256 REPRODUCTION, EVOLUTION, ETC. 



possess a more extensive knowledge of the cytological life cycles 

 among the Chaetophoraceae it would appear futile to speculate 

 further on the origin of the Phaeophyceae, and all that can usefully 

 be done in this chapter is to point out the various possibihties. One 

 further point remains to be added. In the present volume it has 

 been suggested that the similarity in life cycles and phylogenetic 

 histories leads one to the hypothesis that the three groups of algae 

 are perhaps intimately related. At the same time an attempt has 

 been made to indicate that there are other w^orkers who believe 

 that all three groups have had independent origins from different 

 sources, and that the various types of life cycle have evolved in- 

 dependently. At present the decision bet^veen these two courses 

 would seem to be largely a matter of opinion. 



*EVOLUTION 



Rhodophyceae 



It has already been suggested above that the primitive Rhodo- 

 phyceae, in particular the Eu-florideae, may have arisen from a 

 member of the Chaetophoraceae such as Coleochaete. It is only 

 proper, however, to emphasize that this is purely one viewpoint, 

 and that there are other workers who have sought for an origin of 

 the group from among the unicellular organisms, but unfortunately 

 there are very few members of the Protista which can be regarded 

 as possible sources for the red algae apart from Porphyridium 

 cnientum. An alternative hypothesis is that which considers the 

 Rhodophyceae to have been evolved from the Cyanophyceae, the 

 principal argument in support of this view being the resemblance 

 between the colouring pigments, primarily in colour because 

 it has recently been shown that the pigments are not identical 

 chemically. The principal objection to this theory is the absence 

 of any form of sexual reproduction among the Cyanophyceae, a 

 feature which renders the presence of highly specialized sex organs 

 in even the most primitive Rhodophyceae difficult to explain. Apart 

 from these theories, however, there is also the possibility that the 

 Eu-florideae have been evolved from the Proto-florideae, in which 

 case the origin of the latter group becomes of importance. Two 

 possible lines of evolution can be suggested, but it does not appear 

 feasible to discriminate in favour of either one : 



