GEOGRAPHICAL DISTRIBUTION. 877 
2. ON THE GEOGRAPHICAL DISTRIBUTION OF FERNS, 
As already stated, the number of species of Ferns 
amounts to about 2,500, which are widely spread over 
the earth, being found more or less numerous in all 
climates between the most northern and southern limits 
of vegetable life; and within the tropics from the sea 
level to elevations of from 14,000 to 15,000 feet, their 
number in any locality being generally in proportion 
to the degree of atmospheric moisture in conjunction 
with elevation. Few or none are found in dry or flat 
countries, such as grassy, scrubby, or thinly-wooded plains, 
or mountain slopes, extensive tracts of which are often in 
possession of one of the most cosmopolitan and abundant 
of all Ferns, the common Brake (Pteris aquilina). In hot 
and moist plains the number of species is comparatively 
small; even in the extensive lower valley of the Amazon, 
teeming as it does with vegetable life, the number of 
Ferns found by Dr. Spruce after he left the coast Flora at 
Para, in his journey of 2,000 miles, was small, but they 
gradually became more numerous on attaining an elevation 
of 1,500 feet; and in one locality at a higher elevation he 
found 250 species in an area of fifty miles in diameter. 
Another tract with but few Ferns is the dry region of 
Northern Africa and Arabia, and few as yet have been 
recorded from the interior of Australia; while on the 
i contrary, on many elevated coasts and islands they form a 
large portion of their Flora. 
~ They are most numerous both as regards genera and 
` Species in the tropical and sub-tropical regions; and 
. although, as a general rule, the different zones or latitudes 
. Contain species peculiar to the different regions, yet many 
