104 THE WORLD OF LIFE CHAP. 



generation. But if this special superiority has already 

 persisted in the family for several generations, and both 

 parents belong to this same superior stock, then the reversion 

 towards mediocrity is less marked, and the special quality 

 will almost certainly be transmitted, sometimes even in still 

 larger degree, to some members of the family. 



It is by acting on this principle that breeders of animals 

 or plants for special purposes are able to improve the race. 

 In each generation they choose the most perfect individuals, 

 from their point of view, to be the parents of the next 

 generation, rejecting or destroying all the inferior ones. 

 It is in this way that our race-horses, our best milking cows, 

 our heavy-woolled sheep, our quickly fattening pigs, our 

 luscious pears and peaches, and hundreds of others, have 

 been produced. Just in proportion as we have bred only 

 from the best for a long series of generations does the 

 transmission of these qualities become more certain and 

 the " recession towards mediocrity " appear to be abolished. 

 But it is not really abolished. The average to which there is 

 a tendency to return has itself been raised by careful selection 

 of the best for many generations, and the inferior individuals 

 which were once the average of the race are now so far 

 removed that they can exert only a very slight influence 

 on each successive generation. Owing to the numerical 

 law above referred to, after five generations of such selective 

 breeding it is about 100 to i against the inferior char- 

 acters of the original average stock reappearing in the 

 offspring, while if the operation has been carried on for ten 

 generations it is about 2OOO to I against such inferior types 

 presenting themselves. It is for this reason that our great 

 Colonial sheep and cattle breeders find it to their advantage 

 to give even thousands of pounds for pedigree bulls or rams 

 in order to improve their stocks. 



It is by what is substantially the same process, as we 

 shall see farther on, that Nature works to improve her 

 stocks in the great world of life ; and has been thus enabled 

 not only to keep all in complete adaptation to an ever- 

 varying environment, but to fill up, as it were, every element, 

 every different station, every crack and crevice in the earth's 

 surface with wonderful and beautiful creatures which it is 



