» I A K Jiswr.;-. I * S»;ai U; Li»eu.-^1 



Fig. 228 Geographical distribution of Codium sect, bursa. (After 



Svedelius.) 



(4) In general, the Indo-Pacific region is more probable as the 

 home of the various tropical and subtropical genera and they can 

 be classified into : 



{a) Genera with no Atlantic representatives. 



{h) Genera with a few Atlantic species, e.g. Halimeda^ Caulerpa, 



Sargassum, Dictyota, Scinaia, Galaxaura. 

 The following genera are probably of Adantic origin : Dasycladus, 

 Penicillus, Cladocephalus, Batophora. 



(5) Several families in the Laminariales, e.g. Laminariaceae, 

 Alariaceae, are of Boreal Atlantic-Pacific discontinuity. These 

 famiUes must formerly have had a circum-arctic distribution but 

 were pushed south by the onset of the Ice Age and then they re- 

 mained in their new habitat when the ice retreated. In this case 

 change of climate in a former epoch provides a satisfactory ex- 



432 



