EVOLUTION 



265 



gave rise later to the Florideae. These, it is then supposed on 

 fossil evidence together with the similarity in pigmentation, were 

 followed by the Phaeophyceae and Chrysophyceae, the Chloro- 

 phyceae being the last group to appear. It must be pointed out, 

 however, that the absence of fossil remains does not necessarily 

 mean that a group was absent at any given period : many of the 

 Chlorophyceae are delicate forms and would not be preserved so 

 readily as the tougher fronds of the brown and red algae. The 

 oldest fossil Chlorophyceae to be recognized belong to the Sipho- 

 nales and Siphonocladiales and they show a high degree of differ- 

 entiation which suggests that, as a group, they were evolved at a 



Table V. Parallelism in evolution among the advanced types of algae 



^^ery early stage. With very little evidence to support the view, certain 

 workers consider that these siphonaceous fossils represent the 

 primitive green algae, the remainder, principally fresh- water forms, 

 developing much later. Whilst it is doubtful if many algologists 

 would subscribe to this interpretation, it is mentioned here because 

 it is felt that any suggestions, however likely or unlikely, open up 

 fresh fields of thought and investigation. 



On the more orthodox interpretation it would seem as if the 

 Cyanophyceae and unicellular Chlorophyceae were the most 

 primitive algae. Some of the Florideae, especially the Proto- 

 florideae, may have appeared quite early, whilst the Eu-florideae 

 perhaps developed somewhat later at the same time as the 



