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 ability on the part of all those which migrate to follow the favorable conditions 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 organisms 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 original 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 considerable distance from its ordinary locus habitans; and in some cases by seeing 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 habitansduring 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.
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
