

DISTRIBUTION AND TOPOGEArHICAL 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 ^yhich 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. 



