136 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



Europe, in Scotland, in Sweden, and in Russia, 

 where their areas of dispersion meet and overlap 

 reciprocally, we never meet with a cross or tran- 

 sitional form, which proves the existence of a 

 mixiological barrier between the two species. 



The fauna of our European Mammals and Birds 

 would offer us examples without end of this absence 

 of transitional forms between two or several neigh- 

 bouring species living together in the same localities. 

 Every one knows that the flocks of hooded crows 

 which invade our country in winter time include, 

 beside the black, which is the most numerous, 

 other crows ornamented with a sort of light grey 

 mantle on their wings. This is the mantled crow, 

 a species whose real domain is further east. These 

 two species form the same flocks, take part in the 

 same hibernal migrations, but do not mingle 

 genetically. No transitional forms can be ob- 

 served between the black and the mantled crow. 

 In our countries, also, two sparrows, the domestic 

 and the tree sparrow, live side by side, without ever 

 mingling or hybridizing; so do two tits, the large 

 and the small ; two wrens, the Regulus cristatus 

 and the ignicapillus, etc., etc. If at times a certain 

 difficulty is experienced in distinguishing between 

 the species of certain genera, the peewits, the gulls, 

 and the sea-swallows, for example, a careful ex- 

 amination of certain characteristics, the relative 

 length of the wing feathers, the colour of the eyes, 

 the disposition of the scales on the feet, etc., will 

 enable a practised ornithologist to avoid any errors 

 in determining these species. 



To sum up, observation shows us that, in actual 



