CHAPTER IX 



REPRODUCTION, EVOLUTION AND 

 FOSSIL FORMS 



*REPRODUCTION 



In this chapter it is proposed to give a general review of the various 

 reproduction cycles that are to be found in the three principal algal 

 groups, Chlorophyceae, Phaeophyceae and Rhodophyceae. It will 

 also be instructive to ascertain whether such a survey can lead one 

 to any helpful conclusions in considering evolution among and in 

 the different groups. A study of fossil forms is of fundamental 

 importance in any evolutionary or phylogenetic survey, but it 

 must be clearly understood, however, that as the fossil forms of 

 algae are largely confined to certain calcareous genera it is very 

 difficult to draw any decisive conclusions. As a result, hypotheses 

 must be based almost wholly upon living forms and these may have 

 advanced far from their primitive ancestors, and furthermore, 

 evolution may have proceeded at varying rates along the diiferent 

 lines. For this reason the bulk of the material set out in this 

 chapter can only he speculative, and students would do well to hear 

 this in mind. The necessity of basing hypotheses upon living forms 

 also leads to the further complication that different authors 

 inevitably propound schemes, and these may differ widely in repre- 

 senting their views of the lines along which the present living 

 species have evolved. Here again it cannot be too strongly im- 

 pressed upon the student that much of what follows must be 

 attributed to the author's personal opinions, and these are not 

 necessarily shared by other workers. The student should read the 

 additional literature critically and then attempt to work out his own 

 conclusions, and in this connexion it will often be found very 

 helpful to draw up some form of schematic diagram. 



As an essential preliminary it is convenient to recapitulate the 

 principal life cycles to be found in the three groups, pointing out at 

 the same time any problems that may arise immediately from such a 

 survey. The life cycles of representative genera in the Phaeophyceae 



