Page:Popular Science Monthly Volume 77.djvu/80

74 from sea up rivers in breeding seasons; pigeons fly eastward or westward 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 seasons. 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 migrations.

In the first class of cases the migration is voluntary and is performed 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 organisms to appreciate slight changes of favorable conditions of environment and to take advantage of the better conditions during the lifetime of an individual. Shifting of faunas is an expression of the necessity for the perpetuation of the race of certain conditions of environment 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.