S16 Dr Yule on the Physical Distrlhutlon 
the line of snow, at the mean temperature 36 Fahr. Again, un- 
der the parallel 19° SO', Humboldt notes another member of the 
PINE family ( P. australis J, as occupying a zone of the height 
of 6000 feet, on Popoc in Mexico. 
Next to the Pines, the larches approach nearest the line 
of snow. This tribe was observed by Michaux, extending 
from the more elevated parts of the middle countries of North 
America, to the northward of St Lawrence and Hudson'^s Bay. 
within the Arctic Circle ; and other members of the tribe are no- 
ticed by Gmelin, approaching the same parallel in Siberia, al- 
though none of them, except the Cedar (L. cedrus), have 
hitherto been detected farther to the southward than Mounts 
Taurus and Libanus 
There are other tribes of this great series more distantly 
related to, but generally accompanying them, on the northern 
hemisphere. The Juniper and Yew, towards the Arctic Circle ; 
and towards the south, the Thuiacese, including the verticillated 
Cupressi, and their ally, the Taxodium of Richard -j-. 
The differences in the geographical distribution, then, is not 
confined to the distantly related tribes, but extends to those most 
nearly related, namely, distinct species of Pines, Firs, Spruces 
and Larches, found at different degrees of elevation, their distribu- 
tion being influenced, no doubt, by the same general laws, of the 
particular nature of which we are as yet too little informed to 
pronounce with certainty. The seeds of plants are dispersed, 
in a few instances, by man himself, granivorous birds and 
quadrupeds, — by the winds, and, in numerous instances, by the 
tides of the ocean. But even independent of climate, it seems 
evident, that the peculiar structure and economy of the several 
species must, in a great measure, finally regulate the permanence 
of their respective stations on the globe. It is in vain that the 
current from the Gulf of Mexico, flowing along the coast of 
North America, and setting eastward, deposits the seeds of 
various tropical plants on the inhospitable shores of Shetland 
* Is it not likely that other members of this tribe will be found on the Thibet 
Mountains ? 
t Annah's de Museum^ vol. xvi. 
3 
