219 
Arber . — On the Past History of the Ferns . 
to the position of a dominant factor in the vegetation of past or present 
times did not spring into existence abruptly, although in some cases it may 
have reached its dominant phase within a comparatively short geological 
period. It is true that, in the case of the Angiosperms, their advent 
certainly does appear to have been remarkably sudden, but one cannot 
help feeling that this conclusion, however much we may be driven to accept 
it on the present evidence, is really but the expression of our ignorance on 
the subject. 
Our difficulties, as regards the Leptosporangiatae, are increased by the 
fact that there exists, at present, a considerable gap in our knowledge. Of 
all the geological periods from which some of the plant remains are known 
to us, two are especially interesting in that they represent the most critical 
epochs in the past history of the Vegetable Kingdom. The earlier of these 
covers the period in which the change from the Palaeozoic to the Mesozoic 
facies of plant-life took place. The latter holds the secret of the evolution 
of the Tertiary-Recent from the Mesozoic facies. How, during this latter 
period, this transition was effected we know practically nothing at the 
present moment. Of the earlier epoch, our knowledge is somewhat more 
satisfactory, but this flora stands in urgent need of correlation and critical 
revision, and until this has been accomplished, a gap in our knowledge, as 
I have said, will still exist. So far as one may judge from the present 
evidence, it would seem that the flora of the Upper Permian and Trias 
(excluding the Rhaetic) was probably a true Transition flora, in which the 
older Palaeozoic types gradually died out, and gave place to plants of 
a newer facies, which quickly rose to the position of dominant types in 
Mesozoic times. It may be that the thin, subsidiary, incoming-phase of 
the life-line of the Leptosporangiatae is chiefly to be sought for in the rocks 
of this transition period, of which at present we know so little. I do not 
propose, however, to attempt to trace here the evidence of the Lepto- 
sporangiate ferns in these sediments. Such will scarcely be possible until 
the critical revision, above suggested, has been effected. 
If we pass back to the flora of the Carboniferous and Lower Permian, 
can we find any evidence of the Leptosporangiatae, either as a dominant or 
subsidiary factor in that vegetation ? This question is by no means a new 
one. It has been discussed on many occasions, among them in a paper by 
Professor Bower 1 , published in 1 89 1, in the fifth volume of the Annals. In the 
course of that contribution, which was in the main devoted to a considera- 
tion of such anatomical features of the modern Ferns as bear upon the 
question whether the Eusporangiate or the Leptosporangiate is the more 
primitive type of Fern, the author gave some account of the evidence 
derived from the fossil flora of the Palaeozoic Rocks. He concluded that 
‘ though it is not possible absolutely to deny the presence of Leptosporan- 
1 Bower (’91), p. 127 . 
