36 CHARLES D. WALCOTT 



EVOLUTION OF FAUNAS 



That the environment of the faunas of Middle Cambrian time was 

 more favorable for their rapid evolution than that of Lower and Upper 

 Cambrian time is strikingly shown by the stratigraphic distribution 

 of the brachiopods. In the restricted waters of Lower Cambrian time 

 the known brachiopods (of the entire world) were represented by 20 

 genera and 75 species. In the expanding seas of Middle Cambrian 

 time 31 genera and 243 species are known to have existed. With the 

 more uniform conditions of Upper Cambrian time, and the dying-out 

 of the impulse to variation created by both favorable and unfavorable 

 environments in Middle Cambrian time, the brachiopods decreased in 

 variety and numbers, and are represented by only 23 genera and 137 

 species. 



About the same relative numerical ratios are exhibited by the 

 trilobites but the exact statistics are not yet available. The favorable 

 environment of the Middle Cambrian fauna is well illustrated by the 

 development of Ogygopsis, Asaphiscus, and Bathyiiriscus of Cordil- 

 leran Middle Cambrian time,' genera which are so far in advance of 

 contemporary trilobitic genera that they have sometimes been referred, 

 upon biological grounds, to the Upper Cambrian.^ 



The closing of Cambrian time w^as accompanied and followed by 

 changes in the relations of the sea and land upon the continental plat- 

 form that were favorable, like those of Middle Cambrian time, to the 

 evolution of new genera and species, and to the existence of multitudes 

 of individuals of the prolific species. 



This is not the place for a detailed discussion of the faunas and 

 sediments of the lower Paleozoic. Only the broadest generaHzations 

 can be touched upon. I think, however, that sufficient has been said 

 to fix in your minds the following conclusions : (i) That more or less 

 uniform and favorable, even warm, climatic conditions must be 

 appealed to in explanation of the widespread occurrence of almost 

 identical coral-like organisms in the Lower Cambrian, and of the vast 

 number of individuals of various species of trilobites, etc., which existed 

 in Middle Cambrian time; (2) that the rapid and accentuated devel- 



1 See Bull. U. S. Geol. Survey, No. 30, 1886, Pis. XXX, XXXI, and Canadian 

 Alpine Journal, Vol. I, No. 2, 1908, PI. 3. 



2 G. F. Matthew, Trans. Roy. Soc, Canada, 2d ser., Vol. V, 1899, p. 64. 



