1918] 



Mcrriam: Age of Lake Lahontan 



521 



origin of the new fauna replacing it is not so readily conceived. "We 

 may, I believe, assume that the existing fauna of the Lahontan region 

 is not merely a reminder from a large Pleistocene fauna. The origin 

 of the present fauna is not to be accounted for in this manner more 

 easily than one would account for the origin of the existing earth 

 fauna as a remainder from a Mesozoic life series gradually dimin- 

 ishing from period to period. The present fauna is to be accounted 

 for in some part as a remainder and in part as a new development 

 either out of ancestral American forms or out of immigrants from 

 other regions. One might, of course, assume that the existing faunal 

 types arose very recently as mutants developing in response to some 

 great shock given by a physical change such as a great modification 

 of climate, but no evidence has been advanced indicating that such 

 change has ever taken place within a short time measured in terms 

 of years. The rate of change of faunas is fairly known in relation 

 to rate of sedimentation, and there seems to be on record no case 

 in which such an extreme modification of life as is noted between the 

 Rancho La Brea stage and the present has taken place in less than a 

 considerable period measured in terms of deposition. 



If we assume that the present fauna peopled America by migration, 

 we encounter an obstacle no less difficult to overcome than that pre- 

 sented by rapid evolution without change of geographic situation, as 

 migration from any other continent involves the crossing of great 

 barriers of topography, and belts of varying temperature at least as 

 important as hindrances to migration as are those of relief. Such 

 migration as would be required presumably involves nothing less than 

 general specific change of the migrating types, and a biological reor- 

 ganization process which would take place slowly in the extension of 

 the group by migration transverse to physical barriers. 



The problem of fixing the date of the Lahontan fauna seems the 

 same as that for other Pleistocene faunas containing a large percent- 

 age of extinct forms. An interpretation of the history of the Lahontan 

 region as shorter than that commonly accepted, which measures the 

 time in long periods expressed in terms of years, involves explanation 

 of the extinction of a great variety of forms over a wide area, and 

 appears to require the relatively sudden repeopling of this area either 

 by more rapid evolution than we have reason to consider probable, 

 or by a more rapid migration across barriers of temperature or of 

 topography than we have reason to believe could occur without evo- 

 lutionary changes in the migrating fauna. 



