42 THE ORIGIN OF SPECIES n 



guinea-pigs, by a means which he has discovered, 

 is transmitted to their offspring. 1 



But a race, once produced, is no more a fixed 

 and immutable entity than the stock whence it 

 sprang ; variations arise among its members, and 

 as these variations are transmitted like any others, 

 new races may be developed out of the pre-exist- 

 ing one ad infinitum, or, at least, within any limit 

 at present determined. Given sufficient time and 

 sufficiently careful selection, and the multitude of 

 races which may arise from a common stock is as 

 astonishing as are the extreme structural differ- 

 ences which they may present. A remarkable 

 example of this is to be found in the rock-pigeon, 

 which Mr. Darwin has, in our opinion, satisfactorily 

 demonstrated to be the progenitor of all our 

 domestic pigeons, of which there are certainly 

 more than a hundred well-marked races. The 

 most noteworthy of these races are, the four great 

 stocks known to the " fancy " as tumblers, pouters, 

 carriers, and fantails ; birds which not only differ 

 most singularly in size, colour, and habits, but in the 

 form of the beak and of the skull : in the pro- 

 portions of the beak to the skull ; in the number 

 of tail-feathers ; in the absolute and relative size of 

 the feet ; in the presence or absence of the uropygial 

 gland ; in the number of vertebrae in the back ; 

 in short, in precisely those characters in which 



1 [Compare Weismann's Essays Upon Heredity , p. 310, et scq. 

 1893.] 



