26 Evolution and Distribution of Fishes 



One effect of this would be the frequent formation of 

 fissures for the passage outward of chemically active gases, 

 and for the percolation inward of chemically active liquids 

 from the surface. These would hasten the decomposition 

 and denudation of exposed rocks, at the same time that 

 other masses were being deposited in freshwater or in 

 littoral marine areas. Furthermore, the writer has already 

 emphasized {g :2^g;i i^^) ^^^^ ^^^^ restricted areas of 

 the present day as the Yellowstone region, the geysers of 

 Iceland, the terraced hot-springs of New Zealand, and the 

 thermal springs of Japan, may have been of wide and 

 frequent occurrence during early geologic times. Though, 

 owing to subsequent denudation-change, the deposits formed 

 by them may have left small trace behind, it seems to be 

 amid such chemically and physically active centers that we 

 should expect to encounter the first dawnings of life. 



II. Geological and early biological relations. 



Now from the standpoint of organic evolution, it seems 

 necessary to accept that primitive plant organisms began 

 to form during the early part of the Mid-Archaean epoch*; 

 that these gradually gave rise to higher plants, and later to 

 the simpler unicellular animals; while throughout the Or- 

 dovician and early Cambrian periods types had appeared 

 that were, on the animal side, the ancestral originators 

 of the graptolites, corals, brachiopods, trilobites, and mol- 

 luscs of later Cambrian rocks. 



Now alike from the standpoints of morphological de- 

 tail as of neurological complexity, we may regard the fishes 

 of Silurian age as fairly intermediate between simple non- 

 nucleate unicellular animals on the one hand, and anthro- 

 poid apes on the other. The average thickness of beds, 

 from the base of the semistratified or stratified rocks that 

 are classified as mid-Archaean to the base of the upper 

 Silurian may be taken as 70,000 feet, from estimates for 

 the different formations made by Prestwich, Geikie, Haug, 

 de Lapparent, Chamberlin, Grabau, and others. The 



* The writer throughout will use the term Archaean rather than Proterozoic, as it is 

 a noncommittal term, and all available evidence indicates that simple cellular plants ap- 

 peared first, while primitive animal types branched off from these at a considerably 

 later time. 



