42 THE ORIGIN OF SPECIES ae 
guinea-pigs, by a means which he has discovered, _ 
is transmitted to their offspring.! 
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 7 
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 notéworthy 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 vertebre in the back ; 
in short, in precisely those characters in which 
1 Sompere Weismann’s Essays Upon Heredity, p. 310, e¢ seg. 
1893. ] 
