86 BIOLOGICAL ASPECT OF ALTERNATION 
carbon assimilation, protection, propagation, and a variety of more special 
duties. 
In the present work it is not proposed to enter into any detailed 
discussion of those later evolutionary advances, which are more especially 
shown by the higher Seed-Plants. The attention will be chiefly directed 
to the more fundamental features involved in the establishment of the 
sporophyte as a factor in terrestrial vegetation, and chapters will be devoted 
to sterilisation, to the methods of increase and decrease in number of 
sporangia, and to the origin of appendicular organs. The attempt will be 
made to gather from living plants (and from fossils also as far as possible) 
such evidence as there is which. will bear upon the working hypothesis 
sketched in its broad outlines above. For this is the only satisfactory 
way of testing its validity in the absence of more direct evidence. Definite 
proof of the truth of the biological theory of alternation need not be 
anticipated. What is possible is to show such a degree of reasonable 
probability as will justify its acceptance. For this the evidence available 
appears to be sufficient. 
