Mr. H. E. Dresser. 135 



from which subsequent writers have by degrees only rescued 

 .it. Amongst the earliest to realise the importance of collecting 

 together a large series of specimens, so as to illustrate the 

 geographical variation in species, was the late Marquis of 

 Tweeddale, who was indefatigable in securing large series from 

 various countries, and when we commenced the issue of "The 

 Birds of Europe," I, following in Lord Tweeddale's footsteps, 

 used my best endeavours to secure for examination and com- 

 parison as large a number of specimens from different localities 

 as I possibly could, and always had his hearty co-operation. 

 Since then many collectors have done the same, and hence the 

 question of climatic variation is being now more thoroughly 

 worked out than formerly, though very much remains still to 

 be done before much light can be thrown on the subject. We 

 have to deal with individual variations, climatic variations, or 

 such as arise from the nature of the soil inhabited by the birds, 

 and variations that arise from interbreeding; and it maybe 

 well, before going further into these various causes, to take a 

 glance over some of our European species of birds, and note a 

 few facts especially as regards climatic variations; for climatic 

 variations, when continued isolation has taken place for a 

 •long space of time, frequently merge into good species, and 

 hence it is often difficult to determine with any degree of 

 certainty where the variety ends and the species begins. 



Take, for instance, the Redtailed Fieldfare, Tiirdus 

 naumanni) which is a very good and distinct species ; still 

 one finds in eastern Russia examples of this bird, which show 

 much similarity to Tiirdus dichius, and makes one doubt 

 whether they may not be hybrids between the two species ; 

 and, again, I have seen in the Berlin Museum a large series of 

 specimens, showing an almost uninterrupted passage from two 

 other perfectly distinct Thrushes, Tiirdus ruficollis, which has 

 the throat and upper breast bright fox red, and Turdus 

 atrigiilaris, which has these parts deep black, and in this case 

 I cannot but believe that the two species interbreed in localities 

 where their breeding range interosculates, and that the h3'brids 

 are fertile, as is the case in so many instances where allied 

 species interbreed. Dr. Severtzoff, the celebrated Russian 

 ornithologist, believes that these four species of Thrushes are 

 all sprung from one parent stock, and he obtained specimens 

 of a Thrush in Turkestan (which I believe to be a form of 

 Turdus atrigularis), but which he named Turdus mystacinus, 

 and which he believes to be the parent stock of all these four 

 species. 



The Blue Rock Thrush, Monticola cyanus, in Europe is 

 subject to scarcely any variation, but as one goes further east 

 one finds another closely allied form, M. pandoo, and still 



