the more primitive type in the Ferns ? T29 
an exposed habitat, while the more delicate Leptosporangiate 
Ferns are less fitted to withstand exposure in dry air 1 . 
On grounds of comparison, which I now am disposed to 
think were insufficient, I then concluded that the series 
from the Hymenophyllaceae to the Marattiaceae represents 
an ascending series, illustrating the emergence of one phylum 
of the Vascular Cryptogams from the semi-aquatic to the 
aerial habit 2 . But, it may well be asked, have we not in the 
Leptosporangiate Ferns a peculiarly specialised type? They 
are found at the present day chiefly, and the Hymeno- 
phyllaceae, upon which the comparison with the mosses is 
specially based, almost exclusively, under the shade of larger 
growths 3 . Where in the earlier period would such shade 
be found? If, as I have elsewhere suggested 4 , the origin 
and differentiation of the spore-bearing generation may be 
correlated with the migration from water to the land, it 
would be contrary to all reason to suppose that the primeval 
forms were such as we now see specialised to life under dense 
shade. Moreover, the plants which constituted the chief 
terrestrial growths of the primary rocks were not distinctively 
shade-giving plants, such as the more modern Angiosperms, 
under the shadow of which such plants now grow : there is 
thus a presumption against specialised, shade-loving forms 
such as the Hymenophyllaceae being really primitive. I am 
now disposed, on considering all the evidence, to hold that 
the Leptosporangiate Ferns are derivative and specialised 
forms, while the Eusporangiatae represent the more primitive 
type : that the similarities of the vegetative organs, such as 
they are, between the Mosses and the Hymenophyllaceae are 
the result of specialisation along two distinct, but parallel, 
developmental lines, in accordance with similar external con- 
1 On the extreme susceptibility of the Hymenophyllaceae to exposure, and their 
special adaptation to life in wet, densely shady forests, see Giesenhagen, Flora, 
1890, p. 417. 
2 1. c., p. 374. 
3 Those more exclusively acquainted with the British Flora must bear in mind 
that Hymenophyllum tunbridgense, and Wilsoni, are exceptional among Filmy 
Ferns as regards their exposed habit. 
4 Annals of Botany, vol. IV, p. 347. 
