32 
DISTRIBUTION AND TOPOGRAPHICAL ASPECT. 
The species of Ferns known to botanists, including the 
lesser groups sometimes separated from what have been 
called the ^^true'’ Ferns, amount, according to the pub- 
lished estimates, to something more than three thousand. 
Their head-quarters are the humid forests of tropical 
islands, in some of which they acquire a giant size, and 
in their tree-like habit become rivals to the noble Palms. 
The tree Ferns are not, however, numerous, the number 
of species having this habit bearing a small proportion to 
those of shrubby or herbaceous growth. 
From the statistics which have been collected in refer- 
ence to this question, it appears that the Ferns bear a 
higher proportion to the flowering plants both towards the 
equator and towards the poles ; and that their proportional 
number is least in the middle of the temperate zone. They 
reach their absolute maximum in the torrid zone, amid the 
heat, moisture, and shade of the tropical forests ; and their 
absolute minimum on the inhospitable shores of the polar 
regions. 
