DISTRIBUTION OF MAEINE ALG^. 173 



southern hemisphere, and we at once obtain conditions 

 suitable for the migration and mingling of tropical forms, 

 by way of the Cape of Good Hope. This consideration 

 shows us that the continental barriers, great though their 

 antiquity be, are, geologically speaking, modern obstacles 

 to the distribution of tropical forms. A comparison of the 

 tropical species of the Pacific with those of the Indian 

 Ocean, between which are no such barriers, would throw 

 a side light on the discussion of this question. Unfortu- 

 nately our knowledge of the marine flora of the Pacific does 

 not enable us as yet to deal adequately with the matter. 



How effective a barrier is maintained by areas of different 

 temperature in the ocean may be best seen by comparing 

 the widely different floras of the north temperate and 

 south temperate zones, separated as these are by the 

 tropics. Here is a barrier which, though it has shifted its 

 position with variations of climate, has yet been steadily 

 interposed between north and south. Want of light 

 prevents the migration of the temperate species via the cold 

 depths of the ocean beneath the tropics ; and, moreover, 

 the cold currents come to the surface in the warm ocean, 

 there to be heated. The marine floras on either side of 

 this warm barrier are radically different. In the Pacific 

 we have, in the great Laminarian Macrocystis, a striking- 

 example of wide distribution extending from the southern 

 ocean right up to San Francisco, if not beyond. In the 

 Atlantic we have two species of Fuciis (F. vesiculosus and 

 F. serratus) which occur in both north and south temper- 

 ate zones, but not between, and there are others ; but the 

 general truth of the wide difference between north and 

 south temperate zones, in the matter of marine vegetation, 

 is not weakened greatly by special exceptions however 

 striking. 



The barrier interposed by the depths of the sea is not 



