ON NORTH AMERICAN ZOOLOGY. 12& 



Writers on climate have remarked that the eastern coasts of 

 continents in the northern hemisphere have a lower mean tem- 

 perature than the western coasts. This is certainly true in the 

 higher latitudes of North America, for the winters are much 

 milder and the vegetation more luxuriant to the westward of the 

 Rocky Mountains *. On the coast of Hudson's Bay down to 

 the 56th parallel the subsoil is perpetually frozen, and further 

 inland in the 50th degree of latitude the mean annual heat is 

 only 36° F. and the ground is covered with snow for more than 

 six months in the year. Even in the 45th parallel on the north 

 side of the Canada lakes the frost is continuous for more than 

 six months, and the grallatorial and most of the granivorous 

 birds can find no means of support during the winter season ; 

 consequently the migration of the feathered tribes is much more 

 general than in the countries of Eui'ope lying under the same 

 parallels. Occasional frosts occur as low down on the Atlantic 

 coast as the confines of Florida, where during the late war 

 several British soldiers were severely frostbitten; this was 

 near the 30th parallel, or that of Morocco, Cairo, and Suez. In 

 Mexico and Old California there are also sharp frosts even on 

 the low grounds, from local causes. The severity of the winters 

 in the 40th parallel and even lower on the Atlantic coast of North 

 America destroys many evergreens which flourish all the year 

 in Scotland, 18 degrees further noi'th. 



The decrement of mean annual heat on an increase of lati- 

 tude is greater in North America than in Europe, and in the 

 former country there is a wider difference between the summer 

 and winter temperatures ; that is, the isothaeral lines in their 

 passage through America curve convexly towards the pole and 

 the isocheimal lines towards the equator. 



Vegetation (the growth of forests in particular) is more in- 

 fluenced by the amount and duration of heat than by the severity 

 of the winter cold. In countries whose mean heat is below 63°, 

 spring, or the renewal of vegetation, takes place, as Humboldt 

 has shown, in that month which attains a mean temperature of 

 33° or 34°; and deciduous trees push out their leaves when the 

 mean rises to 52°. It follows from this that the sum of the 

 temperatures of the months which attain the latter heat furnish 

 a measure of the strength and continuance of vegetation. On 

 the eastern coast at Winter Island, lying in latitude 64:^°, no 

 month of the year reaches a mean heat of 52°; but in the in- 



* Geologists may find it worth while to inquire how far the superior climate 

 of the Pacific coast is influenced by the active volcanos of the maritime Alps, 

 No recent volcanos exist in the Rocky Mountains or more eastern ranges. 

 VOL. V. 18.J6. K 



