104 Mr Edmondston on the Colymhus Grylle, ^c. 
the perfect species, it is not easy to believe ; and here wha4 
seemed a plausible objection |to the opinion, becomes another 
presumption of its accuracy; 
The final causes of migration have been often too restricted, 
confined almost exclusively to incubation, food, and climate ; 
but these causes hardly apply in the present case ; for the indi- 
viduals of this species that winter in Zetland are in as good 
plight, and possess as great an abundance of food, as at any 
other season. We must recollect that animals do not exist merely 
to preserve themselves, and to continue their species, but to mi- 
nister to similar purposes in others, to become their food, or to 
diminish their number ; and migration facilitates these objects. 
What, for instance, should have become of one of the chief 
sources of the wealth of the Icelander, or of the “ light and 
life” of the Greenlander, if the eider-ducks, in the one case, were 
to remain all the year, where so many pass the winter ; or, in 
the other, if the seals should think proper to prolong their spring 
or autumn jaunts to those mysterious and unknown regions, 
where the hungry and superstitious fancy of the Esquimaux, 
impatient for their return, delights to exhaust itself 
But, the young birds may not be able to obey this useful and 
powerful law of migration, from not possessing strength suffi- 
cient to encounter the same fatigues and hardships as the old “ 
and hence are left behind till they have acquired the perfect 
powers of their kind. 
Temporary and local circumstances may also give rise to 
habits of migration, as well as instincts, proceeding from causes 
of general and permanent operation, and these may be continued 
through many generations, when the accident or necessity which 
first produced them no longer exists. Illustrations of this re- 
mark are familiar to every practical naturalist. 
The Grey Guillemiots found in Zetland in winter, are there- 
fore merely the young of the Black Guillemot remaining behind, 
while the parent birds migrate, and perhaps carry along with 
them a few of the more advanced and vigorous of their young. 
