i899 C. CALLAWAY — ON EARLIEST LIFE 75 



on theoretical grounds, and we know it in some degree 

 by actual indications furnished by the rocks themselves. 

 I will take these two lines of evidence in their order. 



I assume that the types of life found in the Cambrian 

 rocks are the descendants of preceding species. I assume 

 this as a deduction from the theory of evolution, which 

 may now be taken for granted as safely as the law of 

 gravity or the doctrine of the conservation of energy. 

 Before the Cambrian fauna can have commenced to evolve 

 the succession of life which culminates in Man, it must 

 itself have been evolved. It is the result of a series of 

 evolutions extending through vast periods of time. 

 Think what the plastic material of life must have gone 

 through before a brachiopod or a trilobite was born ! 

 Some of the Cambrian trilobites are furnished with com- 

 pound eyes. Yet we regard an eye as one of the most 

 elaborate of all organs. The brachiopod possessed com- 

 plex apparatus for respiration and a digestive system of a 

 comparatively high type, and the gasteropod was more 

 elaborately formed. The conical shells of the Cambrian 

 probably contained creatures of still higher organisation. 



In estimating the time required to evolve Pre-Cambrian 

 life, we must keep in mind the fact that the lower forms 

 change less rapidly than the higher. This law is well 

 seen if we compare the mollusca of the Tertiary epochs 

 with the mammalia. Existing species of molluscs began 

 to appear in the oldest Tertiary strata, but no living 

 mammal is found even so low as the Pleiocene. Since 

 the beginning of the Eocene period the MoUuscan fauna 

 has varied but slightly, very few new genera having been 

 produced ; but the mammalia have varied fundamentally, 

 nearly all the existing orders, to say nothing of families 

 and genera, having come into existence. In the Eocene 

 period, we find the primaeval stock from which all the 

 higher mammals have descended, but the differences 



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