7o THE POPULAR SCIENCE MONTHLY 



remembered, however, that a boreal climate probably did not then mean 

 a frigid climate, and that the differences in temperature were probably 

 not so great as at the present time. 



The conclusions justified by the evidence from fossil invertebrates 

 are: 



1. In the Paleozoic there is practically no faunal evidence of cli- 

 matic zones comparable with those that now exist. 



2. In the Mesozoic there is a more or less definite zonal distribution 

 of faunas which may be in part due to differences in climate but this 

 conclusion in each case should be checked by the study of the floras and 

 all other available lines of evidence. 



3. From the middle of the Tertiary on through the Pleistocene trust- 

 worthy conclusions as to climatic conditions and changes can be made 

 by direct comj)arisons with the distribution of living faunas. 



THE MIGRATION AND SHIFTING OF DEVONIAN FAUNAS 



By Professor HENRY S. W T ILLIAMS 



CORNELL UNIVERSITY 



IN the year 1881 I presented before the American Association for 

 the Advancement of Science the first definite announcement of 

 the theory of recurrent faunas, applying it to the fauna of the Mar- 

 cellus, Genesee and Ithaca black shales of New York, which I then 

 conceived to be represented by the continuous fauna of the black shales 

 of Ohio, Indiana, Kentucky and Tennessee; and also in the same paper 

 the theory of shifting of faunas was applied to the Hamilton and Che- 

 mung faunas of central New York. 1 Since that time a large amount of 

 evidence has been accumulated confirming these hypotheses. 



The two hypotheses are correlated. Recurrence, or the departure of 

 a fauna, its replacement by another and its final reappearance in the 

 same section at a higher level, become the facts upon which the hypoth- 

 esis of shifting of the faunas is based; and only on the assumption of 

 the continuance and shifting of a fauna without losing its character- 

 istics can we satisfactorily explain its recurrence. 



The following facts are among the more imjoortant which have come 

 to light in the course of my studies : 



§ 1. The Catshill sedimentation was shown to be thicker and to start 

 lower down in the geological column in eastern New York than in 

 middle and western New York. In eastern New York it began while 

 the Hamilton marine fauna was still present and cut it off, bringing in 

 estuarian conditions with a brackish water and land fauna and flora. 

 In middle New York no Catskill sedimentation is present until after 



1 Proc. American Association for the Advancement of Science, Vol. XXX., 

 p. 186, etc. 



