428 ILLUSTRATION OF THE BOTANY OF { Filices. 
also explains why they decrease so rapidly from equinoctial regions to a little beyond 
the tropics; for instance, forming 1-9th of the vegetation of Phaenogamous Plants of 
Jamaica; but only 1-971st of the Flora of Egypt; and though the heat so ‘rapidly 
diminishes, yet the proportion of Ferns increases, as they form 1-31st of the vegetation 
of Scotland, 1-10th of Greenland, and 1-7th of North Cape. The further extension 
of Tree-ferns intc the southern hemisphere is thus accounted for, from the greater 
proportion there, of sea; they exist at the southern extremity of Van Diemen’s Land, 
and even at Dusky Bay in New Zealand, while they are not found beyond St. Helena 
and Silhet in the northern hemisphere: the former in 20°, and the other in 27° of north 
latitude. India in the northern parts, Africa, and the Mediterranean region, being 
dry, are therefore unfavourable to the existence of such plants. Hence we might infer 
the more moist state of the ancient world, from the great proportion of Ferns and Tree- 
ferns, which are found in the Fossil Flora. These views are completely borne out in 
India, whence a large number of Ferns have been obtained ; nearly five hundred are 
enumerated by Dr.Wallich in his Catalogue: many of them certainly from the islands 
of the Indian Archipelago, but the majority are from the Malayan Peninsula, 
Silhet, Nepal, and the Himalayas, as well as from the west coast of the Peninsula, 
where there is considerable moisture. Few are found in the plains of India, where for 
a great part of the year dryness prevails. In the author’s collection there are about 
eighty species, obtained chiefly from the tract of the Himalayas, between the Ganges 
and Sutlej rivers, and only one species from the plains of India, that is, Asplenium 
radiatum of Swartz, which is found at Saharunpore and Delhi, and along the banks of 
the Jumna, often on the sides of wells. This species is also found in Arabia and the 
Peninsula of India. The only other Fern I found in the plains was Cheilanthes dealbata ; 
but even this only near the Rajmahl hills, where there are also a few other species, 
as Lygodium microphyllum, &c. 
Among the Ferns, as in several other families, of the geographical distribution of 
which we have treated, there are several genera, common to both the Old and New 
World; of these many are found in the southern parts of Indfa, as the Peninsula, 
Bengal, and Silhet, whence they extend to the lower parts of Nepal, as Acrostichum, 
Hemionitis, Antrophyum, Meniscium, Tenitis, Nothoclena, Blechnum, Lomaria, Vittaria, 
Lindsea, Dicksonia, Cyathea, Ceratopteris, Hymenophyllum, Trichomanes, and Angiopteris. 
Allantodia, found in the Moluccas and New Holland, extends north on one hand to 
Madeira, and to Nepal and Japan on the other. The genera of which species extend to 
the more northern parts of the Himalayas are Grammitis, Cheilanthes, Adiantum, Pteris, 
Davallia, Gleichenia, and Lygodium. In these mountains, likewise, occur species of 
other genera, which are found in most parts of the world, and occurring with the above 
in other parts of India, such as Polypodium, Asplenium, Nephrodium, Aspidium, and 
~Ophioglossum. Pleopeltis, though differing little from Polypodium, has one species, P. nuda, 
common in the Himalayas; the others being found in Tropical America, at the Cape 
of Good Hope, and the Mauritius. Besides these, there are also species of Allosurus 
(Cryptogramma), 
