246 REPRODUCTION, EVOLUTION, ETC. 



are shown in fig. 163, and a study of these enables one to make the 

 following generalizations : 



(i) The life cycle is by no means simple in most of these types 

 and it frequently has no fixed relation to the nuclear cycle or to the 

 cycle of reproductive bodies, and so it has been suggested that the 

 term "Hfe cycle" should be abandoned and replaced by the term 

 "race cycle" because that indicates more clearly the numerous 

 possible variations in the life history of any one species. Lying 



L An INJURIA 



Fucus 



NOTHEl/\ 



Fig. 163. Types of life cycle in the Phaeophyceae and their possible inter- 

 relationships. i?Z) = position of reduction division in the life cycle. 



behind the race cycle is the fundamental nuclear cycle, but this is 

 often obscured by the frequent repetition of any one generation. 

 Whether these variations in the life history of any one species, e.g. 

 Ectocarpus siliculosus, are to be related to differences in environment 

 or whether they are due to genetical differences is a problem that 

 still awaits solution. 



{2 a) Any thallus in the Ectocarpales, whether it be haploid or 

 diploid, can produce an unlimited series of the same generation by 

 means of zooids from plurilocular sporangia. In this connexion it is 

 extremely instructive to compare and classify the Phaeophyceae in 

 relation to the two types of sporangia. In Table II it will be seen 

 that one can distinguish two primary divisions if one regards the 



