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. 



