30 PLANT LIFE 
must obviate the inevitable difficulties of 
existence in another way altogether. Some, 
like fungi and many parasites, have adopted 
the latter alternative; but as regards the 
vast majority of the green plants, we shall 
find that a recognition of these two dominating 
factors, water and light, will serve us in good 
stead, as furnishing an important clue to 
much of the complexity of structure to be 
observed in so many of the more advanced 
types of plants. Such complexity is intimately 
related with a corresponding differentiation 
and specialisation of function, and indeed 
it is largely to this circumstance that many of 
the more striking examples of “ adaptation 
to the environment ”’ are to be attributed. 
The best way of arriving at a clear concep- 
tion as to how the higher plants, with their com- 
plicated structure and high degree of differ- 
entiation, have come into existence in past 
times, is to study the more primitive types 
which illustrate various degrees of advance 
on the simple unicellular stage. The class 
of Algz, which includes the simpler water 
and land plants, will furnish us with excellent 
material wherewith to reconstruct, in outline 
at least, the course of vegetative evolution. 
This must not, however, be taken to imply 
that the simple types in question are to be 
regarded as new forms which are now on an 
upward grade of evolutionary development. 
They are to be understood, rather, as _per- 
manent representations of some of the phases 
