12 THE STORY OF BIRD-LIFE. 



should find that half of the whole number would 

 fall below, and half would rise above a common 

 mean. The mean, of course, would be different 

 for each test. Thus a large number of those 

 who were included as above the average in 

 height might find themselves bracketed as below 

 the average in power of endurance, and so on. 



In this tendency to vary we have the raw 

 material used in Nature's workshop in the manu- 

 facture of new types. 



Man long ago seized upon this fact and turned 

 it to his own advantage, as witness our garden 

 flowers and vegetables, and our domestic cattle. 

 These have only reached their present form by 

 slow degrees, each stage improving upon the last. 

 The result of such a series of progressive changes 

 must be to remove slowly from, and obliterate 

 in, the individuals concerned, the traces of their 

 ancestral likeness. 



In the case of domestic animals we can con- 

 stantly compare the latest variety with the 

 original stock from which it was derived. For 

 instance, all the domestic pigeons are descended 

 from our common rock dove. Selective breed- 

 ing — that is to say, breeding with a definite end 

 in view — has resulted in numerous varieties so 

 unlike one another, and the original, that did 

 they exist in a wild state, we should regard each 

 as a separate ''species." When it was desired, 

 for instance, to obtain birds with enormous fleshy 

 lobes or wattles round the beak and eyes, those 

 young, in which this feature was most developed 

 were selected to breed from, the rest went to 

 supply the table. The offspring of the selected 



