CHAP. II.] BIRDS. 17 
return along the eastern coasts of the United States, and often 
fly from point to point across bays and inlets. They are then 
liable to be blown out to sea by storms, which are prevalent at 
this season ; and it is almost always at this time of year that 
their occurrence has been noted on the shores of Europe. It 
may, however, be doubted whether this is not an altogether 
modern phenomenon, dependent on the number of vessels con- 
stantly on the Atlantic which afford resting-places to the wan- 
derers; as itis hardly conceivable that such birds as titlarks, 
cuckoos, wrens, warblers, and rails, could remain on the wing 
without food or rest, the time requisite to pass over 2,000 miles 
of ocean. Jt is somewhat remarkable that no European birds 
reach the American coast but a few which pass by way of 
Iceland and Greenland; whereas a considerable number do 
reach the Azores, fully half way across; so that their absence 
can hardly be due to the prevailing winds being westerly. The 
case of the Azores is, however, an argument for the unassisted 
passage of birds for that distance ; since two of the finches are 
peculiar ‘species, but closely allied to European forms, so that 
their,progenitors must, probably, have reached the islands before 
the Atlantic was a commercial highway. 
Barriers to the Dispersal of Birds——We have seen that, as a 
rule, wide oceans are an almost absolute barrier to the passage of 
most birds from one continent to another; but much narrower 
seas and straits are also very effectual barriers where the habits 
of the birds are such as ‘to preserve them from being carried 
away by storms. All birds which frequent thickets and forests, 
and which feed near or on the ground, are secure from such 
accidents; and they are also restricted in their range by the 
extent of the forests they inhabit. In South America a large 
number of the birds have their ranges determined by the *ex- 
tent of the forest country, while others are equally limited to the 
open plains. Such species are also bounded by mountain ranges 
whenever these rise above the woody region. Great rivers, such 
as the Amazon, also limit the range of many birds, even when 
there would seem to be no difficulty in their crossing them. The 
supply of food, and the kind of vegetation, soil, and climate 
