336 Causes and Course of Organic Evolution 



Not only then is the evidence for a fresh-water origin of 

 plants preponderatingly perfect, if we regard mere number of 

 species and genera now existing, their entire evolutionary 

 and palseontological history emphasizes this. 



A second feature is that the sum-total of the forces that 

 represent an average land environment constitutes a greatly 

 more stimulating, molding, and proenvironing medium than 

 does sea water. For, though a few botanists have suggested 

 the derivation of sex cells, or even of important plant families 

 of dry land habitat, from the bro\^^l or red algee, nearly all 

 are agreed that a fresh-water ancestry in every respect suits 

 the case. 



The transition from a fresh-water to a marshy and at length 

 terrestrial environment must have been accompanied by 

 changes in the relation of the plants to environal conditions, 

 that could not fail to fundamentally influence, and stamp 

 themselves on, the organisms. The development of cuticular 

 protective tissue, of indurated strengthening strands or bands 

 or even sheets of fiber cells, the formation of transpiration 

 pores, the restriction largely or wholly of water absorption 

 to the lower or geotropic part of the plant, and of sap trans- 

 piration to the upper or heliotropic part, are only a few of 

 many changes that must gradually have been effected. 



But seasonal and environal variations during the annual 

 growth-period are much more pronounced in their action on 

 fresh-water algae than they could possibly be on marine ones; 

 while the crowding of the former on to a drier substratum, 

 or their proenvironal tendency to accommodate themselves 

 to this, would still further accentuate the forces of variation 

 in their action on fresh-water algae. Such suggest an expla- 

 nation, as several writers have pointed out, for the origin of 

 that striking phenomenon, in classes of plants liigher than 

 the algae, that is now known as "alternation of generations." 

 For the phase in the com])lete life-history of members of the 

 moss and fern alliances, that we now call the sexual or gamet- 

 ophytic (characterized also by a reduced or haploid number 

 of chromosomes) generation, suggests throughout adaptation 



