THE EARTH BEFORE THE CAMBRIAN 399 



or indirectly. Yet, with the conditions as they are to-day, almost 

 nothing of this profuse life would be preserved in a fossil state as a 

 record of their existence, since, with few exceptions, 1 fossils are con- 

 fined to such forms as possessed some hard parts, such, for example, 

 as shells or skeletons. The study of embryology teaches that all 

 classes of life were descended from minute, possibly swimming crea- 

 tures. The starfish, coral, shellfish, and other marine animals all 

 began life as minute, free-swimming forms. It seems probable that 

 preceding the Cambrian the oceans were tenanted by such small, 

 soft-bodied animals as those which populate the surface to-day, and 

 that they were in equal abundance. 



The question next to be answered is : Why did any of these animals 

 seek the bottom of the ocean and become stationary forms ? The first 

 settlers on the bottom probably did not secure more or better food 

 than their swimming relatives, but they had one advantage : they were 

 able to devote their superfluous energies to growth and multiplica- 

 tion and thus to become larger and to increase in numbers faster than 

 the swimming forms. Consequently those which first acquired the 

 habit of resting on the bottom soon began to multiply faster than their 

 swimming relatives. But this rapid increase must soon have given 

 rise to crowding and competition which led to a struggle for existence. 

 Thus the stronger forms increased at the expense of the weaker. 



The development of hard coverings, such as the shell of the mollusk 

 (the clam is an example, p. 413) and the crustacean (the crawfish is an 

 example, p. 410), may have been due largely to such competition, 

 since the animal which was protected in some way would have a better 

 chance to escape being devoured. Or the development of hard pro- 

 tective coverings may have been due to the appearance of some es- 

 pecially voracious creature, and the trilobite (p. 410), the largest and 

 most active of the inhabitants of the early ocean bottom, has been 

 suggested as the aggressive animal. Later, however, some animal 

 arose more formidable and active than the trilobite, such perhaps as 

 the ancestor of the fish, and may have caused the development of still 

 heavier armor. 



REFERENCES FOR THE PRE-CAMBRIAN ERAS 



Adams, F. D., — Basis of Pre-Cambrian Correlation : Outlines of Geologic History 



(Willis and Salisbury), pp. 9-27. 

 Brooks, W. K., — The Origin of the Oldest Fossils: Jour. Geol., Vol. 2, 1894, pp. 455- 



479- 



1 Some jellyfish, worm borings, worm casts, trails, etc. 

 CLELAND GEOL. — 26 



