5 88 
Notes. 
indicate decisively which theory is the correct one. The reasons 
which render one or the other view the more probable are bound 
up with the more general question of the course of descent in the 
vegetable kingdom. The question of the relationship between the 
main groups of plants is a very complex one. All that we need 
do here, however, is to recognize the existence of several alternative 
views, and the bearing of these on the two theories of alternation. 
The indications of alternation in the Thallophytes may be first 
referred to. These seem closely comparable to the simplest Liver- 
wort sporogonia, but it has not been suggested that any direct 
relationship exists in any case. The existence of these rudimentary 
sporophytes in various Green Algae, in Cystopus , and in an analogous, 
though distinct form in Ascomycetes and Florideae, is indeed strongly 
suggestive of their independent origin in the Thallophytes of the 
present day, and justifies us in considering it probable that similar 
developments may have occurred in the ancestral Algal forms from 
which the Archegoniates arose. But the further recognition of the 
possibility that the origin of the Archegoniatae may have been 
polyphyletic, and in particular that the Vascular Cryptogams may 
have had a line of descent from Thallophytes perfectly distinct 
from that of the Bryophyta, has a much more important bearing 
on the nature of alternation. The gap between Bryophytes and 
Pteridophytes is wide, and on this view would be an essentially 
natural one ; any attempt to bridge it would involve misleading 
conclusions. I do not wish to enter into the question of the 
polyphyletic origin of archegoniate plants further than to show that 
its possibility must be borne in mind in considering the nature 
of alternation. It may be pointed out, however, that such a view 
would appear to follow naturally from the supposition that the origin 
of the sporophyte was correlated with the spread of aquatic organisms 
to the land. It may be considered probable that a number of 
organisms in different places would have undergone more or less 
similar modifications. The homologies which exist between the 
spore-bearing generations of Mosses and Ferns are no less possible 
results of homoplastic developments than others in favour of which 
direct evidence exists. If the origin of the Pteridophyta has not been 
from the Bryophyta, the comparison between the sporogonia of the 
latter and the simpler sporophytes of the Vascular Cryptogams would 
lose much of its weight, since the two may have proceeded, as Goebel 
