189C.1 175 [Ortmann. 



25. Mammalia of the Uinta Formation. Pt. iii : Tlie Perissodactyla. 



Ft. iv : The Evohition of the Uugulate Foot. Trans. Amer. 



Philos. Soc, 1889. 

 2G. Osboru, H.F., and Wortman, J. L.: Fossil Mammals of the Lower 



Miocene White River Beds. Bull. Am. Mus., Vol. vi, pp. 



199-288. 



27. Perissodactyls of the Lower Miocene White River Beds. Bull. 



Am. Mus., Vol. vii, pp. 343-375. 



28. Schlosser, M.: Beitrage zur Kenntniss der Stammesgeschichte der 



Hufthiere. Morphologisches Jahrbuch, 1886, Bd. xii. 



29. Scott, W. B.: On the Osteology of Mesohippus and Leptomeryx. 



Jouru. of Morph., Vol. v, No. 3. 



30. Evolution of the Premolar Teeth in Mammals. Proc. Acad. 



Nat. Sci., Philadelphia, 1892. 



31. Mammalia of the Deep River Beds. Trans. Amer. Philos. Soc, 



Vol. xvii. 



32. Wortman, J. L.: On the Divisions of the White River or Lower 



Miocene of Dakota.. Bull. Am. Mus. Nat. Hist., Vol. v, pp. 

 95-106. 



33. Zittel, K. : Handbuch der Palseontologie. Band iv : Mammalia. 



Miinchen und Leipzig, 1893. 



On Natural Selection and Separation. 



By Arnold E. Ortmann. 



{Read before the American Philosophical Society, May 15, 1S96.) 



1. It is generally understood that the chief merit of Darwin in creating 

 his theory of the origin of species is the establishment of the principle of 

 Natural Selection, and that by the introduction of this principle the pro- 

 cess of development of organic nature from the conditions existing in 

 former times to the present may be made intelligible, and mostly it is also 

 understood, that natural selection is only one of the factors playing a 

 part in the formation of species. But the proper line of action of natural 

 selection, as conceived by Darwin, is estimated by some other authors 

 very differently. I refer especially lo Weismann, who calls natural 

 selection "all-sufficient," which implies that it is the only factor that 

 forms species ; but I regard this expression only an exaggeration, since 

 Weismann contradicts himself in this respect.* The assertion, however, 

 stands, that natural selection of itself may form different species. On 

 the other hand, Eimer maintains, in opposition to Weismann, that there is 



* See Ortmann, Qrundziige der marinen Tiergeographie, 1896, p. 30. 



