1879.] NATURAL SCIENCES OP PHILADELPHIA. 



A COMPARISON OF THE EOCENE MOLLUSCA OF THE SOUTHEASTERN 

 UNITED STATES AND WESTERN EUROPE IN RELATION TO THE DE- 

 TERMINATION OF IDENTICAL FORMS. 



BY ANGELO HEILPRIN. 



The study of the fauna, whether extinct or living, of any coun- 

 try resolves itself into two distinct methods of investigation, the 

 general and the comparative. In the general method we look upon 

 an assemblage or community of animal forms as constituting an 

 integral part of the country it characterizes, and we then consider 

 it only in relation to that country and to itself ( the animal forms 

 inter se). In the second or comparative method we no longer re- 

 gard this community as constituting a whole or unit, but merely 

 as a part of a more extensive community, and we now view it in 

 the relation of a part to a whole. This comparative system of 

 investigation, which, it will be manifest, involves a thorough gen- 

 eral acquaintance with all or most extraneous faunae as well as the 

 one under special consideration, is one of great difficulty, and one 

 that requires more than an ordinary amount of acumen in its pur- 

 suit, for in the broad facts of geographical distribution are con- 

 nected some of the profoundest biological and physical problems. 

 The study of comparative or geographical zoology constitutes one 

 of the essential factors of biological science, for without a true 

 understanding of the general affinities of scattered groups of 

 animals, our conception of the organic universe would be one of 

 disjointed parts instead of a continuous whole. We know, in fact, 

 little of a whole unless we comprehend its relation to its component 

 parts, and per contra, we know little of a part unless we under- 

 stand the relation it bears to the whole. 



The subject of geographical distribution in its bearings on 

 geology, whether considered in its broader sense as pertaining t^) 

 groups, or in the more limited sense as pertaining to the individuals 

 composing those groups, cannot be over-estimated. It is by the 

 recurrence over broad or scattered areas of certain related animal 

 types, and sometimes even over the most remote areas of identi- 

 cal specific forms, that the palaeontologist is enabled to arrange 

 and classify his strata. One single well-determined fossil will, in 

 the absence of further data, frequently determine approximately, 

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