74 THE POPULAR SCIENCE MONTHLY 



from sea up rivers in breeding seasons; pigeons fly eastward or west- 

 ward in great flocks, or grasshoppers invade a rich country devouring 

 the vegetation in their path, or lemmings migrate across country in 

 great quantities. 



The term in these cases has to do with movements of one kind of 

 animal in relation to the comparatively stable range of feeding-ground 

 for the remainder of the fauna inhabiting the areas concerned. The 

 term is rarely if ever applied to the slower movement of the whole body 

 of animals of a fauna, coincident with great changes of climate, such 

 as the advance of the glacial cover over the northern parts of Europe 

 or America produced during the glacial age, or the advance of an 

 Asiatic fauna across the Bering Straits and down the west coast of 

 North America at some Pleistocene time when an ice bridge furnished 

 means of communication by land from one continent to the other. 

 Perhaps there is no imjn'opriety in extending the application of the 

 term migration to these latter cases in which the whole fauna and 

 flora of a region is affected instead of single or a few species; and in 

 which the change of position of habitat is slow and spread over a great 

 period of time instead of being coincident with annual change of sea- 

 sons. The term may equally well be applied to movements in the 

 seas and movements on the lands. 



There is, however, one reason for choosing a separate name for the 

 movements of the latter kind to distinguish them from typical migra- 

 tions. 



In the first class of cases the migration is voluntary and is per- 

 formed by those organisms which have the power of more or less rapid 

 locomotion. They may be said to do the migrating themselves. In 

 the second case the movements are involuntary and the movement is 

 forced upon all the living organisms of the region and the change in 

 position may be supposed to take place by the contracting on one side 

 of the area of the conditions of possible existence for the species and 

 the extension on the other side of favorable conditions of environment. 

 The movements extend over many generations of life so that relatively 

 sedentary species may gradually adjust their locus liabitans to a given 

 direction of migration. To this latter process of migration I have been 

 accustomed to apply the term " shifting of faunas." 



Migration of species is an expression of the ability of some organ- 

 isms to appreciate slight changes of favorable conditions of environ- 

 ment and to take advantage of the better conditions during the life- 

 time of an individual. Shifting of faunas is an expression of the 

 necessity for the perpetuation of the race of certain conditions of en- 

 vironment and the dying out of the whole fauna in the areas from 

 which the favorable conditions are removed with corresponding spread 

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

 been shifted. 



