THE MIGRATION OF ANIMALS. 127 



former land flights is thus seen to operate in the after ages as an un- 

 erring guide over the changed aspect of affairs, and to lead the 

 migrants safely and securely over the pathless deep. In the case of 

 the carrier-pigeons we probably witness a high development of the 

 same instinct, associated with a special faculty of memory and with a 

 wondrous perfection of sight. 



Inherited habits, induced by changes of food, and these latter in 

 turn produced by alterations in climate and accompanied by changes 

 in the distribution of land and sea, are thus noted to constitute the 

 factors in inaugurating the habit of migration. It seems admissible, 

 however, to suggest, by way of conclusion, the fashion in which 

 another and different set of circumstances in the life of birds might 

 give rise to the adoption of migratory habits, and cause a species to 

 assume a place in the list of migrants. Let us imagine a number of 

 birds to be carried as some species not unfrequently are by con- 

 trary winds into an area differing as widely in climate from their 

 native haunts as Britain does from Northern Africa. The result to 

 the birds, should such an event happen in winter, would be of the 

 most untoward description ; but if the northward and forced flight 

 were taken in summer, the birds finding abundance of insect-fare in 

 Britain, might find in the latter fact, and in the genial climate, an in- 

 ducement to prolong their visit. Imagine, further, that the breeding 

 season of these birds arrived in due course an event which the 

 plentifulness of food might and probably would expedite and we 

 should find the young to be born in the new land ; the production 

 of more than one brood (as in the swallows) being determined pro- 

 bably by the amount of food and the continued geniality of the 

 climate. 



The fact of the young being reared in any particular locality 

 possesses of itself a sufficient and powerful effect in inducing 

 a close association between the bird and the locality. Hence the 

 production of the young in the new home would unquestionably 

 tend to impress the birds in favour of the new locality. The return- 

 ing cold of autumn and the scarcity of insect food would serve as a 

 sufficient cause accounting for the southward migration. And if to 

 the condition of temperature we add the consideration that land may 

 have prevailed where the Mediterranean Sea now exists, the original 

 home of the birds might readily enough be found. Admitting, as 

 before, that of the "finding instinct" of birds we know literally 

 nothing, the idea of a continuous land surface is geologically both 

 possible and probable. The arrival of the young brood, led by their 

 elders, in Africa, would conclude the preliminary conditions for the 

 establishment of the migratory propensity. Then comes the con- 

 sideration of the force of habit and instinct. The instinct of having 

 bred in the northern land would serve as a sufficient incentive on the 



