cert: 
1910] Grinnell—Birds: Alaska Expedition, 1908. 493 
the Prince William Sound region at some remote period. This 
is a reasonable assumption from the seesaw-like elevation and 
depression to which the Alaskan coast has been subject, and 
which is even now in operation. Then the present avifauna is 
composed of birds which have entered the region from elsewhere, 
these having been permitted to do so as the new region became 
inhabitable and as the intervening barriers allowed. An exam- 
ination of the present ornis of the region shows that all the 
species represented may be traced to the two different sources 
already referred to, now geographically contiguous to the Prince 
William Sound region, namely, the Yukon and Sitkan districts. 
These two faunal areas had already become differentiation 
centers at least before the Prince William Sound region had 
become more than a meeting ground of invading species. 
Migrants may pass regularly over a wide area; yet the evi- 
dence teaches us emphatically that migrants do not ordinarily 
stop in their semiannual movements and start breeding centers 
at any point where the conditions become attractive (as is often 
assumed). Birds, whether migratory or sedentary, invade a 
new territory only by gradual extension of their breeding areas 
from season to season, physiographic conditions permitting. 
That there is apparently no geographic variation in the birds 
of the Sound from island to island or from island to mainland 
is possibly due either to (1) the narrowness of intervening water- 
ways, so that they may be so frequently crossed as to swamp by 
inbreeding any developing centers of differentiation; (2) the 
relative uniformity of ecologie conditions throughout the region, 
so that the forces are lacking which would carry on rapid 
divergent speciation; or (3), the submergence of the region and 
resulting isolation of colonies, so recent as not to have allowed 
sufficient time for the differentiation of local forms with per- 
ceptible characters. It is entirely possible that all three of 
these factors have contributed to the condition of uniformity 
displayed throughout the region by such susceptible birds as 
Melospiza and Passerella. 
In conclusion, it is safe to say that the Prince William Sound 
avifauna, althoueh younger than the Sitkan and Yukon ayi- 
faunas, and of a lower rank, is nevertheless well marked. This 
