252 



SCIENCE. 



[N. S. Vol. VIII. No. 191. 



It would seem from these data that the 

 physical condition of the sea and atmos- 

 phere was favorable to the existence of types 

 for aught we know quite or nearly as highly 

 specialized as those of the same classes now 

 in existence. Life and nature in the Pre- 

 cambrian went on, so far as we can tell, 

 much as in Cambrian times. Though lo- 

 cally there are breaks in the continuity of 

 geological processes, yet probably over the 

 world generally there was a continuity of 

 geological phenomena, and on the whole a 

 tolerably unbroken series of organic forms. 



It is obvious, however, that in the re- 

 gions thus far examined, the Precambrian, 

 whether we include the Archean or not, 

 more than at any time since, though the 

 land areas are by some considered to be of 

 small extent, was a period of widespread 

 and profound changes in the distribution of 

 land and sea. While it is generally sup- 

 posed that the extent of the continental 

 areas at the beginning of Paleozoic time was 

 small, forming islands, Walcott is inclined 

 to the belief that it was very considerable, 

 stating : 



" The continent was larger at the begin- 

 ning of the Cambrian period than during 

 any epoch of Paleozoic time, and probably 

 not until the development of the great 

 fresh-water lakes of the Lower Mesozoic 

 was there such a broad expanse of land 

 between the continental platform between 

 the Atlantic and Pacific Oceans. The agen- 

 cies of erosion were wearing away the sur- 

 face of this Algonkian continent and its 

 outlaying mountain barriers to the east- 

 ward and westward, when the epoch of the 

 Lower Cambrian or Olenellus zone began. 

 The continent was not then new. On the 

 contrary, it was approaching the base level 

 of erosion over large portions of its surface. 

 The present Appalachian system of moun- 

 tains was outlined by a high and broad 

 range, or system of ranges, that extended 

 from the present site of Alabama to Canada, 



and subparallel ranges formed the margins 

 of basins and straits to the east and north- 

 east of the northern Paleo-Appalachian or 

 the Paleo-Green Mountains, and their nor- 

 thern extension toward the Precambrian 

 shore- line of Labrador. The Paleo-Adiron- 

 dacks joined the main portion of the conti- 

 nent, and the strait between them and the 

 Paleo-Green Mountains opened to the north 

 into the Paleo-St. Lawrence Gulf, and to 

 the south extended far along the western 

 side of the mountains and the eastern 

 margin of the continental mass to the sea 

 that carried the fauna of the Olenellus ep- 

 och around to the Paleo-Eocky Mountain 

 trough." {I. c. p. 562.) 



Eemarking on the habitat, or nature and 

 extent of the sea-bottom tenanted by the 

 Olenellus or Lower Cambrian fauna, Wal- 

 cott remarks : 



" One of the most important conclusions 

 is that the fauna lived on the eastern and 

 western shores of a continent that, in its 

 general configuration, rudely outlines the 

 North American continent of to-day. 

 Strictly speaking the fauna did not live 

 upon the outer shore facing the ocean, but 

 on the shores of interior seas, straits, or 

 lagoons that occupied the intervals between 

 the several ridges that rose from the conti- 

 nental platform east and west of the main 

 continental land surface of the time." (I. c. 

 p. 556. ) 



Dana had previously (1890) claimed that 

 the earth's features even to many minor 

 details were defined in Archean time (evi- 

 dently referring to all Precambrian time) 

 and that ' Archean conditions exercised a 

 special and even detailed control over 

 future continental growth.' May not this 

 idea be extended to include the life of the 

 Precambrian, and may we not suppose that 

 biological variations and evolutions were 

 predetermined to some degree at least by 

 the geological conditions of these primeval 

 ages? The continental masses were then 



