EFFECT OF SHIFTIXG OX SPECIES 185 



had not been seriously interrupted, suffered little modification. The 

 same rule applies for the Chemung fauna farther west. In McKean and 

 Warren counties, Pennsylvania, where there was presumably a continu- 

 ous succession of species on the same general ground, those species which 

 survived suffered little modification till the stage which, stratigraphic- 

 ally, is equivalent to the Waverh% as shown by the occasional immi- 

 grants from that fauna which appeared in the strata with them. The 

 metropolis for that class of faunas had shifted westward as far as central 

 Ohio, and with the shifting those species which survived suffered muta- 

 tion. 



HOMCEOTOPIC AND HETEROTOPIC FaUNAS 



It is to be noted that whenever fossil faunas are used for determining 

 the equivalenc}" of the formations containing them, t)r, in general, for 

 classif3dng stratigraphic formations on a time basis, it is b}^ means of the 

 likeness or dissimilarity of the fossils that the correlations are made. 

 The faunas of the Hamilton, Ithaca, Chemung, and Waverly formations 

 differ in the species composing them, but a large majority of the species 

 of each fauna belong to the same or closely allied genera, and hence the 

 several faunas are generically alike. Many of the species of each are so 

 similar that it is reasonable to infer that they are geneticall}^ related. 

 The faunas, whenever seen in a continuous section, bear the same order 

 of succession in relation to each other. The center of distribution of 

 each is different and, in general, approximately a hundred miles sepa- 

 rates them. At points between these centers of distribution the species 

 of the earlier fauna blend with those of the succeeding faunas of this 

 group, making it difficult to draw a sharp line of separation between the 

 two, and species which are dominant and are in general characteristic 

 of the separate faunas lap over and are found together in the same strata 

 in intermediate regions. Such a group of faunas may be regarded as 

 owing their similarity chiefly to the fact that they have become adjusted 

 to the same or closely similar conditions of environment, and on account 

 of this cause of their likeness may be called homoeotopic (from the Greek 

 o.aofo?, similar, and ro-09, place or environment). Thus it would be said 

 that the faunas of the Hamilton, Ithaca, Chemung, and Waveiiy forma- 

 tions are homoeotopic, while the faunas of the Hamilton and Genesee ai^e 

 heterotopic. So the faunas of the Chemung and Catskill are heterotopic. 

 It will be evident to those who have followed what I have said so far 

 that two faunas of this latter kind (^heterotopic), which are normally ad- 

 justed to different types of environment, although they may appear in 



XXVI— BcLL. Geol. Soc. Am., Vol. 14, 1902 



