VOL. I. ] Distribution of Land Birds. : 343 
all the same species. The affinity of the island form to Wood- 
house’s is shown by the blue under-tail coverts, by which both these 
two are distinguished from the California jay. In fact, all three 
forms seem to stand in almost perfectly parallel relations to one 
another, and to have been developed from the same stock. The 
most conservative estimates of the time which has elapsed since the 
age of ice, based upon the rate at which Niagara Falls has worked 
backward, place it at from seven to ten thousand years, which fig- 
ures may perhaps give some idea of the length of time which these 
species have required for assuming their present comparatively 
slight differences. 
To return from this digression, it appears that the theory applied 
by Prof. Le Conte in accounting for the distribution of plants will 
bold, tosome extent, with birds. The facts seem to indicate that 
the fauna, like the flora, of the islands: was, at the time of their sub- 
sidence, exclusively Sonoran in character, but that the isolation has 
not been so complete with regard to the birds as with the plants; 
consequently the number of peculiar species of birds is much less 
than of plants. Perhaps of even greater significance in confirmation 
of this theory is the fact told me by Dr. J. G. Cooper, that the shells 
of the island are mostly southern forms peculiar to Lower California. 
Such means of dispersal as effect the distribution of plants and birds 
not being at their disposal, it seems almost certain that they are the 
relics of the Sonoran fauna which once spread over the entire State. 
Ocean currents could not have carried them there, certainly, for the 
Japan current flows to the south. 
With regard to the distribution of birds on the Farallones little 
can-be said, as there is but one land-bird resident there—the rock 
wren (Salpinctes obsoletus)—and the island birds are apparently 
identical with the form found all over the State.* The absence of 
other resident land birds is to be accounted for solely by the bar- _ 
renness of the rocks, which are not capable of affording food and 
shelter to them. Many stray migrants reach the islands, however, — 
‘some of them being species which have never been recorded from 
any other locality in California. This brings us to the subject of 
accidental visitants to California, which will be discussed 1 in the — 
next paper. | 
_ “See. Bryant on Birds and Eggs from the Farallon Islands. Proc. Cal. Atad. 
Sci., 2d etadas i, p. 49. 
