INTRODUCTION TO CRYPTOGAMIC BOTANY. 517 



dance ; and this development attains its maximum where to 

 these conditions is added a wooded, hilly or mountainous, 

 and rocky country. That this is the case in limited areas is 

 familiar to the local observer, and is nowhere more remarkably 

 evidenced than in Britain, as may be seen by a comparison of 

 the Fern floras of the drier eastern and damp western shores 

 of our kingdom, of the firths of Forth and Clyde, or of the 

 estuaries of the Thames and Severn, of Dublin and Killarney ; 

 and that it equally obtains over large areas is obvious from 

 many facts in their distribution. Thus, it is in the moist warm 

 islands of the Indian Ocean, Pacific, and Gulf of Mexico, that 

 the order reaches its maximum ; and whilst the north temperate 

 hemisphere produces scarcely one hundred species and no 

 approach to a tree fern, in corresponding latitudes of the 

 southern hemisphere a luxmriant fern vegetation prevails, as 

 in Tasmania, New Zealand, West Chili, and Juan Fernandez ; 

 and whereas arborescent ferns attain their boreal limits in N. 

 lat. 37° (and this only in the humid valleys of the Himalaya), 

 various species are found in perfection in S. lat. 40° to 50°. 



579. " Shade, although a very necessary condition for most 

 ferns, is yet of secondary importance ; for in a damp soil and 

 moist climate ferns often carpet the soil even when exposed 

 to the full light of the sun ; and a few species of Adiantum,, 

 Fteris, Oynvnogrammd, and Cheilanthes, inhabit the dry 

 rocks of South Africa, India, and Australia 



580. " The Arctic limit of ferns is found in America, at 

 Minto Inlet, lat. 70° N. and long. 120° W., whence Oysto- 

 pterisfragilis was brought by the officers of Captain McLure's 

 expedition on the Greenland coast ; and at Disco, where also 

 Polydichum Lonchites was found by Dr. Lyall. On the 

 east coast of Greenland Aspidium fragrans was the most 

 northern fern found by Captain (now Major-General) Sabine. 

 In Arctic Europe, no ferns have been found in Spitzbergen 

 or Nova Zembla ; only fourteen in Iceland ; and on the 

 mainland (Lapland), about fifteen species. Proceeding east- 

 wards, the increasing drought and inflection of the isothermaJs 

 in Northern Prussia, and still more in Northern and Central 

 Asia, remove the limits of ferns almost as far south as the Altai. 



