ON THE MIGRATION OF BIRDS. 11 
cannot be the winter-retreats of the Periodical Sum- 
mer Birds, as their temperature is not far from the 
maximum when these birds retire, and is near the 
minimum about the time that they begin to appear. 
He then proceeds to observe (pp. 461, 462) that 
“very few arguments will be now required to demon- 
strate the impossibility of the analogy which is sup- 
posed to connect the periodical birds of summer and 
the sleeping animals of winter. It is sufficient barely 
to remark, that the former are never found slumbering 
with the latter near the surface of the earth, and 
deep caverns are proved to be unfit for the reception 
of any creature in the torpid season. Consequently 
the birds in question desert the temperate zones at 
the approach of winter, to seek a better climate in 
lower latitudes.’ These conclusions, however, will 
appear to have been formed rather hastily, when we 
consider what numbers of: Bats become torpid every 
winter in this country, and how rarely they are dis- 
covered in their dormitories. Might they not have 
been derived more satisfactorily from the circum- 
stance of the Summer Birds being seldom or never 
found abroad with the sleeping animals during the 
mild weather which we frequently have in winter? 
Bats, Hedgehogs, &c. usually appear when the mean 
daily temperature is about 50°; but I am not aware 
that there is a single instance on record of any of our 
Periodical Warblers, properly so called, having been 
observed in the cold season, either in a state of active 
