Allen.] pi ics ae [June 19, 
‘North American continent as compared with those of the eastern 
portion. Again, in respect to insular regions, while the above men- 
tioned general laws of climatic variation are there evident, certain 
other exceptional modifications obtain, that seem to specially charac- 
terize those regions. ; . 
A word, in conclusion, respecting hybridity :—When comparatively 
few instances were known, in which specimens combined in various de- 
grees the characters of two quite distinct forms, their synthetic char- 
acter was generally explained by the theory of hybridity; but the 
irrefragibility of the evidence now at hand in proof of the intergrad- 
ation of such forms over large areas,—the transition being so gradual 
as to occupy hundreds of miles in the passage, — and also coincident 
with a similarly gradual change in the conditions of environment, to- 
gether with the demonstrable evidence of the power of climatic influ- 
ence, seems to furnish a far more satisfactory explanation of these -per- 
plexing phenomena. But an advocate of the theory of hybridity 
might still assume that this gradual transition over a wide area is no 
objection to the theory, since the gradual fading out of the impression 
of contact in either direction from the line of junction of the respective 
habitats of two forms is just the result that would be anticipated from 
such a sexual intermingling of the forms.in question. But the real 
objection to the theory — granting the possibility of hybridization on 
such a gigantic scale, which seems really improbable — is, that 
widely different forms occur also at different points in latitude, be- 
tween which each successive stage of gradual differentiation can be 
readily traced, where hybridity can scarcely be supposed to account 
for the gradual change. Furthermore, gradual differentiation is now 
known in so many cases that it amounts to the demonstration of cli- 
matic variation as a general law, by means of which a species may be 
safely predicted to take on a given character under certain specific cli- 
matic conditions. If the theory of hybridity be urged to account for 
the intergradation of forms occurring at localities differently situated 
in respect to latitude, 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 geographical 
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 
