SEA AND LAND FLORAS 201 
Plants are represented in the aquatic flora; wind- 
pollinated flowers of a rather primitive type of 
structure are the rule in our lakes and rivers; which 
points to an early assumption of the aquatic habit, and 
suggests that the land is more favourable than the 
water for the evolution of higher types. 
While the fresh waters of the globe have thus 
acquired from the land an abundant population of 
higher plants, the presence of salt, in water as on 
land, has had a deterrent effect. The sea was at first 
fresh. The primitive ocean derived by condensation 
from a cooling atmosphere in the early days of the 
world’s history contained no excess of salts. Whether 
life arose while this condition still persisted it is not 
possible to say; but as the sea grew salter owing to 
the rivers bringing into it incessantly salts derived 
from the land, the Seaweeds alone of the great groups 
of plants adapted themselves to saline conditions, and 
the ocean is now their unchallenged kingdom. The 
divisions which are represented by the Mosses, Liver- 
worts, Club-mosses, Horsetails, and Ferns, have not, 
and so far as is known never had, a single repre- 
sentative in the sea. Only one or two Fungi—often 
symbiotically combined with Algz to form Lichens— 
and a very few Flowering Plants, have attempted 
marine colonization, after long ages spent on land; 
and they have met with indifferent success. As we 
pass from fresh to brackish water, the population 
decreases rapidly, till in the seas surrounding our 
islands only one Seed Plant—the Grass-wrack, Zostera 
marina—has adopted a habitat which is thoroughly 
marine, and very few are found in other parts of the 
world. A study of the meeting-ground of the land 
