birds will in time become very 

 different from each other. This is roughly the theory of 

 * Natural Selection;' that nature allows only those animals 

 to live which in some way escape the dangers which threaten 

 their neighbours, and thus in time the race becomes altered 

 to suit the life it has to lead. 



There is only one difficulty. It is clear that the strong- 

 winged birds must not pair with the dark-winged birds, or 

 otherwise both peculiarities would come out in the young 

 birds, and the two kinds would no longer remain distinct. 

 And this is the one stumbling-block in the theory ; we 

 have never yet been able to trace out two varieties of an 

 animal which have become so different that they do not pair 



