354 E - M - KINDLE 



appear to have experienced a succession of alternate extensions 

 and withdrawals in the New York Chemung. 



It is essential to a clear understanding of the interrelationship 

 of these distinct but contemporaneous faunas to recognize the 

 fact that zoological provinces were often as distinct in the Paleozoic 

 seas as in those of the present. In the Devonian we know that 

 there were such provinces, but as yet we know but little of their 

 limits in any given epoch. We also know comparatively little 

 about the factors which set the limits to faunal provinces. It is 

 safe to conclude, however, that the recurrence of a fauna has 

 been due to the oscillation or migration of the factors which 

 conditioned its geographical distribution. Attention has been 

 called to one of these factors by Ulrich 1 in discussing the 

 recurrence of a Spergen fauna in the Ste. Genevieve limestone. 

 He conceives one of the conditions inducing the recurrence 

 of this fauna to have been "the subsidence or modification 

 of barriers allowing communication with seas more perma- 

 nently inhabitated by the invading fauna." The probable 

 combination of two factors which are no doubt often effective in 

 controlling recurrence is cited by Bagg in discussing the recurrence 

 of a Cretaceous brachiopod in the Eocene of Maryland. Bagg 2 

 believes this case of recurrence to have been due to a deepening 

 of the sea south of New Jersey, assisted perhaps by cold currents 

 from the north which killed off the other Cretaceous species and 

 encouraged the southward migration of a shell which previously 

 had lived in the New Jersey region. 



While the development or removal of land barriers and changes 

 in the character of sediment have doubtless been at times influen- 

 tial in causing the recurrence of faunas, it is probable that changes 

 in the temperature of marine waters have much more frequently 

 been the direct effective agency in causing recurrence. Among 

 the agencies controlling faunal distribution it is most probable 

 that temperature has in the past, as in the present, been a factor 

 of paramount importance. The recurrence of a species necessarily 

 represents the recurrence of those factors in its environment which 



1 Prof. Paper U.S. Geol. Survey No. 36 (1905), 49. 



2 Am. Geologist, XXII (1898), 272-373. 



