202 CHARLES Dr WALCO 
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 generalizations 
can be touched upon. I think, however, that sufficient has been said 
to fix in your minds the following conclusions: (1) 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- 
opment of the Middle Cambrian faunas was due in great measure to 
enlarged opportunity caused by the extension of the Cambrian seas and 
the consequent shifting of shore-lines and changes in habitat, etc.; 
(3) that the diversification of the Middle Cambrian fauna, as a whole, 
may have been due, in a large degree, to the rapid development of 
narrowly provincial or isolated faunas that were subsequently merged 
into the more widely distributed fauna by the breaking-down of the 
restrictive barriers; and (4) that a free and more or less complete 
interchange of currents in the Cambrian seas was strongly instrumental 
in producing those cosmopolitan faunas so characteristic of the early 
Paleozoic. In other words it is evident that the evolution of the early 
Paleozoic faunas was profoundly influenced by their environment. 
