GEOGRAPHICAL DISTRIBUTION; LIFE FORM 



433 



planation of the present discontinuity. Other genera, however, e.g. 

 Lessonia, Macrocystis, Ecklonia, are of Antipodes-Northern Pacific 

 discontinuity, Macrocystis in particular being primarily circum- 

 antarctic, after which it is absent from the tropics, to reappear 

 again on the Pacific coast of North America and around the shores 

 of South Africa. The two southern species of Macrocystis ap- 

 pear to be identical with the two species in the northern hemisphere 

 so that presumably they have disappeared from the intervening 

 warm zone. Again, it must be concluded that their migration took 

 place at a time when the temperatures of the ocean waters were 

 more equable, unless it is assumed that the species have since be- 

 come less tolerant towards temperature. 



Another interesting feature in geographical distribution, which 

 has been estabUshed by Setchell (1920), is the relation of the 

 various species to the isotherms. The surface waters of the oceans 

 are divided into zones according to the courses of the 10°, 15°, 20° 

 and 25° C. isotheres. The great majority of algal species are con- 

 fined to only one zone, a considerable nimiber occur in two, only a 

 small number occur in three zones, whilst the number extending 



_ Antarctic 



^i-r*^ Cold temperate 

 riiiKut Warn temperate 

 • • • • Sub- tropical 

 Tropical 

 Coastal upweliing 



Fig. 229 Littoral Provinces of the World. (Modified from 



Hedgpeth.) 



