160 NATURAL SELECTION VII 
comparatively short time the standard of our best racers might 
be reached. But that standard has not for many years been 
materially raised, although unlimited wealth and energy are 
expended in the attempt. This is held to prove that there 
are definite limits to variation in any special direction, and 
that we have no reason to suppose that mere time, and the 
selective process being carried on by natural law, could make 
any material difference. But the writer does not perceive 
that this argument fails to meet the real question, which is, 
not whether indefinite and unlimited change in any or all 
directions is possible, but whether such differences as do occur 
in nature could have been produced by the accumulation of 
variations by selection. In the matter of speed, a limit of a 
definite kind as regards land animals does exist in nature. 
All the swiftest animals—deer, antelopes, hares, foxes, lions, 
leopards, horses, zebras, and many others—have reached very 
nearly the same degree of speed. Although the swiftest of 
each must have been for ages preserved, and the slowest must 
have perished, we have no reason to believe there is any 
advance of speed. ‘The possible limit under existing con- 
ditions, and perhaps under possible terrestrial conditions, has 
been long ago reached. In cases, however, where this limit had 
not been so nearly reached as in the horse, we have been 
enabled to make a more marked advance and to produce a 
greater difference of form. The wild dog is an animal that 
hunts much in company, and trusts more to endurance than 
to speed. Man has produced the greyhound, which differs 
much more from the wolf or the dingo than the racer does 
from the wild Arabian. Domestic dogs, again, have varied 
more in size and in form than the whole family of Canide in 
a state of nature. No wild dog, fox, or wolf is either so 
small as some of the smallest terriers and spaniels, or so large 
as the largest varieties of hound or Newfoundland dog. And, 
certainly, no two wild animals of the family differ so widely 
in form and proportions as the Chinese pug and the Italian 
greyhound, or the bulldog and the common greyhound. The 
known range of variation is, therefore, more than enough for 
the derivation of all the forms of dogs, wolves, and foxes 
from a common ancestor. 
Again, it is objected that the pouter or the fan-tail pigeon 
