26o REPRODUCTION, EVOLUTION, ETC. 



Dictyotales, it is obvious that their ancestry is not to be found in the 

 present Hving forms of either group. It would therefore seem best 

 for the time being to derive the Fucales from the ancestral groups 

 of either the Laminariales or Dictyotales, recognizing that there is a 

 definite bridge in both cases, the gap being least perhaps between 

 the Fucales and Laminariales. The common race cycle found in the 

 Phaeophyceae with its irregular alternation of generations must have 

 evolved several times, the course of evolution probably being 

 determined by the morphological changes, e.g. corticated type, 

 reduced ectocarpoid type, reduced cable type. 



It has been pointed out that the Phaeophyceae can be divided 

 into two great groups, the Isogeneratae and Heterogeneratae, and 

 the latest schemes of evolution take these into consideration. In 

 both Iso- and Heterogeneratae there is a gradual transition from 

 isogamy to anisogamy, and on these grounds one can perhaps 

 postulate at least two major lines of evolution. The schema below is 

 an example of what can be obtained employing this line of approach, 

 which is probably more satisfactory than one that is purely morpho- 

 logical : 



Fucales 



Anisogamy 



Dictv^otales? 



Tilopteridales 



Cutleriales 

 Sphacelariales 



Laminariales 

 Desmarestiales 



Sporochnales 

 Advanced Ectocarpales (part) \ 



Chordariales 



Isogamy 



Reduced 

 Ectocarpales 



Anisogamy 

 A 



Dict>'osiphonales 



t 



Punctariales 



Isogeneratae 



Primitive Ectocarpales 



I 



Chlorophycean ancestry Heterogeneratae 



V? 



Isogamy 



The names of the orders given above do not imply that the 

 present living representatives formed the stages in evolution, but 

 that types more or less similar to them existed in the evolutionary 

 sequence. 



