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. 
Guascow, December, 1907. 
