256 INTRODUCTION 
will be to start with the simpler types, and to proceed to the more 
complex. The presumable course of progressive evolution will thus be 
followed, but only in the broadest lines. Paragraphs will be inserted from 
time to time, pointing comparisons from one phylum to another ; and thus 
some general conclusion may be arrived at as to the stability of the 
“working hypothesis.” 
It may be anticipated that the first place in the detailed description 
will accordingly be given to those Algae which show post-sexual develop- 
ments of the nature of a sporophyte, inasmuch as their nuclei have a 
double chromosome-number. But it seems unnecessary to give any more 
detailed account of these than that already embodied in Chapter V.: 
for at best these Algae only show that such post-sexual complications do 
exist among them, while none of them can be accepted as direct lineal 
progenitors of even the simplest of the Archegoniatae. It is therefore 
sufficient for our present purpose to recognise again the fact that they 
suggest how the antithetic alternation seen in the Archegoniatae may have 
originated. 
With these remarks the Thallophytes may be left on one side: it is 
reasonable to expect, however, that in the future a better knowledge of them 
may result in their being drawn more directly into discussions of the origin 
of alternation; but at present they have only a remote, and chiefly a 
theoretical connection with the question of the origin of a Land-Flora. 
Such materials as are available for the elucidation of this question are to 
be sought for in the study of the Archegoniatae, organisms which show 
themselves already fitted in greater or less degree for life on exposed 
land-surfaces. 
In treating the Archegoniatae there will be no need to give any detailed 
description of the gametophyte: this is already adequately done in the 
Mosses and Ferns of Campbell, and in the Organography of Goebel. It 
may be necessary to refer in some special cases to the gametophyte in 
order properly to understand the sporophyte which it bears ; but excepting 
in such cases the gametophyte will be omitted from our descriptions: and 
thus the subject resolves itself into a comparative examination of the 
sporophyte in the Archegoniatae, from the general point of view laid down 
in the foregoing chapters. 
