PREFACE vii 



lies outside the realm of possible proof under present conditions : the 

 theory is submitted as a working hypothesis. Naturally it is applicable 

 with greater readiness to those organisms which are less advanced, but 

 less readily to those which have departed furthest along the lines of 

 adaptation to life on exposed Land-Surfaces. Other opinions on the origin 

 and nature of Alternation have come into fresh prominence in recent 

 years, and especially the view that the present condition of the Arche- 

 goniatae has originated by differentiation of phases of a life-cycle originally 

 Homologous. This theory has not been disproved any more than the 

 theory of Antithetic Alternation has been proved Whatever view be 

 ultimately taken of the prime origin of the alternating generations, many 

 of the conclusions arrived at here as to the morphological progress and 

 phyletic grouping of the Archegoniatae will stand : they have a validity 

 of their own quite apart from any question of the ultimate origin of the 

 sporophyte, which has finally become the dominant factor in the Flora of 

 the Land. 



F. O. BOWER. 



GLASGOW, December, 1907. 



