54 Causes and Course of Organic Evolution 



Some show great capacity for resisting dessication at high 

 temperatures. But, from the standpoint of physico-chemical 

 relations to environment, they along with species of the Schizo- 

 mycetes are unique, in that they can grow and abundantly 

 multiply amid siliceous, calcareous, ferruginous, and sulphurous 

 waters where other organisms are rare or entirely absent. 



But the natural habitats of most existing species also indicate 

 extreme adaptibility. Thus the writer has calculated that 

 about 524 are purely fresh water or soil and rock growers, 24 

 live in brackish water, and 105 species in salt water.* Many 

 of the genera, like Calothrix and Rivularia^ include species 

 that may be fresh water or sea dwellers: some species of Dermo- 

 carpa are epiphytic on sea weeds, while others are normally 

 symbiotic on higher plants. In the genus Nostoc, one species, 

 N. pundiforme, regularly inhabits, and forms a bluish green 

 zone, in roots of Cycadacese or Sago Palms, an ancient group of 

 flowering plants, whose genera are at present distributed over 

 the globe. The writer and his students have studied eight 

 of the genera that now make up the group. These came from 

 Australia, Japan, China, India, South x\frica, Florida, Central 

 and South America, and the Pacific. In all of them the Nostoc 

 organism was abundant. An allied species N. sphcericwn is 

 equally widely distributed, and is associated Tvdth genera of 

 the Scale Mosses or Hepaticse. 



Again in Azolla, an aquatic ally of the ferns, Strasburger 

 has traced the Blue-green Alga, Anabcena azollce, in leaves 

 from specimens native to America, Africa, iVsia, and Australia. 

 Such facts, like all of those above given, conspire to stamp 

 the group as of remote antiquity. 



Excepting some of the Schizomycetes or Bacteria, no other 

 living group presents such evidence of primitive origin or of 

 graded and evolving modifications up to a certain stage. The 

 thermal-spring forms we regard as descended from and still 

 representative of archsean ancestors, while the other species 



* This reckoning has been made on the basis of Hansgirg's, Kirchner's, and 

 Tilden's works. Gomont reduces the number of species considerably, but 

 the ratio would be about the same for his species. 



