Observations on Periodical Birds. 145 
which have two broods in a season, ought to 
be the last that depart: and this is always 
found to be the case: so that whether the de- 
parture of these birds be influenced by a pro- 
pensity to moult or not, it seems to be regu- 
lated, in a great measure, by the cessation of 
their parental cares, and not by temperature 
solely. 
It will be difficult to produce any direct 
evidence of the migration of the periodical 
summer birds, until 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 most sa- 
tisfactorily proved indirectly, by a process of 
reasoning somewhat analogous to that adopted 
by geometricians in investigating such pro- 
positions as do not admit of a direct solution ; 
namely, by shewing that the contrary sup- 
position involves an absurdity. It is absurd 
© Adanson asserts that European swallows pass the winter in Sene- 
gal, 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 resided at Gibraltar; 
(Hist. Sel., p. 87, 88, 139 ;) that many of our periodical summer birds 
may winter in Africa. The length and difficulty of sucha journey, 
are the chief objections that 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. 
a 
