130 Bower . — Is E 11 sporangia te or Leptosporangiate 
ditions : as the Cactaceae and Euphorbiaceae have inde- 
pendently adapted themselves to life in regions of prolonged 
drought, by reduction of leaf-surface, and succulent develop- 
ment of the axis, so may these two distinct stocks have 
adjusted their details of construction to a damp and often 
shaded habitat. The thin texture of the leaf is clearly an 
adaptive character, since it recurs in three distinct families 
of Ferns, in species of Selaginella and Lycopodium , and also 
in a minor degree in certain of the higher plants, in all cases 
a concomitant of shady and damp habitat. The same is 
probably the case with the filamentous prothallus ; this 
seeming especially probable in face of the modifications of 
which prothalli are susceptible when grown in water : when 
the gametophyte is thus simplified in structure, the sexual 
organs must needs project, as they do in the Hymeno- 
phyllaceae. Thus the characters which are commonly cited 
as demonstrating an affinity between Mosses and Hymeno- 
phyllaceae may be recognised rather as being of a more 
directly adaptive nature, such as might be produced in the 
vegetative system of distinct groups of plants exposed to 
similar external conditions. Add to this the absence of any 
support for the affinity in the characters of the sporophyte, and 
the fact that the Mosses are entirely unrepresented in the 
earlier rocks, and do not appear until the Tertiaries, and the 
improbability of a true affinity between the Mosses and Hy- 
menophyllaceae becomes in my opinion very strong indeed. 
If this affinity be once given up, then the search for a point 
of attachment of the Filicineous series is necessarily trans- 
ferred to the Liverworts, with the further result that the 
closest similarity to be traced is then with the Eusporangiate 
Ferns: in the anatomical characters of the sorus and 
sporangium a measure of support may be found for the 
suggested affinity, which, however, I would put forward 
only with that reservation which is necessary where the 
facts are but scanty. The sporophyte of the Fern corre- 
sponds to the sporogonium of the Liverwort, and the isolated 
archesporial cells of the former to the united archesporium 
