546 Mr. D. A. Bannermaii on the [Ibis, 



evolution of birds will not find anything new in the following 

 pages, yet to some the facts contained may be of interest, 

 and I sliall therefore attempt to describe the factors which 

 have brought aliout such a remarkable dift'eientiation of 

 forms as we find in the Canary Islands to-day. 



Let ns suppose for a moment that a siuall flock of 

 Passeiine birds which have been wintering on the Guinea 

 coast are wending their way north to breed in Europe. 

 Strong westerly winds prevail, and the birds which usually 

 hug the coast of Africa are blown corisiderably out to sea, 

 till by chance they sight the island of Tenerife and make 

 towards it. Having gained the island they find themselves 

 in an environment which, from their point of view, is all 

 that can be desired. An abundance of fruit and of insect 

 life, plenty of food, no lack of cover, a climate very similar 

 at this time of early spring to that which they iiad hoped to 

 find at tlieir journey's end — hundreds of miles farther north. 

 Is it to be wondered that of this little flock, when the wind 

 drops and the rest set out again on their long journey, a 

 pair or two should remain and breed in the island. 



Having escaped the worst enemies in the form of 

 Sparrow-Hawks, a brood is successfully reared ; and in 

 course of time the species increases and spreads to other 

 islands of the group. These uevvly arrived immigrants 

 at once come under a host of fresh influences. In course 

 of ages variation takes place in more than one direction 

 brought about by the varied factors which they meet with 

 in the dift'erent islands, some of which we shall now discuss. 



Darwin has shown tliat the Presence of Enemies or the 

 presence or otherwise of other species, with which an immi- 

 grant has to compete in its struggle for existence, lias as 

 much to do with the diiferentiation of a species as the 

 physical conditions of the country. Had the Canary Islands 

 been of continental origin, we should probably have found 

 terrestrial mammals and rodents, as well as snakes, living in 

 most of the islands, which would have preyed on the ground- 

 nesting birds, and in various ways influenced the avifauna. 

 But it is a remarkable fact, and one whicli strongly negatives 



