550 Mr. C. Dixon on the 



one. The same remarks apply to the various subspecific 

 forms of Sitta europaa. 



But whatever theory we may adopt to account for the 

 differences in the colour of nearly allied species from different 

 localities, the fact that interbreeding takes place remains; 

 and it is this fact which I wish to press upon the attention 

 of ornithologists. 



The case of the Crows and the Goldfinches, where the 

 extreme forms interbreed, is exceptional. The case of the 

 Shrikes and the Dippers, where each extreme form inter- 

 breeds with an intermediate form, may also be exceptional; 

 but the cases where the individuals of each valley interbreed 

 with their immediate neighbours, and where the range is 

 great enough to make the sum of a series of small differences 

 show a large difference in the extremes, is by no means un- 

 common. What I wish to emphasize is the fact that all 

 these are cases of interbreeding, the difference in the three 

 modifications of interbreeding which I have cited being one 

 of degree and not of kind. 



XLI. — Notes on the Birds of the Province of Constantine, 

 Algeria, By Charles Dixon. 

 (Plate XIV.) 

 The following notes refer to the birds either identified or 

 collected during a short trip to Biskra and the Aures moun- 

 tains made in company with Mr. Elwes. Considering that 

 our stay was such a brief one, and that travelling took up at 

 least half of the month we were away from England, our 

 success was far beyond our highest expectations. Amongst 

 our captures were a dozen specimens of the rare little Algerian 

 Coal Titmouse, Pariis ledoucii ; specimens of Saxicola lugens 

 with the sex carefully ascertained, settling the much-vexed 

 question as to the difference of plumage in the sexes of this 

 bird; several examples of Emberiza saharce-, Phylloscopus 

 honelli ; a female in breeding-plumage of the rare Sylvia deser- 

 ticola ; and, last but not least, we secured two specimens of 

 a hitherto undescribed Chat. 



