STRATIGRAPHIC CLASSIFICATION PALEONTOLOGIC CRITERIA 497 



the early Trenton submergence, which is often cited as the greatest 

 known, comprises several distinct submergences, differing in direction of 

 invasion and in the composition of the faunas that were brought in with 

 the different waters. The fact of greatest significance brought out by 

 study of numerous sections containing middle Ordovician formations is 

 that the several faunas, even where overlap of northern and southern 

 facies occurs, remain perfectly distinct. Although published lists of Mo- 

 hawkian species, say in Kentucky and Minnesota, include, besides the 

 quasi-cosmopolitan forms, many other species common to both areas, it is 

 found that nearly all of the latter are confined to formations or faunal 

 zones that extend either northward from Kentucky to Minnesota (the 

 Lowville) or southwardly from Minnesota to Kentucky. True inter- 

 mingling of faunules characterizing the Mohawkian in these two States 

 does not occur. In some cases intermingling was impossible because beds 

 found in either one of these States are not represented by deposits in the 

 other, while in the case of the remaining, possibly contemporaneous 

 faunas, intermingling failed because the submergence was insufficient to 

 accomplish confluence of the northern and southern basins. 



As may be gathered from various comments in this work on the 

 Magaran submergences (see pages 485 and 558), the facts in this case 

 are essentially as in the preceding Mohawkian invasions. It is somewhat 

 different, however, in the case of the late Hamilton submergence, during 

 which some intermingling of southern and western faunas occurred in 

 the vicinity of Lake Michigan. But even here there is little or nothing 

 to suggest that this intermingling resulted in expansional evolution. 



Organic evolution being largely dependent on physical conditions 

 which are ever changing, and thus doubtless a continuous process, is less 

 a matter of inherency and volition than of necessity. Naturally, then, 

 organic mutation as a rule is in proportion to the degree of diversity in 

 physical condition prevailing in a given time and given region. The 

 stresses incident to the withdrawal of waters from continental areas, the 

 struggle for existence under the resulting restriction of favorable habitat : 

 these obviously might well stimulate the inherent tendency to modify. 

 But the Paleozoic continental basins, which when occupied by large and 

 varied faunas, as is established by the unanimous testimony of all criteria 

 bearing on the question, were much more uniform than the oceanic, 

 basins in depth and most other physical conditions, except temperature 

 of waters, implied by the term environment. In these basins it would 

 seem that organic evolution was more likely to be retarded than stimu- 

 lated, and that the changes noted in their faunas are to be ascribed 

 chiefly to other causes than evolution in the basins themselves. 



