ARBORESCENT FERNS. 46 
o 
favourable to the growth of Ferns are humidity, 
shade, and heat. These circumstances are most 
frequently combined in the highest degree in 
small and lofty tropical islands, where the air is 
charged with humidity, which it is continually 
depositing on the mountains, and thereby im- 
parting freshness to the soil. Thus in Jamaica 
Ferns are to the Phanerogamiae nearly in the 
proportion of 1 to 10 ; in New Zealand as 1 to ; 
in Taiti as 1 to 4 ; in Norfolk Island as 1 to 3 ; in 
St. Helena as 1 to 2 ; in Tristan d'Acunha (extra- 
tropical) as 2 to 3. Ferns are also the most abun- 
dant Plants in the Islands of the Indian Archi- 
pelago. 
It appears still further, that not only are certain 
Genera and Tribes of Ferns peculiar to certain 
climates, but that the enlarged size of the arbores- 
cent species depends in a great degree on Tempe- 
rature, since Arborescent Ferns are now found 
chiefly within, or near the limit of the Tropics.* 
From the above considerations as to the cha- 
racters and distribution of living Ferns, M. Ad. 
Brongniart has applied himself with much inge- 
nuity, to illustrate the varying condition and cli- 
mate of our Globe, during the successive periods 
of geological formations. Finding that the fossil 
* The few exceptions to this rule appear to be confined to the 
southern hemisphere, and one species is found in New Zealand 
as far south as lat. 46°. See Broivn in Appendix to Flinders' s 
Voyage, 
