DIFFERENTIATION, MIGRATION, AND MIMICRY. 375 



ever noticed to outstrip an express train on the Great Northern 

 Railway. 



Observation has shown us that, while there is a regular and 

 uniform migration in the case of some species, yet that, beyond 

 these, there comes a partial migration of some species, immi- 

 grants and emigrants simultaneously, and this, besides the 

 familiar vertical emigration from higher to lower altitudes and 

 vice versa, as in the familiar instance of the Lapwing and Golden 

 Plover. There is still much scope for the field-naturalist in 

 observation of these partial migrations. There are also species 

 in which some individuals migrate and some are sedentary; e. g. in 

 the few primeval forests which still remain in the Canary Islands, 

 and which are enshrouded in almost perpetual mist, the Woodcock 

 is sedentary, and not uncommon. I have often put up the bird 

 and seen the eggs ; but in winter the number is vastly increased, 

 and the visitors are easily to be distinguished from the residents 

 by their lighter colour and larger size. The resident never leaves 

 the cover of the dense forest, where the growth of ferns and 

 shrubs is perpetual, and fosters a moist, rich, semi-peaty soil, in 

 which the Woodcock finds abundant food all the year, and has 

 thus lost its migratory instincts. 



But why do birds migrate ? Observation has brought to light 

 many facts which seem to increase the difficulties of a satisfactory 

 answer to the question. The autumnal retreat from the breeding 

 quarters might be explained by a want of sufficient sustenance as 

 winter approaches in the higher latitudes ; but this will not 

 account for the return migration in spring, since there is no per- 

 ceptible diminution of supplies in the winter quarters. A friend 

 of mine, who was for some time stationed as missionary at 

 Kikombo, on the high plateau south-east of Victoria Nyanza 

 Lake, almost under the equator, where there is no variation in 

 the seasons, wrote to me that from November to March the 

 country swarmed with Swallows and Martins, which seemed to 

 the casual observer to consist almost wholly of our three species, 

 though occasionally a few birds of different type might be noticed 

 in the larger flocks. Towards the end of March, without any 

 observable change in climatic or atmospheric conditions, nine- 

 tenths of the birds suddenly disappeared, and only a sprinkling 

 remained. These, which had previously been lost amid the 

 myriad of winter visitants, seemed to consist of four species, of 



