THE PALEONTOLOGIC RECORD 75 



Shifting of faunas is an expression of the inability of the species 

 of the fanna to survive under the changed conditions of environment 

 which have overwhelmed them in the original habitat; but of an abil- 

 ity on the part of all those which migrate to follow the favorable con- 

 ditions as they shift from one area to another. 



In both typical migration of species and shifting of faunas change 

 in the environmental conditions of life constitute the stimulus to change 

 of habitat on the part of the organisms; and the movement of the or- 

 ganisms is a direct response to the stimulus — those organisms in the 

 first case which migrate showing their greater vitality compared with 

 their neighbors who stay at home; while those who stay at home show 

 a greater power of endurance and organic adjustment to wider range 

 of environmental conditions. 



In the case of the shifting faunas those which endure without 

 change of characters exhibit an acquired closeness of adjustment to 

 some particular combination of environmental conditions which they 

 are forced to follow or die and suffer annihilation. The evidence of 

 their endurance is indicated by return and reoccupation of the same 

 area at a later geological stage when by their reappearance, the orig- 

 inal condition of environment may be assumed to have recurred. 



In the case of living organisms evidence of migration is found in 

 the actual presence of the species at one time in a region at a consider- 

 able distance from its ordinary locus liabitans; and in some cases by see- 

 ing the species in the process of migration, as for instance the temporary 

 alighting in fatigued condition of flocks of northern land birds on 

 Bermuda Island on their migration southward. 



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

 the presence of a number of species representing an earlier fauna in a 

 stratum in the midst of rocks containing a different and dominantly 

 later set of species. 



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 recur- 

 rent species are found. 



This theory of the shifting of place and the recurrence in time of 

 the same fauna involves certain conceptions as to the nature of species 

 and the laws of evolution which it is important to consider. 



§ 9. Evidence of 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 I have been studying the moving body is a 



