43 
on the migration of birds . 
to the same spot to build their nests ; and at the same 
time that any inference drawn from this fact in support 
of a state of torpor, would be fallacious upon physiologi- 
cal principles. That certain periodical changes of the testes 
and ovaria, are the inciting causes of migration. I have 
stated many facts, hitherto, I believe, unnoticed, chiefly 
with respect to the cause which excites the migrating bird, 
at certain seasons of the year, to quit one country for an- 
other, (viz.) the enlargement of the testes in the male, and 
ovaria in the female, and the need of a country where they 
can for a while be better accommodated with succours for 
their infant brood, than in that from which they depart. 
It is also attempted to be shown, that their departure 
from this country is not in consequence of any disagree- 
able change in the temperature of the air, or from a scar- 
city of their common food, but the result of the accom- 
plishment of their errand, i. e. the incubation, and rearing of 
their young, and the detumescence of the testes and ovaria. 
That successive arrivals of migrating birds are attributable 
to the progressive developement of the generative system in 
the male and female : that progressive developements are 
wise provisions of nature ; that premature arrivals and de- 
partures are frequently to be accounted for on tbe same 
principle ; that the departure of the spring migrators is owing 
to a change in the testes and ovaria, the very opposite to that 
which took place in the spring ; that the departure of the 
young birds is not guided by the parent, but the result of an 
unknown principle. 
In the second part of this paper, some observations are 
made on the winter birds of passage ; that they quit their 
