THE CAMBRIAN PERIOD. 301 



The abrupt appearance of the Cambrian fauna. — The explanation of 

 the apparent suddenness of the appearance of the Cambrian fauna 

 is one of the open questions of geology. In a general way, it may 

 be said that the previous beds were subjected to distortion and meta- 

 morphism and that this was liable to destroy any fossils that might 

 have been preserved; but this is not wholly satisfactory, for some 

 of the earlier beds are not greatly changed, and seem quite capable of 

 receiving and retaining organic impressions. In later formations that 

 have been considerably disturbed and changed, fossils are sometimes 

 retained. It is equally true that later beds that seem quite capable 

 of receiving and retaining organic impressions are devoid of them. 

 On the whole, geologists seem inclined to refer the scantiness of pre- 

 Cambrian fossils, and hence the apparent abruptness of the intro- 

 duction of the Cambrian fauna, to unfavorable conditions for fossili- 

 zation, combined with subsequent changes in the rock. This makes 

 the abruptness merely a matter of preservation, not of evolution or 

 real introduction. Two suggestions have, however, been made which 

 entertain the thought that the coming of the Cambrian life into the 

 shallow seas about the continents, and the taking on of its character- 

 istic forms, were really, in some measure, abrupt, though the general 

 evolution of life elsewhere and under other conditions had long been 

 in progress. 



Brooks' hypothesis. — The first of these * postulates that the earlier 

 forms of life originated in the surface-waters of the open ocean, as 

 one-celled organisms, and that, because of the simple and monotonous 

 conditions there prevalent, the pelagic plant life has remained simple 

 and unicellular in the main to the present time and still constitutes 

 the chief food of all marine animals. There was little occasion for 

 unicellular organisms to develop into aggregated forms, for in the 

 water separate cells had larger contact with the food-supplies and 

 with the sunlight than aggregated cells. It was only when the life 

 became attached to the bottom or shore, or crept out upon the land, 

 that aggregation of cells and the development of complex organisms 

 became a marked feature in evolution. At first the conditions were 

 unfavorable for life-development on the land and on the sea-borders; 



1 W. K. Brooks, The Origin of the- Oldest Fossils and the Discovery of the Bot- 

 tom of the Ocean, Jour. Geol., Vol. II, 1894, pp. 455-479. Some features have been 

 here added to the hypothesis. 



