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 
Enteromorpha (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 Alge. 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- 
