174 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



so effective, but it is yet a marked one. There is a very 

 considerable difference between the two sides of the 

 Atlantic. It is bridged in a way by the great currents. 

 The main Equatorial current — the parent of the Gulf 

 Stream — first crosses the Atlantic from east to west under 

 the tropics, and may there be considered a good bridge 

 since the distance traversed is not enormous and the 

 temperature is steady. But we know too little of the 

 marine floras of the African and South American coasts 

 to enable us to judge certainly of its effects. The Gulf 

 Stream crosses from west to east with too great a slant 

 to the north-east to have a very marked effect — it has had 

 time to cool somewhat, and the distance it travels is great 

 --but yet we have noted above that its effect is beyond 

 question. 



Sand makes deserts in the sea as on land. Every one 

 has observed such coasts of small extent, but they occur 

 over wide areas. For example, an enormous desert 

 extends along the north coast of the Gulf of Mexico from 

 Florida to Yucatan. Another occurs, as observed by 

 Kjellman, in the Siberian Sea; and these areas combine 

 with the depths of the ocean in forming barriers to distri- 

 bution. I think I hear some one saying, " why is it 

 necessary to lay down now and here these fundamental 

 conditions that hedge about the distribution of marine 

 Algae? Surely all that has been done long ago." It is true 

 that much that I have said, though true is not by itself 

 new, but my experience of the study of phycology is that 

 the idea of there being a distribution of Algae at all is in its 

 infancy. In the pages of the older writers we frequently 

 meet with remarks on the local distribution of Algae, in 

 standard modern works like those of the venerable Agardh, 

 the distribution of each species is carefully noted, but so 

 far as I know, very little has been done with the subject 



