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 impropriety 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 habitans 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. 
