THE MIGRATION OF ANIMALS. 117 



that the salmon rarely, if ever, feeds whilst in the rivers a case of 

 abstemiousness from food apparently paralleled by that of the fur 

 seals, which are currently believed to abstain from food during their 

 breeding season a period extending over three months. The skill 

 of the salmon-fisher, indeed, is taxed by the care with which he has 

 to prepare the bait for the fishes whilst they inhabit the rivers, and 

 the remarkable habit of voluntarily depriving themselves of food, 

 correlates itself in a very singular fashion to the instinct which pri- 

 marily leads the salmon to seek a change of abode. 



The migration of birds introduces us to instances of habit, not 

 only of regular and definite kind, but which moreover present a basis 

 for the exercise of that reasonable speculation which in its most 

 valued aspect leads us towards an appreciation of the "causes of 

 things." The periodical and for the most part regularly-timed flights 

 and disappearance of many birds from one land to another, were 

 noticed by the early observers of natural phenomena. So marvel- 

 lous, indeed, did the disappearance of many birds appear to their 

 minds, that theories which accounted for their absence on the 

 idea of their lying torpid at the bottom of lakes, or within the 

 kindly shelter of caves, were gravely discussed. And amongst 

 the thoughts concerning the causes of the flights of birds which 

 were ventilated, may be mentioned that of the Scandinavian 

 poet, who maintained that they migrated in the search for "light." 

 However poetic the fancy that birds sought " more light " may be, it 

 is unfortunately dispelled by a reference, not merely to the facts of 

 migration, but to those connected with the ordinary variation and 

 changes of the seasons. The southward flight of many birds begins, 

 or may even be ended, before the autumnal equinox ; the migrants 

 in such a case actually flying towards shorter days than towards 

 " more light." Similarly, many birds fly northwards before the spring 

 equinox, and thus find themselves in a land of shorter days and " less 

 light." Temperature is no doubt a very prominent and important 

 condition for consideration in connection with migration ; but this 

 subject has to be regarded in the light of other conditions, and in 

 any case is but one cause amongst many others which have operated 

 in producing and perpetuating the habits under discussion. 



The cuckoo is probably one of the most curious of migratory 

 birds, as well as one of the most familiar. It presents the well- 

 known habit of depositing its eggs in the nest of another bird, and 

 departs immediately, after it has thus secured foster-parents for its 

 young. Northern Africa appears to be the principal home-centre of 

 the cuckoo, and from its Ethiopian residence it flies northward to 

 Britain in March or April. Some three months altogether are spent 

 in the north, and the cuckoo then flies southwards in the early 

 autumn to its African home. This bird, it must be noted, deposits its 



