Influence of Cold on Migrations of Birds. 123 



appear to be only about forty resident birds, including thirty 

 land and ten water birds, while there are no less than 296 

 migratory species, including also of course such as only tarry 

 for a short time on their way to and from more boreal regions. 



Seeing that winter lags to May, and sharp frosts set in 

 before the middle of October, it is evident many birds must 

 be expeditious in their movements, especially such as breed 

 in the country. I think with the exception of such hardy 

 wanderers as the migratory thrush, snow bird, chipping 

 sparrow, and a few more early arrivals, very few of the other 

 migrants remain long enough to enable them to rear two 

 broods; indeed several have barely time to bring up the first 

 before the approaching cold warns them to be off to more 

 genial regions. The flights of many birds from this and other 

 northern lands are often very rapid. One cold night may 

 make a difference, as I have often noticed, especially in the 

 case of the swallow tribes, more especially if accompanied 

 by a north wind, when the flock retreats before it. Then 

 many a rural scene is suddenly robbed of some its most 

 attractive objects. 



Now when we think of generations on generations of birds 

 being subjected constantly to the influences of this climate, 

 and the expeditious movements it engenders in comparison with 

 others, or even members of the same species that sojourn in 

 regions where they have not to endure such vicissitudes, 

 and the bird is put to no unusual exertion in procuring food, 

 we may well believe that the strongest and most vigorous 

 individuals are likely to be found among members who have 

 had their constitutions tried, and that these will, through 

 natural selection, attain advantages over the others ; hence 

 may be the origin of the differences in the outward appear- 

 ance, etc., of animals from different regions, as I shall point 

 out in the case of the woodpeckers, etc. 



It will be observed that in animals which change to white 

 in winter, especially the hare, weazel, etc., that the whiskers 



