MIGRATION. 37 



speak of as "sedentary," it is only the adults whicli maintain their ground 

 throughout the year. Birds of prey drive away their offspring so soon as the 

 young are able to shift for themselves, for the simple reason of the impossibility 

 of both parents and progeny getting a livelihood in the same vicinity. Many 

 other species do the same, and with some, the time of expulsion is deferred from 

 the end of summer or the autumn to the following spring. As food grows scarce 

 towards the end of summer in the most northern limits of the range of a species, 

 the individuals affected thereby seek it elsewhere. Thus doing, they press upon 

 the haunts of others, and these in like manner upon that of yet others, and so 

 on, until the outward movement which began in the far north is communicated 

 to the individuals occupying the extreme southern range of the species at that 

 season, though but for such an intrusion these last might be content to stay some 

 time longer in the enjoyment of their existing quarters. 



This may explain the southward movement of all migratory birds in the 

 northern hemisphere, but when we consider the return movement some six months 

 later, it is doubtful whether scarcity of food can be assigned as its sole or sufficient 

 cause. On one side it may be urged that the more equatorial regions, which in 

 winter are crowded with emigrants from the north, though well fitted for the resort 

 of so great a population at that season, are deficient in certain necessaries for the 

 nursery. And it may be assumed that, even if such necessaries are not absolutely 

 wanting, the regions in question may not supply sufficient food for both parents 

 and offspring, the latter being, at the lowest computation, twice as numerous as 

 the former, unless the numbers of both were diminished by the casualties of travel. 



(If the relative proportion of land to water in the southern hemisphere were 

 at aU such as it is in the northern, we should no doubt find the birds of southern 

 continents beginning to press upon the tropical and equatorial regions of the 

 globe at the season when they were thronged with emigrants from the north, 

 and in such a case it would only be reasonable that the latter should be acted 

 upon by the force of the former, according to the explanation given of the south- 

 ward movement of northern migrants. But though we know almost nothing of 

 the migration of birds of the other hemisphere, yet, when we regard the com- 

 parative deficiency of land in southern latitudes all round the world, it is obvious 

 that the feathered population of such as nowadays exists can exert but little 

 influence, and its effects may be practically disregarded.) 



On the other hand, we must remember the pertinacity with which birds return 

 to their accustomed breeding places, and the force of this passionate fondness for 

 the old home must be taken into account, even if we do not allow that in it lies 

 the whole stimulus to undertake the perilous voyage. 



Mr. Wallace suggests (Nature, x. p. 459) that " survival of the fittest " has 

 had a powerful influence on the manner in which the habit of migration has 

 come to be adopted. "Let us suppose that in any species of migratory bird, 

 breeding can as a rule be only safely accomplished in a given area ; and further, 

 that during a great part of the rest of the year sufficient food cannot be obtained 

 in that area. It will follow that those birds which do not leave the breeding 

 area at the proper season will suffer and ultimately become extinct, which will 

 also be the fate of those which do not leave the feeding area at the proper time. 

 Now if we suppose that the two areas were (for some remote ancestor of the 

 existing species) coincident, but by geological and climatic changes gradually 



