Darwin's Artificial and Natural Selection 95° 
with feathered feet have skin between the outer toes, and 
those with short beaks have small feet, and vice versa. 
Another source of variation is that of reversion, or the 
reappearance in the offspring of characters once possessed 
by the ancestors. Finally, Darwin thinks that a source of 
variation is to be found in modifications due to the influence 
of a previous union with another male, or, as it is generally 
called, telegony. As an example Darwin cites the famous 
case of Lord Morton’s mare. “A nearly purely bred Ara- 
bian chestnut mare bore a hybrid to a quagga. She subse- 
quently produced two colts bya black Arabian horse. These 
colts were partially dun-colored and were striped on the legs 
more plainly than the real hybrid or even than the quagga.” } 
This case, however, is not above suspicion, since it is well 
known that stripes [often appear on young horses, and the 
careful analysis made later by Ewart, as well as his other 
experiments on the possibility of the transmission of influ- 
ences of this sort, puts the whole matter in a very dubious 
light. 
These citations show that Darwin recognized quite a num- 
ber of sources of variation, and, although he freely admits that 
“our ignorance of the laws of variation is profound,” yet 
some at least of these sources of variation are very question- 
able. Be this as it may, it is important to emphasize that 
Darwin recognized two main sources of variation, — one of 
which is the indefinite, or fluctuating, variability that appears 
constantly in domesticated animals and plants, and the other, 
definite variability, or a change in a definite direction, that can 
often be traced to the direct action of the environment on 
the parent or on its reproductive cells. It is the former, 
ze. the fluctuating variability, that, according to Darwin, has 
been used by the breeder to produce most of our domestic 
races. In regard to the other source of variation, the 
definite kind, we must analyze the facts more closely. 
1« Animals and Plants under Domestication,” Chap. IX. 
