MIGRATION. 29 



able to shift their quarters, so as to live throughout the 

 whole year in a temperature congenial to their constitution. 



3. Periodical Migration. Quadrupeds in general, from 

 their limited power of locomotion, cannot migrate from one 

 country to another, with ease and safety, in order to avoid 

 the variations of temperature which accompany the changes 

 of the seasons. In the same country partial migrations may 

 take place, as we witness in the stag and the roe, which 

 leave the alpine regions at the approach of winter, and seek 

 protection in the more sheltered plains. In America some 

 species of the genus Dipus perform still more extensive mi- 

 grations. Those quadrupeds y however, which have the fa- 

 culty of flying, as the bats, or of swimming, as the seals 

 and whales, may overcome the obstacles which oppose a 

 change of place, and execute safely periodical migrations. 

 Individuals of these tribes are accordingly observed to 

 avoid the extremes of heat and cold, by shifting their situa- 

 tions according to the seasons. The great bat (Vespertttio 

 noctula), which inhabits England during the summer, is 

 known to spend its winters in a torpid state in Italy. The 

 Greenland seal forsakes the icy shores which it has fre- 

 quented during summer, and migrates southwards, at the 

 approach of winter, to Iceland. 



The facts which have been ascertained in reference to 

 the periodical migration of quadrupeds, are too few for en- 

 abling us to point out the laws by which they are regulated. 

 The movements of birds, however, furnish more interesting 

 results. 



The migrations of the feathered tribes have been the ob- 

 ject of popular observation, since the days of the prophet 

 Jeremiah : " For the stork in the heaven knoweth her ap- 

 pointed times ; and the turtle, and the crane, and the swal- 

 low, observe the time of their coming." (ch. viii. ver. 7), 



