Evolution of Plants 303 



day, a number of well-known instances might be cited such 

 as Lingula from, the cambrian beds, Crania and Rhynchonella 

 from the next higher or ordovician beds, all three being brach- 

 iopod genera; while the gasteropodous mollusc Capulus and 

 representatives of other groups might be cited. This is direct 

 proof that organisms still living may afford fairly accurate 

 data in some cases, regarding the main lines of evolutionary 

 advance in the past. But alongside this we must ever place 

 the great and demonstrable fact that wholesale destruction 

 of many thousands of genera has been effected during the 

 evolution process. 



We will now bring together statistics and structural details 

 regarding living plant species, and try to link these, so far 

 as possible, with groups of the past. 



The Blue-green Algae or Cyanophycese, which we have al- 

 ready regarded as including many of the simplest plants, con- 

 sist of 81 genera and 653 species. Of these, 47 genera are 

 purely fresh-water, 26 genera consist mainly of fresh-water 

 but in part of marine or brackish species, while 9 genera alone 

 are marine. The total number of fresh- water species (accept- 

 ing average results from the works of Gomont, Kirchner, 

 Tilden, and others) is 548, and of marine 105. Of the last 

 31 species are unicellular or loosely joined into threads, and 

 the remaining 74 belong to the nostocaceous, oscillarioid, and 

 higher groups. 



It thus appears that fully five times as many species are 

 fresh-water — including under this hot-spring and related areas 

 — as marine, and that of the latter the great majority are more 

 evolved and so later in origin. The primitive development 

 of the entire group therefore in land-locked fresh-water areas 

 is strongly suggested. But, when about 41 species (13: 253) 

 are met with in thermic waters at a temperature of 50-75° C., 

 their primeval origin in thermic areas amid the volcanic activi- 

 ties of the mid-archsean epoch seems highly probable. 



We need not sui)pose that even the simi)lest living thermic 

 species absolutely resemble their primeval ancestors. It is 

 by no means unlikely that the latter were of simi)ler structure 



