548 Mr. D. A. Bannerman on the [Ibis, 



been brought out by the desert-like conditions appertaining 

 in the eastern Canary Islands. 



But the islands of the western group, perhaps excepting 

 Hierro, are fairly similar in climate and in their geographical 

 nature to one another. Why, then, are the Titmice and 

 Chaffinches (I do not here refer to the Blue Chaffinches) so 

 distiibnted in the live islands of the western group? It has 

 been remarked elsev/iiere that the distrilnition is similar in 

 each case. 



This state of things points to the Tits and the Chaffinches 

 having arrived in a single immigration in the islands of 

 Tenerife, Gomera, and Gran Canaria^ where identical forms 

 of Ijoth Tit and Chaffinch are found. The islands of Palma 

 and Hierro would then have received their Chaffinch and 

 Titmouse (distinct forms of each bird being found in both 

 islands) at widely separated periods, and in the case of the 

 Tits the immigration has extended to Fuerteventura and 

 Lanzarote, vidiere still another race has been formed. 



Had the Titmice and Chaffinches spread over all the 

 islands of the Arcliipelago in a single immigration, we should 

 doubtless have found that if differentiation had taken place, 

 only one form existed, at any rate in the western islands ; 

 but the movement having been gradual {'i.e., having taken 

 place at widely separated })eriods) the birds have met Avith 

 fresh organisms in the different islands where they have 

 gained a footing. They have thus been influenced by the 

 various factors with which they have come in contact, those 

 factors having themselves altered considerably through lapse 

 of time. 



Instances of the case just mentioned are afforded by those 

 species which are peculiar to the Archipelago, and which 

 are universally distributed amongst the islands, but of 

 which there exists only one race respectively in all the 

 Archipelago. The Canaiian birds in point 'are Anthus 

 hertlieloti bertlieloti, Sylvia melanocephala leucogastra.^ Em- 

 heriza calandra thamieri, Biiteo huteo insiilarum, and Columba 

 livia canariensis. Darwin's explanation of this fact would 

 have been that these species emigrated in a body, so that 



