REVIEWS AND BOOK NOTICES. .541 
from such a sexual intermingling of the forms in question. But 
the real objection to the theory—granting the possibility of hy- 
bridization on such a gigantic scale, which seems really improb- 
able—is, that widely different forms occur also at different points 
in latitude, between which each successive stage of gradual differ- 
entiation can be readily traced, where hybridity can scarcely be 
supposed to account for the gradual change. Furthermore, grad- 
ual differentiation is now known in so many cases that it amounts 
to the demonstration of climatic variation as a general law, by 
means of which a species may be safely predicted to take on a 
given character under certain specific climatic conditions. If the 
theory of hybridity be urged to account for the intergradation of 
forms occurring at localities differently situated in respect to lati- 
tude, as has sometimes been done, it evidently falls under the 
Weight it has to support; and yet there seems to be little better 
evidence in its behalf in cases where the intergrading forms 
happen to be differently situated in respect to longitude. 
. To describe in detail, or even to give illustrations, of geograph- 
ical modification would require more space than would be proper 
to use in this connection, especially since a preliminary exposition 
of the facts upon which the preceding generalizations have been 
based, has already been presented in two papers in the Bulletin 
of the Museum of Comparative Zoology (Vol. ii, No. 3, April, 
1871, and Vol. iii, No. 6, June, 1872). 
REVIEWS AND BOOK NOTICES. 
RECENT PUBLICATIONS oN OrniTHOLOGY.— Like the pages of the 
Narorauisr with which our readers are of course sufficiently fa- 
miliar, recent issues of nearly all our scientific institutions show 
notable activity in ornithology, and a number of papers have 
accumulated on our table. In the Philadelphia Academy’s Pro- 
ceedings, Mr. Thos. G. Gentry has described peculiarities in the 
Nidifications of Sayornis fuscus (1873, p. 292) and Vireo solitarius 
(op. cit., 354); Mr. B. R. Hoopes has published a new variety, 
Krideri, of Buteo borealis (op. cit., 238, pl. 5) from Iowa, a pale 
race of the dry interior, apparently as distinct as some others now 
Currently recognized. In the Boston Society's Proceedings (xvi, 
