EVOLUTION OF EARLY PALEOZOIC FAUNAS 37 



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. 



Note. — Since this paper was written in November, 1908, I have made a 

 detailed examination of the genera and species referred to the Mesonacidae 

 (Smithsonian Miscellaneous Collections, Vol. LIII, No. 6, 19 10) and referred 

 Holmia meeksi of the original paper to the genus Nevadia and limited its range 

 to the basal Lower Cambrian fossiliferous strata in Nevada. The Holmia rowei 

 fauna (p. 31) is now closely limited in stratigraphic range and is below the 

 Archaeocyathinae-heanng strata. 



May, 1910 



