THE TEETH OF MAMMALS. 269 



surface of land and sea has been of pretty nearly its present 

 form, to say nothing of the enormously longer earlier geo- 

 logical epochs to profoundly modify the breeds of dogs, of 

 horses, of numbers of plants, all of which are absolutely 

 known to have had a common origin, so in nature forces are 

 and ever have been in perpetual operation, which effect the 

 same thing. 



A pigeon fancier wants a pigeon of particular plumage, 

 with a few feathers a little different from any pigeon he has 

 ever seen or heard of^ 1 ) he knows by experience that little 

 variations are for ever arising, and that by watching a 

 sufficient number of young ones, and rigorously picking out 

 those which at all tend in the direction of what he wants, 

 he will get what he wants, and will even tell you with confi- 

 dence that in so many years he will make a breed with the 

 peculiarity desired. And exactly as the plumage that was 

 wanted is got, so in nature the tooth that is "wanted," 

 i.e., the dentition that is excellently well adapted to do its 

 work is manufactured by the operations of that law known 

 as " survival of the fittest." 



It is quite enough that one of the small variations for 

 ever arising in animals shall be of advantage to it, for us to 

 see that the peculiarity is likely to be transmitted and 

 intensified in successive generations. 



The question has been well presented by Mr. Wallace, 

 who points out that we must not think so much of varia- 

 tions in individuals as in groups of individuals : for instance, 

 it is a familiar fact that people vary in height, so that any 

 hundred persons may be divided into fifty taller and fifty 

 shorter. Now if a little extra height were of advantage, 

 many or most of the fifty would experience it ; though some 

 might not. In the same way if we grouped one hundred 



(*) An eminent pigeon fancier, Sir J. Sebright, told Mr. Darwin that he 

 could produce any given feather in three years. 



