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70 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 climatic 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 trustworthy conclusions as to climatic conditions and changes can be made by direct comparisons with the distribution of living faunas.



N 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 Marcellus, 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 Chemung faunas of central New York. 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 hypothesis of shifting of the faunas is based; and only on the assumption of the continuance and shifting of a fauna without losing its characteristics can we satisfactorily explain its recurrence.

The following facts are among the more important which have come to light in the course of my studies:

§ 1. The Catskill 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