ON THE MIGRATION OF BIRDS. 21 
their winter retreats are well ascertained* ; but from 
what has been already advanced respecting these birds 
it will be seen that this fact may be satisfactorily 
proved indirectly, by a process of reasoning somewhat 
analogous to that adopted by geometricians in inves- 
tigating such propositions as do not admit of a direct 
solution, namely, by showing that the contrary sup- 
position involves an absurdity. It is absurd to sup- 
pose that the Summer Birds become torpid with an 
increased or an increasing temperature, or that they 
can change their feathers in such a state when the 
organs of secretion are known barely to perform their 
several offices; or that, under such circumstances, 
scarcely more than one fourth of those birds which 
withdraw in autumn should reappear in spring, 
though the same birds almost constantly return to 
the same haunts: these suppositions, I repeat, are 
* Adanson asserts that European Swallows pass the winter in 
Senegal, but does not particularize the species (see his ‘ Histoire 
Naturelle du Sénégal,’ p. 67); and it appears probable, from the 
observations of Mr. White’s brother (the Rev. J. White), who re- 
sided at Gibraltar (‘ Nat. Hist. Sel.’ pp. 87, 88, 139), that many 
of our Periodical Summer Birds may winter in Africa. The length 
and difficulty of such a journey are the chief objections which 
have been urged against this opinion; but they will cease to be 
looked upon as serious obstacles when we reflect that these birds 
may pass hence to the equator without crossing any great extent 
of sea, and that as they are continually advancing into better 
climates they are enabled to travel leisurely, there being no 
necessity for extraordinary haste. 
