Evolution of Plants 333 



remained almost wholly a fresh-water or terrestrial group of 

 organisms that have only rarely adapted themselves to marine 

 life. Such we regard as one of many proofs that sea- water 

 and a marine life are conserving and disseminating agents, 

 but permit of limited variation and slow advance by any one 

 group, and even of frequent extinction of whole genera or 

 tribes, through such environal changes as rise or fall in tem- 

 perature of sea water, volcanic submarine activity, discharge 

 of submarine gaseous effluvia, and other causes. A fresh- 

 water or land origin for all of the dominant lines of advance 

 is thus correlated ^^'ith a steady increase in species of saprophy- 

 tic and parasitic bacteria and fungi, that are closely associated 

 "VN-ith such lines. 



We have already accepted it that, while the green, the brown, 

 and the red algse have all descended from a cyanophyceous 

 fresh-water ancestry, the latter two early passed — possibly 

 in archsean times — in large measure into a marine environment 

 and have there slowly undergone subsequent variation, evo- 

 lution, devolution, or extinction. The green algse continued 

 in fresh- water surroundings and in preponderating numbers, 

 so that now there are IT'^ fresh- water to 59 marine genera, 

 and 1924 fresh-water to 391 marine species, or there are about 

 three times as many genera and five times as many species of 

 green fresh- water as of marine algae. So it is to fresh- water 

 green algae that botanists have turned in recent decades for 

 the origin of higher plants. 



An attempt will now be made to trace the main evolving 

 lines, and to note morphological modifications that have accom- 

 panied these. Here it can be at once clearly and unhesitat- 

 ingly postulated that rapid and advancing migration of plants 

 took place from a shallow fresh-water to a marshy and later 

 to a dry-land situation, and that this must have been effected 

 largely during later archa?an times, and accentuated during 

 the Cambrian epoch. For the appearance of scorpions and 

 insect-life, along with highly organized plant remains, in the 

 Silurian rocks necessitates a long previous period of evolu- 

 tionary activity on land. 



