433 Rev. H. B. Tristram on the 



and are possibly occurring still. We know so very little of the 

 causes which in the majority of cases make species rare or com- 

 mon, that there may be hundreds of others at work, some even 

 more powerful than these, which go to perpetuate and eliminate 

 certain forms " according to natural means of selection." But 

 even these superficial causes appear sufficient to explain the 

 marked features of the Desert races which frequently approach 

 30 very closely the typical form, and yet possess such invariably 

 distinctive characteristics, that naturalists seem agreed to elevate 

 them to the rank of species. The differences in size may be 

 yet more simply explained by the facility or difficulty of sustain- 

 ing existence in varying localities. On similar principles we 

 may account for the existence of such a bird as Galerida macro- 

 rhynfiha in the warm, genial climate of the Oases, where, winter 

 being unknown, and food always abundant and close at hand, 

 every stimulus is afforded to a vigorous development, while its 

 prey being generally hidden in the soft open mould of the gar- 

 dens and barley patches, any tendency to elongation of the bill 

 is fostered and encouraged, until we find a race two inches longer 

 than Galerida isabellina, and with a bill exactly double in length 

 (1 inch instead of 'S). 



A process precisely similar may be supposed to have developed 

 the various species of Desert Chats, until we find in the desert of 

 Souf that all distinctive trace of colour has been scorched out, 

 and instead of the brightly clad Saxicola stapazina, we have no 

 more cheerful representative of the genus than S. homochroa. 

 Widely as these two extremes appear to be separated, yet a well- 

 chosen series of the numerous African species of the class will 

 exhibit a range of transitions so imperceptible, that it will be 

 found very difficult without careful comparison to draw a line 

 between one species and the next. 



I cannot but hope that ere long ornithologists will systema- 

 tically recognize, what is already admitted in a great degree by 

 conchologists, the clear distinction between species and race. I 

 do not see any difficulty in taking as the true definition of a 

 species all the individuals who may resonably be presumed to 

 have a common origin, though among them there may exist 

 races differing from one another even in a considerable degree. 



I do not mean for a moment to imply that such birds as Rham- 



