24 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



riers of sea or land, climate or vegetation, the migratory flight takes them 

 over wide areas of the earth's surface in a general north-and-south direction. 

 Should any attempt be made in a later geological period to correlate the 

 swamp deposits of to-day by the presence of the remains of the wild duck or 

 the blackbird, for instance, it would be necessary to assume geographical 

 conditions far from those which really exist. Here the time element would 

 be correct, but all implications of geographical similarity would be utterly 

 wrong. The changes of plumage which frequently accompany migratory 

 movements or the changing seasons are superficial and no traces would 

 remain in the fossil state. 



Similarly, but over less distances, some grazing forms move with the 

 seasons, following the grass, or water, or temperature changes. Carnivorous 

 forms always follow the herds. Here the implication both of climate and 

 geographical similarity might hold true whenever the remains were found 

 in a determinable natural habitat, but, as suggested above, such forms 

 might readily be swept far beyond their usual limits by flooded rivers, as 

 when herds of bison were overcome on the Missouri or Mississippi Rivers 

 and the cadavers swept away. The inference to be drawn from the occur- 

 rence of such forms is the work of experts. 



(c) Character of the Fauna. 



The nature of the beds and their surroundings is revealed by the con- 

 tained fauna in large measure — marine, brackish, or fresh water; fluviatile, 

 swamp, or terrestrial; arid or humid; plains, plateaus, woodland, or forest, 

 etc. There facts are revealed by the structure of the animals which inhabit 

 them. The general facts in such an interpretation are easily recognized, 

 but the final interpretation depends on minute and exact knowledge. The 

 best treatment of the matter is found in Abel's Paleobiologie and Lull's 

 Organic Evolution.^ 



(d) Phylogenetic Relations of the Fauna. 



The genetic relations of fossils are of the utmost significance not only in 

 placing the beds in their proper position in the geological column, but for 

 an understanding of the position of the fauna. The place of origin of the 

 fauna, the connecting links which reveal the route of migration, and the 

 separation of the constituent elements of a mixed fauna depend upon an 

 understanding of their phylogeny. The mixed Devonian faunas known at 

 Rochester, New York, or Milwaukee, Wisconsin, could only have been 

 separated into their constituent parts by this means and the routes of 

 migration of the different elements and the movements of the epicontinental 

 seas of the time traced out. 



1 Abel, Grundzuge der Paleobiologie der Wirbelthiere, Stuttgart, 1912. 

 Lull, R. S., Organic Evolution, 1917. 



