COLOUR, DISTRIBUTION, ETC. 21 



The form and character of a particular species are 

 frequently much changed in specimens grown either at a 

 greater or less depth than usual. In the majority of 

 instances there would be a tendency to increase in luxu- 

 riance pari passu with the increase of depth. A plant 

 whose ordinary habitat was near high-water mark, would 

 attain a larger size in deep water, and one from beyond 

 the limit of the tide would be dwarfed by transfer to a 

 shallow pool. There are, however, instances where the 

 change would be in an opposite direction, and the de- 

 nizen of the shore would become stunted in deep water. 



The bed of the ocean resembles in a great degree the 

 surface of the earth. It has its mountains and its 

 valleys, its plains and deserts, its various kinds of rocks 

 and soils ; and these physical peculiarities affect vegeta- 

 tion in the sea very much in the same manner as they 

 do on land. Submarine mountains, valleys, or deserts 

 hinder the diffusion of the spores of Sea-weeds from one 

 coast to another, just as those on land interrupt the 

 spread of seeds. On the other hand, spores have the 

 water as a means of transit, and seeds the air ; but 

 while the former can all float, only a portion of the latter 

 are furnished with the means of flight. Hence the 

 barrier opposed to the migration of Sea- weeds is less 

 complete than it is to that of terrestrial plants. 



The temperature of the sea is less liable to variation 

 than that of the air, and the effect of climate on Sea- 

 weeds is not in consequence great. Some Orders and 

 genera are more or less confined to cold or temperate, 

 and some to warm regions, while others are more 

 generally diffused ; the different species of the same 

 genus, and occasionally even the same species, extending 

 over a wide range of latitude. Nature, it would seem, 



