MIGRATION 555 



The mode in which the want of sustenance produces Migration 

 may best be illustrated by confining ourselves to some of the un- 

 questionably migrant Birds of our own northern hemisphere. As 

 food grows scarce toward the end of summer in the most northern 

 limits of the range of a species, the individuals aff'ected thereby 

 seek it elsewhere ; in this way they press upon the haunt of 

 other individuals : these in like manner upon that of yet others, 

 and thus 



' ' The waves behind impel the waves before, " ^ 



until the 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 seems satisfactorily to explain the southward movement of 

 many migrating Birds in the northern hemisphere ; but when we con- 

 sider the return movement which takes place some six months later, 

 doubt may be entertained whether scarcity of food can be assigned 

 as its sole or sufficient cause, and perhaps it would be safest not to 

 come to any decision on this point. 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 

 gTeat a population at that season are deficient in certain necessaries 

 for the nursery. Nor does it seem too violent an assumption to 

 suppose that even if such necessaries are not absolutely wanting, yet 

 that the regions in question would not supply sufficient food for 

 both parents and offspring — the latter being, at the lowest com- 

 putation, twice as numerous as the former — unless the numbers of 

 both were diminished by the casualties of travel.- But on the 



2"ilace, but it is without invitation on his part, and the only particular bond of 

 union not entirely selfish which keeps them together is the cry of alarm with 

 which a stranger is greeted. 



1 In regard to Migration the word ' ' wave " is only allowable as a poetical 

 figure of speech, since the particles composing a real wave do not necessarily 

 move onward. 



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

 all 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 Avith the emigrants from the north, and 

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

 by the force of the former, according to the explanation given of the southward 

 movement of northern migrants. But, though we know almost nothing of the 

 Migration of birds of the other hemisphere, yet, when we regard the comparative 

 deficiency of the land in south latitudes all round the world, it is obvious that 

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

 and its effect may be practically disregarded. 



