59o TEE POPULAR SCIENCE MONTHLY 



rock layer in which it occurs and which it characterizes, long and suc- 

 cessfully employed by the invertebrate paleontologist, must be recog- 

 nized and used by us also. 



I need hardly refer to the fact that the determination of the geolog- 

 ical age and the successful correlation of many North American forma- 

 tions, ranging from Mesozoic to Pleistocene, depend in large measure, if 

 not entirely, on vertebrate fossils. I need only contrast the American 

 series of Pleistocene glacial and intergiacial stages, determinable at 

 present only by the strictly stratigraphic method of superposition, with 

 the carefully worked-out series in Europe, where each epoch of ice 

 advance and retreat is characterized by its particular fauna and flora. 

 That even the beginnings of stratigraphic paleontology, as contrasted 

 with the morphological, will lead to immediate and valuable results, is 

 strikingly shown by Professor Calvin's 1 recent paper in the Bulletin 

 of our parent society, the Geological Society, in which he describes the 

 Aftonian mammal fauna from the earliest of American intergiacial 

 stages. 



While readily admitting that the slow-moving invertebrate, living, 

 it may be, in the very mud which is destined to become the matrix of 

 its fossil remains, enjoys advantages as a prospective horizon-deter- 

 miner which the agile vertebrate can more readily, and does most will- 

 ingly, escape, still the short life of vertebrate species, and their com- 

 paratively rapid evolution, fit them for use as index fossils quite 

 admirably. The localization of mammalian faunas, their inability to 

 cross barriers such as ocean basins and great mountain ranges, their 

 dependence on temperature, etc., are comparable to similar conditions 

 circumscribing the free migration of invertebrates. We should not 

 expect to find in the distribution of vertebrate faunas the analogues of 

 the cosmopolitan graptolite zones of the Ordovician or the ammonite 

 zones of the Trias, but we can work out our major zones as recognized 

 by the great migrational movements among vertebrates, expressed in 

 changes in the faunas and the rock succession, which will give us a 

 world scale, and then, by interpolation, fill in the minor and local sub- 

 divisions which we probably shall not be able to correlate at once, but 

 which there is every reason to believe we may be able to do later. 



The attempt will be accompanied by difficulties which are not appre- 

 ciated by the invertebrate paleontologist, and I speak feelingly and from 

 experience, for there is a difference between collecting, on the one hand, 

 from a layer a few inches thick, crowded with shells, and, on the other, 

 tramping miles up hill and down over beds hundreds of feet thick, to 

 be rewarded by a few teeth, a lot of useless bone fragments or nothing. 

 Horizons based on vertebrates must include larger stratigraphic units 



1 Bulletin of the Geological Society of America, Vol. 20, pp. 341-356, Pis. 

 10-27, October, 1909. 



