690 JAMES PERRIN SMITH 



province and near together. But intermittence of occurrence is 

 occasionally noted when it cannot be due to difference of facies. 

 Just such a case is the reappearance of a Chouteau fauna in the 

 Osage horizon of Missouri, 1 or the reappearance of Devonian 

 types in the St. Louis beds at a number of places in the United 

 States. In the Jura of northern Europe, according to Neumayr, 2 

 the genera Lytoceras and PJiylloceras appear only sporadically, 

 being lacking in sixteen zones ; and even the known species 

 there do not belong to a genetic series. But in southern Europe 

 these genera appear plentifully in all the zones, and seem to 

 represent genetic series. Their migration northward at several 

 successive periods is thus clearly established. In these same 

 beds AmaWieus also appears intermittently, but no region is yet 

 known where AmaWieus developed continuously. Among the 

 Jurassic ammonites of northern Europe there are a number of 

 other cryptogenic types, many of which coincide with the Amal- 

 tlieidae in their appearance, and thus probably came from the 

 same region. 



Today, when the struggle for existence becomes too severe 

 for a species it disappears. In geologic history, too, a species 

 has a certain length of life and dies out, never to reappear. This 

 has given rise to the theory that species, like individuals, have a 

 limited life, and that in time they reach a stage of development 

 where they can go no further, and then of necessity die out. 

 This would all be very well if species dropped out one at a time 

 and contemporaneously all over the earth, but in reality they 

 come and go by faunas. A study of the successive fossil faunas 

 of the Pacific coast region has shown that while there may be a 

 nearly perfect stratigraphic series, the faunal succession is 

 broken, so that each fauna appears unheralded, in a way that 

 would have delighted the heart of Cuvier. But we often find 

 the forerunners of these unheralded faunas in older beds in other 

 regions ; this gives the rational explanation of the phenomenon, 



1 C. R. Keys : Amer. Jour. Sci., Dec. 1892, p. 447. 



2 Jahrbuch K. K. Geol. Reichsanstalt, Wien. Bd. 28, 1878. Ueber unvermittelt 

 auftretende Cephalopodentypen in Jura Mittel-Europas. 



