Differentiation, Migration, and mimicry. 365 



which has facilitated frequent immigrations from the continent, 

 the process of Differentiation is only partially accomplished. Yet 

 there is scarcely a resident species which is not more or less 

 modified, and this modification is yet further advanced in the 

 westernmost islands than in those nearest to Africa. In Fuerta- 

 ventura and Lanzarote, waterless and treeless, there is little 

 change, and the fauna is almost identical with that of the neigh- 

 bouring Sahara. There is a Whinchat, Pratincola dacotice, 

 discovered by my companion, Mr. Meade-Waldo, peculiar to 

 Fuertaventura, which may possibly be found on the opposite 

 coast, though it has not yet been met with by any collectors there. 

 Now, our Whinchat is a common winter visitant all down the 

 West African coast, and it seems probable that isolation has 

 produced the very marked characters of the Canarian form, while 

 the continental individuals have been restrained from variation 

 by their frequent association with their migratory relations. A 

 similar cause may explain why the Blackbird, an extremely 

 common resident in all the Canary Islands, has not been modified 

 in the least, since many migratory individuals of the same species 

 sojourn every winter in these islands. Or take the Blue Titmouse, 

 Our familiar resident is replaced along the coast of North Africa 

 by a representative species, Parus ultramarinus, differentiated 

 chiefly by a black instead of a blue cap, and a slate-coloured 

 instead of a green back. The Titmouse of Lanzarote and Fuer- 

 taventura is barely separable from that of Algeria, but is much 

 smaller and paler, probably owing to scarcity of food and a dry 

 desert climate. Passing, 100 miles further to sea, to Grand 

 Canary, we find in the woods and forests a bird in all respects 

 similar to the Algerian in colour and dimensions, with one excep- 

 tion — the greater wing-coverts of the Algerian are tipped with 

 white, forming a broad bar when the wing is closed. This, 

 present in the Fuertaventura form, is represented in the Canarian 

 by the faintest white tips, and in the birds from the next islands. 

 Tenerife and Gomera, this is altogether absent. This form has 

 been recognised as P. tenerifce. Proceeding to the north-west 

 outermost island, Palma, we find a very distinct species, with 

 different proportions, a longer tail, and white abdomen instead of 

 yellow. In the Ultima Thule, Hierro, we find a second very 

 distinct species, resembling that of Tenerife in the absence of the 

 wing bar and in all other respects, except that the back is green 



