INTRODUCTION. Q7 
approach it more closely than any other species 
that occurs in Britain; therefore, assuming that two 
thousand ascertained ferns constitute a connected 
chain, it follows that in Britain a hundred links 
are wanting at that part of the chain where. the 
True Maidenhair is placed. No wonder, then, that- 
when we look at any list of ferns we are unable 
to understand the reason for placing the True Maid- 
enhair and its next-door neighbour side by side. But 
there is still another mode of accounting for some 
of the monstrous gaps observable in our _best- 
arranged series of ferns. The changes continually 
occurring in the condition of the earth’s surface} 
render large tracts of land incapable of sustaining 
any longer certain plants which formerly concealed 
the soil beneath their luxuriant foliage i we know 
that thousands of species formerly existing, but now 
existing no longer ‘as living things, are preserved for 
ever in the solid rock, on which their history is written 
by Nature's own hand. The hypothesis that Nature 
is compensating herself by new creations, or new 
developments, for the losses thus sustained is now 
very popular; but with the sober-minded it requires 
