Environment and Adaptation 187 



algae, probably not very different from some of the 

 existing fresh-water forms. The extraordinary 

 persistence of the motile reproductive cells, found 

 as high up as the lowest of the seed-plants, indi- 

 cates that the ancestors of the modern land plants 

 were algae, whose pedigree can be traced back to 

 free-swimming unicellular plants, resembling some 

 of the existing Volvocales. These green " Monads " 

 may be assumed to have abounded in the seas of 

 the earlier geological epochs. It has been assumed 

 that the water of the primordial seas was fresh or 

 only weakly saline, and would, therefore, have been 

 adapted to the existence of forms like the modern 

 green algae, which at present are mainly fresh-water 

 species. With the increasing salinity of the ocean, 

 many of these more primitive green algae probably 

 retreated to the smaller fresh-water bodies, where 

 they have persisted, perhaps but little changed, from 

 the remotest times ; while in the more saline ocean 

 water, the two groups of typical seaweeds, the red 

 and brown algae, have developed and taken the 

 place of the primitive green forms, their peculiar 

 characters becoming more and more pronounced 

 with the increasing salinity and other changes of 

 their environment. 



Uniform Conditions in Fresh Water. Except 

 for differences of temperature the conditions of life 

 in fresh water are very uniform, and it is not re- 

 markable that the range of structure in the fresh- 

 water algae is relatively slight. The most primitive 



