1909 | Grinnell—A New Cowbird. 279 
emphasize is that artemisiae is in all particulars a uniformly 
large edition of obscurus, and not nearly as closely allied to ater, 
although nearer the latter in the matter of bulk. <A survey of 
relevant cases among birds leads me to argue that as a rule mere 
uniform increase or decrease in size signifies less of phylogenetic 
separation than do changes in proportion of parts. In other 
words artemisiae has closer affinities with obscurus than with ater. 
This being the interpretation of the facts in the present case, 
then artemisiac is a derivative from obscurus stoek, from which, 
but more remotely, ater may have also been derived. The Y- 
shaped. geographie divergence of artemisiae and ater from 
obscurus is exhibited in the present ranges of the three forms. 
The northern limit of the range of obscwrus extends clear across 
the Mexican border of the United States, very close to the breed- 
ing range of artemisiae at the west, and very close to afer, in 
Texas, at the east. 
Should it be found that the ranges of artemisiae and ater 
adjoin somewhere across the Rocky Mountains, and should series 
of specimens be forthcoming from various localities from the 
west to the east, and should these demonstrate the existence of 
intergradation in all characters continuously between artemiisiac 
and ater, I believe there would still be little reason for consider- 
ing artemisiae a subspecies derived from ater (and in process of 
‘ 
invasion) or vice versa. The existence of such ‘‘intergrades,’’ 
where eastern and western races have met, might with good 
reason be looked upon as the result of long-continued hybridiza- 
tion. 
It would seem to me that the cowbirds afford a condition of 
affairs hke that so clearly demonstrated by Stejneger with regard 
to the origination of the various forms of the hairy woodpecker 
in North America. (See Auk XXIII, July, 1906, pp. 265-270). 
Stejneger says of the hairy woodpeckers: ‘‘We may then assume 
that previous to the Glacial period there lived in North America 
a hairy woodpecker small of size. The advance of the glaciation 
pushed the woodpeckers southward and in combination with the 
transgression of the Gulf of Mexico effected a separation of the 
southern Alleghany region from the western portion of our con- 
tinent which then as now formed the northern continuation of 
