28 
The late Dr. Jenner 
in the spring. These parts now begin to shrink,* the dispo- 
sition for raising a farther progeny ceases, and the nuptial 
knot is dissolved. What inducement have they to stay lon- 
ger in that country where, I think, it clearly appears their 
chief object is to multiply their species ? This being now 
effected, they retire to different parts of the globe, doubtless 
better suited to their general dispositions and wants, when 
disengaged from parental duties. In many of the migrating 
species, indeed in the far greater number, the disposition for 
farther incubation, and the season for their procuring a far- 
ther supply of insect food, cease at the same time. It is pretty 
evident from the habits of the cuckoo and the swift, that quit 
us in the summer as soon as their nesting is at an end, that 
swallows, martins, and those birds that disappear in the au- 
tumn, would depart at an earlier season, even though their 
supplies were to continue, if the rearing of their young were 
perfected. Indeed, as has been before observed, so strong 
does this propensity now and then appear, that it overcomes 
even the obligation of rearing their young when hatched late 
in the season, and they are sometimes left in a callow state 
to perish in their nests. This premature departure, probably 
arises from a reverse of that stimulus which occasions the 
too early migration of the spring birds, as has been noticed 
in a former part of this essay, namely, a change which 
takes place in organization. 
One of the most singular occurrences in the history of 
migration, is the mode of departure of the young birds from 
the country where they were produced. It may be conceived 
* I examined a female cuckoo the first week in July, and found the oviduct shri- 
velled, and all the eggs disposed of. 
