PART III. 

 CONCL USION. 



CHAPTER XLI. 



ALGAE AND BRYOPHYTA. 



THE general theory which may be based on the occurrence of anti- 

 thetic alternation in Archegoniate plants has been expounded in the 

 First Part of this book : the Second Part has been devoted to the 

 examination of those facts which specially bear upon the theory, as 

 they are seen in the several groups of Bryophytes and Pteridophytes. 

 It now remains to draw these facts together into a collective statement, 

 and to see how far they uphold the hypothetical position : at the same 

 time, the attempt may be made to formulate some general morpho- 

 logical and phyletic conclusions. It must be remembered, however, 

 while doing so, how fragmentary the series of genera and species, living 

 and fossil, actually is, and how incomplete the knowledge of the details, 

 especially in the fossils, in which developmental facts can rarely be 

 observed. These considerations will restrain any tendency to dogmatism, 

 and make such statements as are offered rank rather as tentative con- 

 clusions than as matters susceptible of ultimate demonstration under 

 present conditions of knowledge. 



It must be admitted at the outset that the theory of initiation of the 

 sporophyte by amplification of the zygote, by repeated cell-division in its 

 products, by sterilisation of some of them so as to form vegetative tissue, 

 and consequently by deferring of the tetrad-division, with its concomitant 

 reduction of chromosomes is not fully demonstrated by comparison of the 

 representatives of any one series of living organisms : there is no known 

 phylum which exemplifies all of these several steps ab initio. Nor is it 

 likely that there should be, if the biological advantage following on the 

 multiplication of spores in land-growing organisms were such as has been 

 suggested in Chapter VI. ; for it is not probable that those land-growing 

 organisms in which the sporophyte was nascent would have stood per- 

 manently still in the earlier phases of it : the probability would be that 

 all surviving forms would have proceeded some considerable length in the 

 direction of that biological advantage which follows upon a multiplication 



