186 
that period.* This may be true to some extent; but the first 
appearance of birds in England must have been earlier, as their 
remains occur in the London clay of the Isle of Sheppey, where 
they are mixed up with vegetables and fruits.+ 
It is unnecessary, however, to say much on this part of our 
subject. Such of our native birds as were not in this country at 
the time of its final disunion with the continent, probably 
entered soon afterwards—or what is not unlikely—flitted back- 
wards and forwards at will, especially the migrating species. 
Now suppose a certain number of such birds were to arrive in 
this country from the Continent for the first time and at about 
the same time ; say nightingales, which species I select, for 
illustration of what I have to say, as being one whose range over 
England has perhaps been better determined than that of any 
other of our summer migrants. They would arrive in the first 
instance on the shores of the eastern and south-eastern counties, 
where they would settle down immediately—if they found all that 
was necessary for their support and comfort. What would they 
require and look for? Climate—of a temperature agreeable to 
their feelings ; food—such as they had been used to ; shelter—and 
especially such shelter as would be suitable for nidification. IEf 
the place in question supplied all these wants there they would 
remain; though the seasons might not be all alike, and they 
might be compelled to shift their ground in winter from cold or 
want of proper food, and go further S. returning to their former 
haunts in spring. 
Suppose now additional birds of the same species were to 
arrive in course of time on the same parts of our coast. It is 
evident that an unlimited number of birds of the same species 
could not be maintained at the same place. Those that came 
last might be as it were crowded out, and be compelled to push 
* See Elements of Geology pp. 158—160; also Antiquity of Man, 
chap. xiv, + Elements, p. 290. 
