GEOGRAPHICAL DISTRIBUTION. 377 



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 

 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 



