ARBOR ESCF.NT FERNS. 4ii:i 
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 thei’eby im- 
parting freshness to the soil. Thus in Jamaica 
d^erns are to the Phanerogamioe nearly in the 
proportion of 1 to 10 ; in New Zealand as 1 to 6 ; 
*0 Taiti as 1 to 4 ; in Norfolk Island as 1 to 3 ; in 
^^t. Helena as 1 to 2 ; in Tristan d’Acunha (extra- 
fropical) as 2 to 3. Ferns are also the most abun- 
rdant 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 
eliniates, but that the enlarged size of the arbores- 
‘^ent species depends in a great degree on Tempe- 
‘'ature, 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. 
^rongniart has applied himself with much inge- 
^ruity, to illustrate the varying condition and cli- 
*riate of our Globe, during the successive periods 
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 
far south as lat. 46". See Brown in Appendix to Flinders's 
^°yage. 
