INTRODUCTION 11 



The inquiry is worth prosecuting whether man 

 affects the distribution of seaweeds. Iron vessels are 

 much less adequately protected against fouling by 

 the growth of seaweeds than wooden ships, which 

 secure a considerable degree of immunity by the 

 exfoliation of their copper sheathing, and in spite of 

 many ingenious devices, the iron ship and steamer 

 require frequent docking, and when sluggish, a 

 greatly increased expenditure of coal for driving. 

 Though cosmopolitan species like the forms of 

 Enteromvrpha (the " grass " of seafaring language) 

 abound near the water-line, many others, often sea- 

 weeds of large size, occur beneath, especially when 

 the vessel has been long at moorings. With such 

 vessels traversing the sea in all directions, it is more 

 than probable that the acclimatisation of aliens 

 occurs, especially when the passage is from and to 

 similar regions. Vessels making the passage from the 

 Cape of Good Hope to this country across the 

 tropics arrive with cosmopolitan forms merely, as 

 might have been expected; but the Atlantic pas- 

 sage between this country and North America, for 

 example, deserves watching, and still more the Suez 

 Canal passage. 



In this way man may aid the ocean currents in 

 bridging the depths of the sea, which offer a barrier to 

 the distribution of coast Algae. Coast deserts of sand, 

 mud, and very friable rock in the sea are barriers 

 frequently of great extent. There are such for ex- 

 ample in the Gulf of Mexico from Florida to Yucatan, 

 in the Siberian Sea, and along the muddy coast of 

 western tropical Africa. Great irruptions of fresh- 



