500 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



in the proximal parts of the path of invasion is always so small that it is 

 reasonably ascribable to fortuitous circumstances in original distribution 

 and collecting. Evidently, then, the corals and bryozoa advanced only 

 so far as the currents entering from the Gulf of Mexico maintained their 

 efficiency as distributing agents. The truth and great significance of 

 this statement will be apparent when we add that beyond the first con- 

 siderable break in the continuity of their occurrence they, and indeed all 

 representatives of their classes, are seen no more. The conclusion there- 

 fore is inevitable that the evolution of the corals and bryozoa was prac- 

 tically accomplished in the oceanic basins and their further mutation in 

 the continental seas correspondingly insignificant. 



Continuing the argument, if expansional evolution had prevailed, and 

 if we do not confine the process of evolution to saltation of specific grade, 

 the rocks should be filled with intergrading links. But do we find any- 

 thing like this in the fossil faunas? A well known occurrence that 

 might be said to fairly satisfy the requirement is that of the fresh-water 

 shells in the upper Miocene at Steinheim, Germany. Except for the 

 uncertainty attending the preservation of land deposits of any sort, such 

 occurrences would probably be common enough since, on account of their 

 relatively strenuous existence, the shell faunas of lakes and tributary 

 streams are not only more liable to rapid mutation, but they are also 

 much less sensitive to temperature changes than marine shells. But, 

 aside from such considerations it is to be observed that under ordinary 

 circumstances minor organic mutation in these unstable land waters took 

 place within them and not, as I believe of the marine faunas of the 

 continental seas, in the relatively permanent oceanic basins. 



In the case of the Paleozoic marine faunas in the continental seas the 

 above query must be answered with an emphatic no. If the deposits of 

 these seas contained anything like the number of intergrading mutations 

 that might reasonably be expected in the evolution of species the paleon- 

 tologist's work in stratigraphic correlation would be hopeless. There are 

 individual peculiarities in plenty, some of them suggesting reversion 

 while others may be anticipatory; and there are local varieties; but 

 altogether these departures from type rarely if ever constitute a satis- 

 factorily complete transition from one to another species. We find also 

 many species and varieties that are intermediate in character between' 

 other species, and the position of these in the evolution of a genus or 

 group of species is more or less certainly determinable; but almost in- 

 variably we deal with the nearly finished product of a process of mutation 

 that was begun and established before the new phase invaded areas now 

 accessible to the student of fossil faunas. Apparently the mutations 



