236 OEIGIN OF LIFE IN AMEEICA 



than they are now, hardly seems well founded. Another still 

 stronger objection to Dr. Wallace's * theory is that the 

 northern forms alluded to as occurring in Chile and Patagonia 

 belong almost all to different spedes, sometimes even to dif- 

 ferent genera, from their northern relations. If storms had 

 anything to do with this distribution they could only have 

 acted during very long intervals of time so as to produce such 

 specific and generic differences. Moreover, how could winds 

 or storms affect the distribution of Carabus, which is a flight- 

 less ground insect living under stones ? How could these 

 agencies have transported fresh-water species across the im- 

 mense tropical area, for several Chilean fresh -water forms 

 exhibit a similar northern affinity ? These are some of the 

 problems that present themselves to us. There are numbers 

 of others. Why should the family of tortoises, Dermate- 

 mydidae, which is known to have inhabited the North 

 American continent since Cretaceous times, have become ex- 

 tinct there and be now confined to Central America ? 



The scarcity of land and fresh-water fossils in Central 

 America obliges us to resort to zoogeography and to the meagre 

 geological information we possess in elucidating these and 

 other problems. Before dealing with 'the general faunistic 

 features of Central America, a few remarks on some of the 

 more important geological characters will be of interest. 



The long neck of Central America from the isthmus of 

 Tehuantepec to Panama, which joins North and South 

 America to one another, has a length of about one thousand 

 five hundred miles. We are sometimes apt to forget that it 

 does not lie in a north and south direction, but almost east and 

 west. Nearer South America the neck of land starts in a 

 due westerly direction and only gradually turns somewhat 

 towards the north and finally north-westward. Very little of 

 this immense stretch of land has as yet been geologically sur- 

 veyed. Nevertheless, some valuable hinits as to its geological 

 history have been gathered. In his essay on the geology of 

 the isthmus of Panama, Professor Hill f tells us that, pos- 

 sibly before the vast accumulations of more modern igneous 

 and sedimentary rocks of Tertiary and post-Tertiary age were 



* Wallace, A. E., " Distribution of Animals," Vol. 11., p. 45. 

 t Hill, E. T., " GeologioalHistory of Panama," pp. 241—257. 



