4 2 
The late Dr. Jenner 
purposes, would keep an accumulation of the proper powers 
in store, which, in the case of the cuckoo and swift, would be 
entirely useless. 
Whether there be a regular gradation in the size of the testes 
(that of the bird itself being considered), throughout the whole 
race, in proportion to the time taken up in pairing, I cannot 
determine, not having had an opportunity of subjecting the 
matter to a full investigation. However, I thought the fact 
already shown of sufficient importance in natural history, to 
be worthy of communication, as it forms a kind of sequel to 
Mr. Hunter’s paper on the subject. 
With due deference to the late Dr. Darwin, I am inclined 
to think that the opinion he set forth respecting the pairing 
of cuckoos, was taken up hastily, and that the birds which 
his friend saw were not cuckoos feeding their nestlings, but 
goat-suckers, whose mode of nesting corresponds with the 
relation given, and whose appearance might be mistaken for 
them by one not perfectly conversant with the plumage and 
the general appearance of cuckoos when on the wing. Is it 
probable that the cuckoo, which is invariably a polygamist, 
and never pairs, nests, or incubates in this part of the island, 
should fall into opposite habits in another part ? 
To recapitulate the substance of my observations. I have 
first adduced some arguments in support of migration, the 
fact itself not being generally admitted by naturalists of ce- 
lebrity, and also against the hypothesis of a state of torpor, 
or what may be termed the hybernating system. I have 
represented that the swallow tribe, and many other birds 
that absent themselves at stated periods, return annually 
