20 
INTRODUCTION. 
IV. 
the waters which flow over submarine sands will carry the spores of the Algae with 
less injury than the winds of the desert will convey the seeds of plants from one 
oasis to another. It cannot, however, be doubted that submerged sands do exercise 
a very material influence on the dispersion of Algae, or their 
GEOGRAPHICAL DISTRIBUTION. 
Climate has an effect on the Algae as upon all other organic bodies, though its 
influence is less perceptible in them than in terrestrial plants, because the tempera- 
ture of the sea is much less variable than that of the air. Still, as the temperature 
of the ocean varies with the latitude, we find in the marine vegetation a corre- 
sponding change, certain groups, as the Laminarice, being confined to the colder 
regions of the sea ; and others, as the Sargassa , only vegetating where the mean 
temperature is considerable. 
These differences of temperature and corresponding changes of marine vegetation, 
which are mainly dependent on actual distance from the equatorial regions, are 
considerably varied by the action of the great currents which traverse the ocean, 
carrying the waters of the polar zone toward the equator, and again conveying 
those of the torrid zone into the higher latitudes. Thus, under the influence of the 
warm waters of the Gulf Stream, Sargassum is found along the east coast of 
America as far as Long Island Sound (Lat. 44°). And again, the cold south-polar 
current which strikes on the western shores of South America, and runs alone; the 
coasts of Chili and Peru, has a marked influence on the marine vegetation of that 
coast, where Lessonia , Macrocystis, Durvillcea , and Iriclcea , characteristic forms of 
the marine flora of Antarctic lands, approach the equator more nearly than in any 
other part of the world. 
The influence of currents of warmer water is also observable in the submarine 
flora of the west coast of Ireland, where we find many Algse abounding in lat. 53°, 
which elsewhere in the British Islands are found only in the extreme south points 
of Devon and Cornwall. These, and other instances which might be given, are 
sufficient to show that average temperature has a marked influence in determining 
the marine vegetation of any particular coast. 
Seasons of greater cold or heat than ordinary have, as might be inferred, a cor- 
responding action. This is particularly noticeable among the smaller and more 
delicate kinds which grow within tide marks, and are found in greater luxuriance 
or in more abundant fruit in a warm than in a cold season. And the difference 
becomes more strongly marked when the particular species is growing near the 
northern limit of its vegetation. Thus in warm summers, Padina Pavonia attains, 
on the south coast of England, a size as large as it does in sub-tropical latitudes ; 
while in a cold season it is dwarf and stunted. 
In speaking of the difference in colour of Algse, I have already noticed the pre- 
valence of particular colours at different depths of water. A corresponding change 
of specific form takes place from high to low water mark ; and as the depth 
increases, the change is strikingly analogous to what occurs among land plants at 
different elevations above the sea. Depth in the one case has a correspondent 
