MIGRATION OF SPECIES AND SHIFTING OF FAUNAS CONTRASTED 291 



tion. To this latter process of migration I have been accustomed to 

 apply the term "shifting of faunas/^ 



Migration of species is in its typical sense an expression of the ability 

 of some organisms to appreciate slight favorable changes in the condi- 

 tions of environment and to take advantage of the better conditions 

 during the lifetime of an individual. Shifting of faunas is an expression 

 of the felt necessity, for the perpetuation of the race, of certain conditions 

 of environment, resulting in the dying out of the whole fauna in the 

 areas from which the favorable conditions have been removed and the 

 spread of the fauna into new areas into which the favorable conditions 

 have been shifted. 



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

 the fauna to survive under the changed conditions of environment which 

 have overwhelmed them in their 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. 



Changed Environment Cause op both Migration and Shifting 



In both typical migration of species and shifting of faunas, change in 

 the environmental conditions of life constitutes the stimulus inducing 

 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 show their greater vitality, compared with their 

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

 greater power of endurance and of organic adjustment to a wider range 

 of environmental conditions than do those which migrate. 



Close Adjustment of Species of a shifting Fauna 



In the case of the shifting fauna, those species 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 their return and reoccupation of the same area at a later 

 geological stage when, by their reappearance, the original condition of 

 environment is shown to have recurred. 



Evidence of Migration 



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 



