THE PALEONTOLOGIC RECORD 75 
Shifting of faunas is an expression of the inability of the species 
of the fauna 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 habitans ; 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 habitans 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 Continuwity.—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 
