On Species <§• Varieties in Birds. 137 



Cyanecula wolfi, which has a white patch in the centre of the 

 blue, inhabits western Europe and breeds numerously in 

 Holland. In the winter season the two species meet in Africa, 

 but as a proof that they keep apart during their migrations I 

 may name that in Heligoland, where they occur by thousands 

 when on passage, Mr. Gatke informs me only the Red-Spotted 

 Blue Throat occurs, and during the forty years he has been 

 there he has only seen one or two White-Spotted Blue Throats, 

 which were evidently stragglers driven out of their course. 

 One curious circumstance recorded by Mr. Gatke is the fact 

 that these Blue Throats evidently pass right over Europe in a 

 single night, for they have never been recorded on the spring 

 passage between north Africa and Heligoland, and taking all 

 the data respecting their passage into consideration, I believe 

 that he is correct in his surmise. That they can traverse the 

 distance in one night I fully believe, for birds migrate at great 

 altitudes, frequently from two to three miles above the earth, 

 where the air is rarified, and offers but little resistance, and 

 that, when at great altitudes, they can and do travel at a 

 speed that will enable them to traverse the distance in the time 

 named his researches all tend to prove. This gentleman is 

 now busy at a most exhaustive work on migration, and I am 

 looking forward with great interest to seeing its publication. 



Amongst the Warblers there are many instances of eastern 

 and western forms, as, for instance, Sylvia curruca, of which, 

 an eastern form has been treated as distinct under the name 

 oi Sylvia affinis, Sylvia orphea, of which a large-billed eastern 

 form exists, and Sylvia melanocephala, which is replaced in 

 the east by a very closely allied form, Sylvia momus ; Phyllos- 

 copus collybita, also is replaced in the east by Phylloscopus 

 tristis ; Hypolais opaca, by Hypolais olivetorum ; Hypolais 

 pallida, by Hypolais caligata; and ^don galactodes, hy ^ don 

 familiaris, all of which are climatic races, which have, in the 

 course of time, become valid species. The genus Hypolais is 

 peculiar, inasmuch as the birds themselves resemble their 

 allies belonging to the genus Acrocephalus so closely that it is 

 often very difficult to separate them generically, but if one sees 

 the nest and eggs there can be no shadow of doubt, as those 

 of a Hypolais differ so widely from those of an Acrocephalus. 



Amongst the Titmice one finds several climatic varieties 

 which have merged into distinct, though closely allied species. 

 Thus one finds Acredula caudata in northern Europe, Acredula 

 rosea in Great Britain and m continental Europe down to the 

 Pyrenees and Appenines, below which it is replaced by 

 Acredula irbii, and again in south-eastern Europe and Asia 

 Minor there is a very distinct form, Acredula tephronota. 

 Parus ater, which inhabits continental Europe, is replaced in 



