THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 35 



thistle, or the prickly pear cactus of to-day, which have spread into regions 

 differing notably in climate, soil, and altitude. To infer anything in par- 

 ticular from the location of such forms would be to strike the mark only 

 very broadly. One is inclined to believe that the Paleozoic brachiopod 

 Atrypa reticularis^ may have been equally hardy. Such widely distributed 

 forms can not be considered as good indices of local conditions unless they 

 possess some known character which has determined their distribution. 

 On the other hand, sparsely distributed forms may reveal much if they are 

 correctly understood. Sparseness of a given form may be due to either 

 rapid evolution or to restricted powers of adaptation. 



In the first case, forms which are undergoing rapid change may appear 

 uncommon because of the really limited numbers of individuals referable 

 to a given species. A classical example is the large series of ammonites in 

 the Mesozoic. The discovery of but a few individuals or a single species, 

 or to find them in a single unit, does not necessarily imply that they were 

 restricted to any given locality by a definite set of conditions; they may 

 have had a very wide range, but have been only locally preserved under 

 favorable conditions and have disappeared by actual evolution before such 

 conditions arose in other places. If, however, the forms are not a part 

 of a rapidly changing series, but are highly specialized members of a normally 

 stable group, their value as indices is high. 



(c) Distribution Dependent on the Inorganic Environment. 



Forms may be restricted or dispersed by entirely extrinsic forces, though 

 this may be in part due to the nature of the animal or plant. Such floating 

 forms as some aquatic plants, pelagic animals, colonies, as of graptolites, 

 etc., may be closely confined within certain limits of the temperature and 

 food-supply of currents, but due to the space covered by such currents and 

 to the shifting of the currents they might come to be widely dispersed over 

 the earth and occur in a great variety of deposits of a very different character. 

 Could we understand them fully they would tell much concerning the peculiar 

 environment which favored them, but it would be very erroneous to assume 

 similarity of conditions over broad areas in all places where they are found 

 fossil. The shifting of the Gulf Stream and the polar currents are well 

 known, and they bear a life peculiar to themselves; but it is obvious that 

 they drop the remains of the fauna and flora peculiar to themselves among 

 widely different assemblages of more fixed forms. 



Other less mobile or movable forms found widely dispersed are clearly 

 indicative of similar conditions over wide areas, as the fauna of Niagara time. 



^ See also Ruedemann, Rudolf, The Paleontology of Arrested Evolution, N. Y. State Museum 

 Bull. 196, 1916. 



