4 INTRODUCTION 
has been in the water, and that the higher forms of either kingdom 
have assumed such structural and physiological characters. as enable them 
to subsist in greater independence of aquatic surroundings than their 
simpler progenitors. The present attempt will be to fill in certain of 
the details into this general scheme, as applied to the vegetable kingdom, 
and to present some connected story of how the transition may have 
come about, as it may be seen reflected in the plants themselves, 
whether of the present day or of the remote past. 
