166 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Hald., A. cincinnatiensis Anthouy, and PomaUo])sis lapidaria Say. It 

 is plain that this marl is from the Bonneville beds of Mr. Gilbert, 

 containing shells which lived in the lake when the waters were 

 at the level of the mouth of the cave. Prof. F. V. Hayden, in 

 1870, found in these beds Fluminicola fusca Hald., Valvata sincera 

 Say, Limncea catascopium Say, L. desidiosa Say, Amnicola Umosa Say,* 

 Pomatiopsis cincinnatiensis. Afterward Mr. Gilbert found the follow- 

 ing additional species : — Pomatiopsis lustrica Say, Succinea lineata Binn., 

 and a Cypris (■?). This formation was regarded as Quaternary by Dr. Hay- 

 den. Mr. Gilbert regards the deposit as a lacustrian one, thrown down 

 during the Glacial epoch, when " the great climatal revolution which 

 covered our Northeastern States with ice was competent to flood the dry 

 basin of Utah". The cave, then, is of very recent origin, and as it is 

 only perhaps 200 feet above the present level of the lake, the highest 

 terrace or raised beach being 1,000 feet above the present level, Clin- 

 ton's Cave was not excavated until the latter half or last third of the 

 Quaternary epoch, and it was not until some time after then that the 

 ancestors of the present inhabitants obtained a foot-hold, and that 

 nearly the present relations of the existing fauna of Utah were estab- 

 lished. That this was the case is further supported by the fact that the 

 species of animals found in the cave are such as may have been descend- 

 ants of an assemblage which flourished when the country was more 

 humid than now. 



These species have been nearly as highly modified as the cave animals 

 of the Eastern States, and now that we know the exact geological age of 

 this Utah cave fauna, we seem warranted in assuming, as has been sug- 

 gested by Professor Copet and the writer (American Naturalist, v, 758, 

 1871), that the caves of Kentucky, Indiana, and Virginia were formed 

 during the Quaternary period, and that they were first tenanted late in 

 this period. 



This fact — for it is not simply a theory — is important in its bearings 

 on the evolution theory. The modifications undergone by these animals, 

 which consist chiefly in the absence of eyes or their partial develop- 

 ment, the elongation of the appendages of the mouth and thorax, and 

 the loss of color, are changes probably wrought with comparative sudden- 

 ness, namely, after perhaps a few hundred generations, rather than a great 

 number of generations, such as are demanded by extreme followers of 

 Mr. Darwin. Two sets of causes, it seems to us, have, by their resultant 

 action, produced the present cave forms, — first, we have the characters 

 inherited from their out-of-doors ancestors ; and, second, those super- 

 added by a cave life. Those due to the latter cause are slight compared 

 with those due to inheritance-force, since the former have evidently 

 acted for a brief period and have little of the cumulative force due to 



*Mr. Tryon writes me that Amnicola galbana Hald. was collected by Dr. Haydeu from 

 an ancient lake-terrace on Salt Lake. 



t Proceedings of the American Philosophical Society, April, 1871. 



