290 H. S. WILLIAMS SHIFTING OF DEVONIAN FAUNAS 



This leads us to consider the general principles of migration, and in 

 particular those which affect marine organisms. 



Migration of Species and Shifting of Faunas contrasted 



Migration as commonly applied in natural history means the move- 

 ment of large numbers of the same species from one place to another in a 

 general definite direction at more or less regular periodic times. So 

 birds migrate northward with the advance of warm weather; some fish 

 migrate from sea to rivers in breeding seasons; pigeons fly eastward or 

 westward in great flocks; grasshoppers invade a rich country, devouring 

 the vegetation in their path, or lemming migrate in great hordes from 

 mountain to lowlands. 



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

 animal in relation to the comparatively fixed range of feeding ground 

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

 term is rarely 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 communi- 

 cation 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 are affected 

 instead of a 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 migra- 

 tion: 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 

 on all the living organisms of the region. The change in position may 

 be supposed to take place by the contraction on one side of an 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 hahitans in the given direction of mo- 



