70 HISTORICAL GEOLOGY 



the highest forms (simple Arthropods) of Cambrian time, and 

 all of this development has been gradually accomplished since 

 Cambrian time. 



Passing upward in the Cambrian system, the fauna shows a 

 gradual progress toward more highly developed or organized forms. 



Apparent Suddenness of Appearance of the Cambrian Forms. 

 — The apparent suddenness of appearance of so many highly devel- 

 oped organisms even in the early Cambrian has caused much dis- 

 cussion by way of attempted explanation. Geologists are agreed 

 that this seeming sudden appearance of so many forms is due to 

 imperfection of the record either because of unfavorable conditions 

 for the preservation of fossils in the pre-Cambrian sediments, or 

 because fossils, though once present in those rocks, have been 

 obliterated by subsequent changes or metamorphism. Further, 

 it is agreed that the first organisms were plants because animal 

 life is ultimately dependent upon vegetable matter as a food 

 supply. 



It should be recognized that the metamorphic, or crystalline, 

 character of all Archean and most Algonkian rocks is obviously 

 unfavorable for preservation of determinable fossils. Thus, 

 Archean sedimentary rocks have flakes of graphite (carbon) dis- 

 seminated through them and, though such carbon is of organic 

 origin, the original organic structures have been entirely obliterated 

 so that crystallized carbon only remains after the intense meta- 

 morphism. Such an explanation, however, does not by any means 

 answer the whole question, because, at a number of localities, 

 thousands of feet of non-metamorphosed pre-Cambrian strata are 

 known and, except in very few cases in the later of these rocks, 

 distinct fossil forms are not known. 



Brooks 1 has advanced the hypothesis that the early living 

 forms (plants and animals) were single celled, and that they 

 originated and lived in the surface portions of the ocean. Because 

 of the lack of severe struggle for existence in such environment, 

 pelagic (free-swimming) plants have to this day remained largely 

 primitive or single celled. For similar reasons the unicellular 

 animals long failed to evolve higher forms because of easy existence 

 in contact with much food and sunlight. Such forms were of ge- 

 latinous consistence and not favorable for preservation as fossils. 

 Not until the attachment to the bottom or along shore were con- 

 i W. K. Brooks: Jour. Geol, Vol. 2, 1894. 



