206 ON THE GEOGRAPHICAL 



do not seem to inhabit a great extent of territory ; such 

 is the Xenodon Michaelis of Spain, the Psamniophis 

 Dahlii of Dahnatia, which is also found in Greece, and 

 which approaclics, by its slender form, to the tree-snakes ; 

 the Dipsas fallax of the same countries, which should bo 

 considered as an anomalous species of the genus ; lastly, 

 the Tortrix oryx, which only occurs in Greece, and of 

 which the deserts of Africa and of Asia are the true na- 

 tive regions. In comparing the observations which the 

 other animals of Europe furnish, with those wdiich we 

 have announced on the reptiles of this part of the world, 

 we may deduce some analogies. We see that the animals 

 of the northern regions are often replaced in the centre 

 of Europe by others Avhich form local varieties, or some- 

 times even races ; and the comparison of the animals of 

 central Europe, with those of its southern regions, often 

 presents similar results. We may cite numerous facts 

 to support this opinion ; and I shall state a few. Our 

 Raven is replaced in the Feroe Isles, by a variety with 

 a mixture of black and white feathers. The Hooded 

 Crow, and the Black Crow, are two races of the same 

 genus which represent each other mutually ; the first be- 

 longs to the northern parts of Europe. It is well known 

 that the same thing holds with respect to the Sturnus 

 volgaris, and S. unicolor ; the last of which, more espe- 

 cially, inhabits the south of Europe. Our Emberiza 

 schoeniculus is replaced, in Dahnatia and in Italy, by the 

 Emberiza palustris, which has ordinarily a much stronger 

 bill ; but its existence as a distinct species cannot be 

 proved, because we often find individuals exactly inter- 

 mediate between the two races.* Everybody knows the 

 local races which our Sparrow produces among the Alps 



* AVe must not imagine that each of these races is, in relation to the 

 place of habitation, perfectly separated from the race which it replaces ; 

 very often they mingle in their migrations, or live in the same places, 

 disappearing insensibly as their representatives increase ; it happens, 

 also, that individuals of the two races propagate together, as happens 

 with the Corvus cornix and C. corone — a fact which I have ascer- 

 tained by numerous observations, made in the neighbourhood of Dres- 

 den. Consult, on these questions, the excellent works of M. Gloger of 

 Breslau. 



