External and Internal Factors in Evolution 307 
a moss-rose on a Provence rose is a retutn to a former 
state, ... nor can the appearance of nectarines on peach 
trees be accounted for on the principle of reversion.” It is 
said that bud-variations are also much more frequent on 
cultivated than on wild plants. 
Darwin adds: “These general considerations alone render 
it probable that variability of every kind is directly or indi- 
rectly caused by changed conditions of life. Or to put the 
case under another point of view, if it were possible to expose 
all the individuals of a species during many generations to 
absolutely uniform conditions of life, there would be no 
variability.” 
In some cases it has been observed that, in passing from 
one part of a continent to another, many or all of the forms 
of the same group and even of different groups change in 
the same way. Allen’s account of the variations in North 
American birds and mammals furnishes a number of strik- 
ing examples of this kind of change. He finds that, as 
a rule, the birds and mammals of North America increase 
in size from the south northward. This is true, not only 
for the individuals of the same species, but generally the 
largest species of each genus are in the north. There 
are some exceptions, however, in which the increase in size 
is in the opposite direction. The explanation of this is 
that the largest individuals are almost invariably found in 
the region where the group to which the species belongs 
receives its greatest numerical development. This Allen 
interprets as the hypothetical “centre of distribution of the 
species,” which is in most cases doubtless also its original 
centre of dispersal. If the species has arisen in the north, 
then the northern forms are the largest; but if it arose in 
the south, the reverse is the case. Thus, most of the species 
of North America that live north of Mexico are supposed 
to have had a northern origin, as shown by the circumpolar 
distribution of some of them and by the relationship of 
