498 E. O. ULRICH REVISION OJh' THE PALEOZOIC SYSTEMS 



This assertion may not harmonize with theoretic considerations re- 

 specting organic evolution, but that is of little consequence so long as it 

 is in accord with observed facts concerning the distribution and integrity 

 of fossil species. I venture to say that the Cretaceous and Tertiary 

 faunas of our coastal regions, which certainly were favored with very 

 considerable expansion of shallow-water conditions as compared with the 

 present stage, do not bear out the idea of expansional evolution. Neither 

 can I find any convincing indication of unusually stimulated evolution 

 in any of the Paleozoic submergences of interior continental depressions. 

 Nor have I found that local or provincial evolution increased with the 

 advance of an invading fauna. 



While it is true that many species occur at Cincinnati that are unknown 

 in corresponding beds in middle Tennessee, the fact may still mean no 

 more than that they have not yet been found there. Many species again 

 are very common at one locality and very rare or quite absent at another, 

 but this probably has nothing to do with their evolutional origin. On 

 the contrary, all species are more or less local in distribution, especially if 

 we consider the matter of abundance of individuals. The discovery of 

 previously unknown species, or of new geographic occurrences, is a matter 

 of daily experience to the energetic collector. Finally, we find species at 

 Chicago that are either the same as, or close allies of, Baltic types. Though 

 they may not have been found in the intermediate areas, no one will 

 doubt that the path of migration passed over certain as yet unworked 

 parts of these areas. If every outcrop of Silurian rocks in Canada could 

 be searched like a Chicago quarry the apparent anomalies in geographic 

 distribution of northern Silurian species would be mostly accounted for. 



MUTATION OF FOSSIL MARINE FAUNAS ACCOMPLISHED BEFORE THEY 

 INVADED CONTINENTAL SEAS 



As a result of my investigations I believe the faunas of the continental 

 seas consist almost entirely of organisms that have periodically and very 

 frequently migrated from their permanent oceanic habitats into these 

 inland seas. The organisms passed into and about in these basins when 

 and where conditions were favorable for their existence, and they re- 

 mained out when the conditions were not favorable. But on account of 

 their shallowness even slight climatic changes effected such extremes in 

 the waters of these inland basins that the faunas were often exterminated 

 locally. The supply, however, was inexhaustible and ever ready to take 

 advantage of opportunities to replenish the shortage, and there was 

 ample time for every purpose. New things came in with each return of 

 the fauna, but many of the preceding tenants remained out forever or 



