38 Bulletin Wisconsin Natural History Society [Vol. 12, Nos. 1 & 2 
as it ingresses other regions may encounter conditions more nearly 
optimum and may increase in productiveness, and may increase 
or decrease in size of individuals or remain constant. Neither 
increase nor decrease in size necessarily implies that the species is 
any the less adapted for successful existence. Could one contend, 
for example, that the little shrew (Sorex personatus) , the white- 
footed mouse {Peromyscus maniculatus) , or the chickadee would 
be “better adapted^’ for existence by a slight increase -in size? 
Or if slight increase in size were noted in any one of these animals 
in a certain locality, would this be reason for designating said 
locality a center of its dispersal? I think that in the case of verte- 
brate animals evidence is too scant for determining the center of 
dispersal by the location of greatest productiveness, and of maxi- 
mum size of individuals; I have, therefore, paid little attention to 
these two factors in my determinations of centers of dispersal. 
The location of great abundance of individuals is often of impor- 
tance in determining centers of dispersal, especially when the area 
of great abundance is relatively large. Here again, however, as a 
species ingresses other regions it may encounter ecological condi- 
tions more nearly optimum and may increase in abundance. An 
example of this is furnished in the distribution of Blarina hrevi- 
cauda. This shrew is without doubt of southern origin; conver- 
gence of lines of dispersal and location of closely related forms 
(both specific and generic) point clearly to a southern center of 
dispersal; yet Miller writes of its occurrence at Peninsula Harbor, 
Ontario, the most northerly known point of its central range, 
thus: “Although the range of Blarina hrevicauda has been supposed 
to be limited by the southern edge of the Canadian zone, the 
abundance of the species on the north shore of Lake Superior 
shows that it must be considerably extended. I have nowhere 
seen Blarina hrevicauda more abundant and destructive than at 
Peninsula Harbor ....’’ (Miller, 1897, p. 38). The loca- 
tion of greatest abundance of individuals is, however, probably 
more indicative of the center of dispersal, especially of recently 
established forms, than it is of the origin of distribution, as wit- 
ness those classic examples of the enormous increase of individuals 
following the introduction of the English sparrow, the starling, 
and the German carp into America, the rabbit into Australia, 
and the mongoose into the West Indies. Of course, in these cases 
