84 
THE DESCENT OF MAN. 
Part I. 
preservation. Nevertheless the migratory instinct is so 
powerful that late in the autumn swallows and house- 
martins frequently desert their tender young, leaving 
them to perish miserably in their nests . 18 
We can perceive that an instinctive impulse, if it be in 
any way more beneficial to a species than some other or 
opposed instinct, would be rendered the more potent of 
the two through natural selection ; for the individuals* 
which had it most strongly developed would survive in 
larger numbers. Whether this is the case with the 
migratory in comparison with the maternal instinct, 
may well be doubted. The great persistence or steady* 
action of the former at certain seasons of the year 
during the whole day, may give it for a time para- 
mount force. 
Man a social animal . — Most persons admit that man 
is a social being. We see this in his dislike of solitude,, 
and in his wish for society beyond that of his own 
family. Solitary confinement is one of the severest 
punishments which can be inflicted. Some authors sup- 
pose that man primevally lived in single families ; but 
at the present day, though single families, or only twc 
or three together, roam the solitudes of some savage 
lands, they are always, as far as I can discover, friendly 
with other families inhabiting the same district. Such 
families occasionally meet in council, and they unite 
18 This fact, the Rev. L. Jenyns states (see his edition of 4 White’s 
Nat. Hist, of Selborne,’ 1853, p. 204) was first recorded by the illus- 
trious Jenner, in ‘ Phil. Transact.’ 1824, and has since been confirmed 
by several observers, especially by Mr. Blackwail. This latter careful 
observer examined, late in the autumn, during two years, thirty-six 
nests ; he found that twelve contained young dead birds, five contained 
eggs on the point of being hatched, and three eggs not nearly hatched .. 
Many birds not yet old enough for a prolonged flight are likewise 
deserted and left behind. See Blackwail, 4 Researches in Zoology,’ 
1834, pp. 108, 118. For some additional evidence, although this is not 
wanted, see Leroy, 4 Lettres Phil.’ 1802, p. 217. 
