RELATION OF PLANT PROTOPLASM TO ENVIRONMENT. 259 



must have occurred on a large scale. There and then environmental conditions 

 must have existed, that were eminently favorable for the upbuilding of complex 

 colloidal bodies, and so there and then the remote and probably simpler ance^ on 

 of our existing thermophilic algae may well have originated. 



It should further be added that, while extensive areas of hot spring activity 

 probably — almost certainly — existed, the geologist has advanced clearest 

 evidences to show that in the mid or late Archaean epoch, lands of denudation, 

 lagoon expanses, sea cliffs, and ocean areas were then established. 



If we attempt with the physicist and geologist to estimate the time-limit 

 that elapsed, say from the mid-Archaean to the close of that epoch, it is generally 

 agreed that the period is a long one. Thus Haeckel, using the data of various 

 geologists, considered that the Archaean and Cambrian rocks together averaged 

 63,000 ft. in thickness, and required about 52,000,000 of years for their forma- 

 tion, a length of time greater than that proposed by him for all of the later 

 formations up to this date. Geikie has said "The geological evidence indicates 

 an interval of probably not much less than 100,000,000 years, since the earliest 

 forms of life appeared upon the earth, and the oldest stratified rocks began to 



be laid down." 



Even if we limit the period from the mid-Archaean to the beginning of the 

 Cambrian to 20,000,000 of years, the active changes proceeding then, as testified 

 by the structure of the earth's crust, would be most efficient factors in evolving 

 and modifying plant species. So an abundant hot spring flora of primitive type 

 may slowly have originated and become distributed over wide regions during 

 the mid and late Archaean or Proterozoic epoch, and from this may have evolved 

 adaptive types of land, cool lake, lagoon, estuarine and sea dwellers. 



This brings us to the consideration of evidence in favor of adaptation and 

 modification of such thermophilic algae from a warm to a gradually cooler environ- 

 ment. With the exception of thermophilic bacteria, that are discussed later, no 

 other group of plants is capable of passing its entire life cycle at temperatures 

 between 50°-75° C. Yet even at the present day there exist 40 or more such 

 species of wide geographic distribution. A remarkable and unique feature of 

 the Schizophyceae, however, is that not only the genera, but even many of the 

 species, are capable of flourishing under most diverse environmental conditions. 

 In saying this the writer bears in mind that wrong specific identifications can 

 eadily be made in this group, since diagnostic descriptions are limited often to a 

 few characters. But discounting a wide margin of possible error here, the state- 

 ment just made seems eminently correct. 



Thus the genus Inactis includes species that grow in freshwater ponds and 

 there form calcareous pebbles, or that coat wet rocks or live in rushing cataracts, 

 that grow in hot water or that cling epiphytically on marine alga?. Species of 

 Spirulina may grow in brackish or salt marshes, in hot sulphur springs, on mAT \™ 

 algae, or in pools of fresh water. The single species 8. subtilissima is recorded 

 from thermal waters of Italy and Africa, from a salt creek in Nebraska, from a 



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