iM j3 



PROCEEDINGS 



OF THE 



4MEETCAN PHILOSOPHICAL SOCIETY 



HELD AT PHILAIIELPniA FOR PROSOTISG USEFUL KNOTODGE. 



Vol. XLI. April-December, 1903. No. 171. 



THE GEOGRAPHICAL DISTRIBUTION OF FRESHWATER 



DECAPODS AND ITS BEARING UPON 



ANCIENT GEOGRAPHY. 



BY DR. A. E. ORTMANN. 



(/^eai/ Aj>rt7 3, 1903.) 



INTRODUCTION. 



During the last decennium Zoogeography has developed in a very 

 peculiar direction, which, in a large part, is directly opposite to 

 the methods introduced by Wallace. The professed aim of the 

 latter was the creation of a zoogeographical division of the earth's 

 surface into regions, realms and the like, the purpose of which was 

 the subordination of the facts of animal distribution under a fixed 

 scheme ; and since it was self-evident from the beginning that the 

 distribution of animals ought to express the physical conditions of 

 the earth's surface, it was assumed that the proposed zoogeographi- 

 cal divisions correspond to the chief features of the distribution of 

 the conditions of life. 



Soon, however, it was discovered that it is impossible to give a 

 division of the earth's surface that could claim general recognition. 

 It is true that each of the proposed schemes was actually supported 

 by more or less numerous instances of distribution, and that in 

 many cases the physical factors influencing and explaining these 

 divisions were easily understood ; but there was always alongside of 

 the supposed normal conditions a number of exceptional cases, 

 where the actual distribution of certain animals or animal groups 

 was directly the opposite. One of the chief causes of this fact has 

 already been recognized and carefully studied by Wallace. It is 



PROC. AMER. PHILOS. SCO. XLI. 171. R. PRINTED NOV. 19, 1902. 



