40 
The late Dr . Jenner 
III. 
Mr. John Hunter, my late valued friend, and honoured 
preceptor, under whose roof I first caught a gleam of that 
light which so successfully conducted him through the ob- 
scure paths of nature, first demonstrated the different sizes 
of the testes of birds at different seasons of the year. On a 
farther investigation of this subject, a fact presented itself to 
me, which may not be unworthy of the attention of this 
Society, and, as it is in some measure connected with the 
preceding observations, I have taken the liberty of annexing it. 
In those birds that remain but a short time paired with the 
female, there appears a vast disproportion in the size of the 
testes, compared with those that live in the connubial state 
much longer. The cuckoo and the swift point out the fact 
most obviously. The common brown wren, which remains 
united with its female from the early part of spring, until the 
autumn, exhibits testes very far exceeding in size, either 
those of the cuckoo or the swift. The cuckoo, although a 
polygamist, may here be considered in the same point of 
view as the birds that pair. The time which he devotes to 
the female being so very short, more so indeed by some 
weeks than even that of the swift, the testes are formed ex- 
tremely small in proportion to the size of the bird. I never 
saw them exceed in size the common vetch, while those of 
the wren were full as large as a common sized garden pea. 
The medium weight of the cuckoo is about four ounces and 
a half, that of the wren but little more than three drachms.* 
* Ornithologists might easily have given us the weight of a bird with greater 
precision, by divesting the stomach of its contents, previous to the bird being 
