John Francis Walker — On Species. 15 



1892. " On two Heads and a Skin of the Yarkand Stag": Proc. ZooL Soc, 



pp. 116-117. 



1893. " On a Stag {Cervus thoroUi) from Tibet, and on the Mammals of the Tibetan 



Plateau" : Proc. Zool. Soc, pp. 444-449. 

 1893. "On the Scientific Name of the Indian Cuckoo": Proc. Zool. Soc, 

 pp. 315-319. 



1893. " On some Genera of Oriental Barbets " : Ibis (6), v, pp. 234-240. 



1894. " On the Scientific Names of the Imperial and Spotted Eagles, and on the 



Generic Names of Bonelli's Eagle and of the Black Eagle " : Ibis (6), vi, 

 pp. 283-219. 



1895. " The Fauna of British India, etc." : Birds, iii, pp. xiv, 450. 

 1898. " The Fauna of British India" : Bu'ds, iv, pp. xxxi, 500. 



1898. "Notes on Lepus oiostolus and L. liallipes from Tibet, and on a Kashmir 



Macaque": Proc. Zool. Soc, pp. 357-362. 



1899. " On some Species of Shells of the genera Streptaxis and Ennea from India, 



Ceylon, and Burma" : Proc. Zool. Soc, pp. 764-770, 1 plate. 



1900. " On a particular Form of Surface, apparently the Eesult of Glacial Erosion, 



seen on Loch Lochy and elsewhere " : Journ. Geol. Soc, Ivi, pp. 198-203. 



1901. "Eeport on the Geological Congress of Paris, 1900": Mem. Geol. Surv. 



India, xxx, pp. 225-230. 

 1901. "The Distribution of Vertebrate Animals in India, Ceylon, and Burma": 

 Phil. Trans. Eoy. Soc, ser. B, cxciv, pp. 335-436. 



II. — On the Formation of a Species. 



By John Francis Walker, M.A., F.L.S., F.G.S., etc. 



fpHE object of this communication is to try to give a theory of 

 X what takes place when one species is changed into another. 

 When the species A is converted into the species B there must 

 be an intermediate transitional stage in which the collection of 

 individuals is neither the species A nor the species B. I have 

 already defined a species as a centre round which individuals are 

 thickly clustered, and the spaces between these centres may be 

 either devoid of individuals or contain here and there an abnormal 

 form (Davidson's " Suppt. Jurassic Brachiopoda," Pal. Soc, 1876, 

 p. 122). 



This idea may be expanded thus : — The true type of a species 

 is its centre, where the individuals are most thickly clustered and 

 most closely resemble each other ; those further from the centre 

 differ less or more widely from the type as they approach nearer 

 the boundary of the species. 



As a species moves, either in geographical space or geological 

 time, the position of its centre will gradually alter, if its environ- 

 inent be different, so that forms like those contained near the 

 boundary of the species A, and therefore not typical of it, may 

 become the centre of the species B and typical of it. The specimen 

 first figured, the so-called type of a species, may be characteristic of 

 it if it happens to be an individual near the centre of the species, 

 Ibut it is not so if it is a specimen near the boundary of the species ; 

 &, named figured specimen is only a fixed point. 



The so-called species are of unequal value ; they may have been 

 named from an abnormal form of growth, or from an immatui'e 

 specimen in which the characters have not been fully developed, 



