22 THE AGE OF MAMMALS 



extreme in his idea of the total depopulation of the continents through 

 great physical revolutions ; we have no evidence that such sweeping changes 

 have ever occurred, yet he was not far from the truth, and it is certain that 

 in these specific illustrations, cited above, he clearly thought out and 

 furnished some of the chief ideas underlying our modern work, namely, 

 of the invasions of great groups of mammals through the forma- 

 tion of new routes for migration and of wide resulting con- 

 trasts between the existing and the extinct forms of life, or 

 faunas in all continental areas. 



As regards mammalian origin and descent it is well known that Cuvier 

 was not an evolutionist, but on the contrary a convinced believer in special 

 creation. This belief kept him from fully anticipating the ground work 

 of modern palaeontology. He did not consider the Age of Mammals as 

 furnishing the source of any animals now existing. He recorded his dis- 

 covery of the mammals of the Gypse de Montmartre as a revelation of a 

 phase of mammal life which he believed to belong early in the Age of Mam- 

 mals (it is now known to be Upper Eocene), but he did not seek among 

 these mammals ancestors of existing forms. Although he believed that 

 all these older forms had become extinct, he did not appeal to new crea- 

 tions to produce the species now existing, but maintained that such species 

 were existing elsewhere, that is, in other parts of the world. This in- 

 genious and interesting feature of Cuvier's theories as to the replacement 

 of faunas has not been understood sufficiently because, as recently pointed 

 out by Deperet,^ he has been credited generally with a theory which really 

 arose in the imagination of some of his followers rather than his own, 

 namely, of a succession of extinctions followed by a succession of creations. 

 Cuvier rather believed that an extinction on one continent or in one region 

 was followed by repopulation through migration from another region, and 

 he illustrated his meaning very clearly in the hypothetical cases cited 

 above of the possible invasion of the sea over the continent of Australia 

 and subsequent repopulation from Asia. 



The Law of Adaptive Radiation 



The law of evolution even as crudely perceived by Buffon added another 

 element of fascination to the ideas of centers of origin and of migration, 

 namely, that of modification of mammals under new and strange con- 

 ditions of environment. Such general modification was about as far as 

 Buffon's thoughts w^ent. Those of Lamarck went farther, namely, to 

 adaptation to new conditions of life, and with this idea is coupled his con- 

 ception of the principle of divergence or radiation in the formation of differ- 

 ent habits and the search for different kinds of food. 



' Deperet, L'evolution des Mammif^^es tertiaires; I'importance des migrations (Eocene). 

 C. R. Acad. Sci., Paris, Vol. CXLI, Nov. 6, 1905, p. 702. 



