PRESERVATION OF INTEGRITY OF A FAUNA 183 



geographical relation to each other at each successive chronologic stage, 

 and, as has been noted in describing them, each is crowded to the west- 

 ward as we pass up the series. It is this gradual movement of the 

 whole set of faunas, coordinate witli the changing of the sediments, to 

 which the term shifting of faunas is applied. It will be readily seen 

 that this shifting differs from what is generally spoken of as migration 

 of species. By migration I would understand such movements of species 

 as are seen in the cases of the entrance into these faunas of Spirifer levis, 

 Piignax pugnus, Productella hallana, forms which are common in the 

 Iowa Devonian fauna and dominate the Devonian faunas of ISevada 

 and Arizona, but have no forerunners in the eastern Middle Devonian, 

 entering it abruptly at the time of the Ithaca fauna of New York 

 province. 



Preservation of the Integrity of a Fauna 



In shifting, the faunas do not lose their integrity, but the whole body 

 of species becomes slightly modified. So long as the fauna occupies the 

 same ground, with its center of distribution or metropolis the same, the 

 species suffer very little modification and range through a thousand feet 

 or more without variation. Where the shifting has moved the metrop- 

 olis 50 miles or more, the decided modification of species is recognized, 

 though some species are affected more than others. It is the keeping 

 together of the great bulk of the species (a fact which makes it difficult 

 to recognize their movement) which characterizes this shifting of faunas. 



Evidences on which this Investigation rests 



It has required the accumulation of a vast number of local, individual 

 faunules and their exact analysis and comparison to make it possible to 

 demonstrate the fact of shifting in the present case. Over 5,000 such 

 faunules in the Devonian laboratory of the Survey and reports by com- 

 petent investigators of thousands more constitute the basis of evidence * 

 upon which the statements here made rest. The second group of these 

 faunas, associated with the Hamilton, Ithaca, Chemung, and Waverly 

 formations, is the one whose statistics are most fully gathered. 



Relation of Extent of Range of Species to Metropolis of Fauna 



In each case the species of each stage range higher in the region of its 

 center of distribution, or metropolis, than at its edges. West of Ithaca 



*A fuller presentation of the facts and their discussion will appear in a forthcoming bulletin (210) 

 of the U. S. Geological Survey. 



