540 GEOGRAPHICAL VARIATION IN NORTH AMERICAN BIRDS. 
the reverse than otherwise of what is commonly supposed to he 
the result of sexual selection. 
Freely admitting, however, that both natural selection and sex- 
ual selection are causes of modification in the gradual differentia- 
tion of animals, I am led to regard them as secondary rather than 
primary elements, and that climate and other environing condi- 
tions take a larger share in the work than the majority of evolu- 
tionists seem willing to admit. Evidently no single law will 
explain all the phases of modification by descent, and in addition 
to those above alluded to, doubtless what Hyatt and Cope, among 
American zoologists, have termed the laws of acceleration and re- 
tardation are among the other causes of the modification. In 
birds, even, phenomena are apparent that cannot be strictly ad- 
mitted into the category of geographical or climatic variations, 
but seem singularly to combine some evident features of this a 
acter with a retention of a few embryonic characteristics, especially 
in respect to coloration, of allied intergrading forms, as OOE e 
some of the birds of the middle portion of the North American 
continent as compared with those of the eastern portion. Agam 
in respect to insular regions, while the above mentioned general 
laws of climatic variation are there evident, certain other a 
tional modifications obtain, that seem specially to characterize ! 
those regions, ; 
A word, in conclusion, respecting hybridity : — When mene 
tively few instances were known, in which specimens on 
various degrees the characters of two quite distinct p e és 
synthetic character was generally explained by the theory e E 
bridity ; but the irrefragability of the evidence now at h a 
proof of the intergradation of such forms over large son a 
transition being so gradual as to occupy hundreds of miles 10 e 
passage, —and also coincident with a similarly gradual per K 
the conditions of environment, together with the dom i "o 
evidence of the power of climatic influence, seems to farnish a 
more satisfactory explanation of these perplexing po that 
` But an advocate of the theory of hybridity might still pee’ the 
this gradual transition over a wide area is nd es contat 
theory, since the gradual fading out of the impression " atit 
in either direction from the line of junction of the pi patol 
habitats of two forms is just the result that would be an 
