BOOK V. 
GEOGRAPHY. 
497 
" In separating cryptogamous plants into three groups, we 
observe that ferns are more numerous, the denominator of 
the fraction being smaller in the frozen than in the temperate 
zone. Lichens and mosses also increase towards the frozen 
zone. The geographical distribution of ferns depends upon 
the combination of local circumstances of shade, humidity, 
and moderate warmth. The maximum (that is to say, the 
place where the denominator of the fraction of the group 
becomes the smallest possible) is found to be in the moun- 
tainous parts of the tropics, especially in small islands, in 
which the proportion rises to ^, and even higher. Not dis- 
tinguishing the plains from the mountains, Brown finds the 
proportion of ferns in the torrid zone to be : in Arabia, 
India, New Holland, and Western Africa (within the tropics) 
it is 2V • our American herbaria only indicate : but ferns 
are rare in the wide valleys and arid table-land of the Andes, 
where we were constrained to reside a long time. In the 
temperate zone ferns are Vo? in France J3, in Germany, ac- 
cording to recent observations, The group of ferns is 
extremely rare on Atlas, and is almost entirely absent from 
Egypt. [In Sicily, Presl finds them q\; in Sweden, according 
to Wahlenberg, they are about tt&-] In the frozen zone 
ferns appear to increase to [There are none in Melville 
Island.] 
" MONOCOTYLEDONES. 
The denominator becomes progressively smaller in going 
from the equator to 62^ N. lat. ; it again increases in still 
more northern regions, on the coast of Greenland, where 
Gramineae are very rare. [Brown remarks that, in the 
list of Greenland plants, Dicotyledones are to Monocoty- 
ledones as 4 to 1, or in nearly the equinoctial ratio; and in 
Spitzbergen, as well as can be judged, the proportion of Di- 
cotyledones appears to be still further increased. This inver- 
sion was found to depend as much on tlie reduction of the 
proportion of Gramineai as on the increase of certain dicoty- 
ledonous families, especially Saxifrageae and Cruciferae. The 
flora of Melville Island is, however, very different, Dicoty- 
ledones being to Monocotyledones as 5 to 2, or in as low a 
K K 
