297 
of the European Ornis, and its Causes . 
ornithologists to Brehm's position since the year 1855 : between 
Thienemann and Bonaparte there is a difference of 100; between 
Thienemann and Brehm of 540 species. The gap between these 
numbers must have its origin in principles, or in a diversity of 
views as to the separation of species. 
If we examine the group of smaller numbers, we find that it 
contains essentially four different sections :— 
1. Thienemann, with 401 species as the minimum. 
2. Keyserling, Blasius, Schlegel, and De Selys, with 424-430 species. 
3. Bonaparte, 1850, and Des Murs, with 463-466 species. 
4. Bonaparte, 1856, with 500 species. 
Amongst the species cited by Thienemann there are but few 
that are open to doubt,—such, for example, as Sitta uralensis , 
Licht.; Sylvia sarda, Marm.; Anthus cervinus , Pall.; Glareola 
melanoptera, Nordm.; Podiceps arcticus, Boie ; and Uria hring - 
via, Briinnich,—in respect of the determination of the species, 
and locos ohscurus, T., and Lanius tschagra , Vieill., with regard 
to the domicile. In the union of the Crossbills he goes very 
sharply to work, but not entirely without reason. In other 
unions he is decidedly wrong,-—for example, in that of Circus 
cineraceus, Mont., and C. pallidus, Sykes. Numerous new dis¬ 
coveries since the year 1846 would have brought his list pretty 
accurately to the position of those under No. 2. This position 
is exceeded by the lists under No. 3 by about 40 species, and by 
that under No. 4 by about 80 species. 
After carefully comparing all known facts, I find that the 
European Ornis consists, in round numbers, of 425 indubitable 
species of birds breeding in Europe [Brutvogeln) , besides 60 
varieties or races, which are frequently regarded as species, and 
about 100 exotic species. These 60 varieties, which are some¬ 
what increased in Bonaparte's Catalogue, form the still doubtful 
specific element in the European fauna, if we follow the majority 
of ornithologists. They constitute almost exactly the eighth part 
of the total valuation. By the application of rigorous principles 
the number may be still greatly reduced. 
Up to the year 1820, and, with few exceptions, even up to 
the year 1840, ornithology, in its conception of species, was 
developed exactly in analogy with the rules followed in other 
VOL. hi. x 
