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 

 J/dsses 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. 



