Related Geological Conditions 27 



thickness of beds from the base of the Devonian to those 

 of recent date may be given approximately as 85,000 feet. 

 This would indicate that the beginnings of life date well 

 back into mid-archean or even older rocks. Such is the 

 conclusion that Patten {10: 23^) has already reached. All 

 such records, however, are confessedly imperfect and ap- 

 proximate. The want of record of organic remains in 

 Archean and even in some Cambrian rocks seems to be 

 wholly due to the soft perishable nature of the primitive 

 organisms. Only when sponges, hydroids, or actinioids be- 

 gan to secrete calcareous or siliceous or chitinous spicules or 

 tests, when previously protoplasmic echinoderms began to 

 form increasingly heavy tests as in the cystoids, or when 

 brachiopods and molluscs started, while in the "phylem- 

 bryo" or in the "veliger" stage of their phylogeny to 

 secrete a chitinous and later a calcareous shell, did the 

 likelihood occur of their being preserved as fossils. 



This circumstance greatly complicates the question as 

 to whether primitive organisms originated in a freshwater 

 or in a marine environment. The writer has already 

 shortly set forth his views (7:410) that certainly most 

 and probably all of the great animal groups as well as plant 

 groups started as inhabitants of lakes and swamps. Only 

 secondarily and still later did they migrate into marine 

 areas, or on to the land. In regard to fishes he hopes 

 in the subsequent context to prove clearly that such is emi- 

 nently true for them. With abundant and ever-increasing 

 material for study of the Crustacea, Arachnida, MoUusca, 

 and other groups, he hopes also to prove its correctness 

 in detail for them, as he already has in general outline. 



If, then, a decidedly more plastic state of the evolving 

 crust existed up to Silurian or even Carboniferous times, 

 such would give rise to several important conditions that 

 would all be highly favorable to organic development and 

 change. Thus, as compared with present distribution of 

 land and sea, a much more extensive expanse of land and 

 of freshwater areas must have existed above the sea-level, 

 while the ocean itself — existing as a highly plastic but 

 heavy and continuous mass — would tend to form sags or 

 depressions around or between the land areas. These de- 



