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 affected 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,” 4 
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 
great 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—aunless the numbers of 
both were diminished by the casualties of travel.2_ But on the 
place, but it is without invitation on his part, and the only particular bond of 
union not entirely selfish which keeps them together is the ery of alarm with 
which a stranger is greeted. 
1 Tn 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. 
2 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 with 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. 
