XXVIII INTRODUCTION. 
islands, Australia, and India, nay even in the New World; for 
A. serra, L. & R., and its relatives belong to the latter, A. 
cuneatum, Lam., to the former. Here one or another of the 
middle forms of our Hawaiian and corresponding Indian and 
S. Polynesian species approach each other, so as to show no 
specific difference, while those next below or above in the series 
may diverge. Thus, New Zealand and Hawaii have A. caudatum, 
but A. falcatum, Lam., of the former island is replaced in the 
Hawaiian flora by my A. nitidulum; the lowest links, though, 
appear to be identical. What species in the other group 
corresponds to our first link pseudofalcatum elsewhere I have 
not been able to make out, but it is certain that its next higher 
neighbor, A. sphenotomum, has not been observed yet elsewhere, 
nor to my knowledge the next lower, A. lobulatum, Metten. 
Now the answer to the questions raised by the. preceding 
considerations is this, that the A. affine of the Hawaiian Islands 
neither derives from Mauritius, Ceylon, or any part of India, 
nor vice versa has been sent there, that the presence of A. cau- 
datum and A. horridum in countries where they are associated 
with A. contiguum does not need to be explained by the agency 
of birds, wind, or other conveyance, but that is has been evolved 
in loco; that originally spores of a few simple species have been 
diffused by known agencies over various countries, there have 
undergone slight modifications in character and evolved a series 
of higher forms; that in the evolution of these higher forms the 
latest modifications gained in prominence from generation to ge- 
neration, but that on the whole the form of the new generations 
was predetermined by the structure of the original immigrant, 
consequently they run parallel in the different countries whither 
the original species drifted; that therefore a gradual divergence 
took place in the offshoots of the original species, which divergence 
increased with the higher grades of development; that nevertheless 
according to the law of atavism exceptionally rapprochement 
occurred even in the higher forms. 
This theory would not be incompatible with the supposition 
that spores of a higher order of development reached a new 
country and the species pursuing its evolution upward reverted 
iso to the lower forms. 
The most interesting group of closely connected species is 
that of our Lindsayae, which stands unique, and, fortunately, has 
not been vitiated by wrong synonyms. 
