292 



H. S. WILLIAMS SHIFTING OF DEVONIAN FAUNAS 



distance from its orclinar}^ locus liabitans; and in some cases the species 

 may be seen in the process of migration, as, for instance, the temporary 

 alighting, in fatigued condition, of flocks of northern land birds on Ber- 

 muda Island on their migration southward. 



In the case of fossil species the shifting of a fauna is recognized by the 

 presence in a stratum of rocks of a number of species representing an 

 earlier fauna surrounded by a different and, dominantly, later set of 

 species. 



Eecurrenoe 



The fauna is then said to recur, and it is the recurrence of the fauna 

 which forms the basis for the inference that the fauna has shifted its 

 locus liabitans during the period of time represented by the sedimentary 

 deposits separating the formation in which the fauna is dominant from 

 the zone in the higher formation in which the recurrent species are found. 



Theoeetical Problems involved 



This hypothesis of shifting of place and recurrence in time of a fauna 

 involves certain conceptions as to the nature of species and the laws of 

 evolution which it is important to consider. 



Evidence of faunae Continuity 



To establish evidence of motion in migration, as in any other kind of 

 motion, it is all important to know that the body or bodies to which the 

 motion is ascribed is continuously the same. 



In the Devonian case the moving body is a fauna. Not only is it neces- 

 sary to establish identity of the species in the recurrent zones with those 

 of the initial zones, but it is essential to show that the faunas as a whole 

 are the same. To put this in another mode of statement, we must estab- 

 lish the fact that not only the individual species have retained their spe- 

 cific characters, but the further fact that the equilibrium of adjustment 

 to each other in the faunal community has not been changed, in order to 

 prove that a supposed recurrent fauna is actually the direct successor of 

 a fauna represented in the rocks at a lower horizon. 



Eare and dominant Species 



This has led to the distinction between rare and dominant species. 

 Only as the comparative frequency of the species in the faunal combina- 

 tion is maintained can we be sure that we are not considering an acci- 



