80 THE DESCENT OF MAN. [Part I. 



tion ; as wlicii a dog rushes after a hare, is rebuked, 

 pauses, hesitates, pursues again or returns ashamed to his 

 master ; or as between the love of a female dog for her 

 young puppies and for her master, for she may be seen to 

 slink away to them, as if half ashamed of not accompany- 

 ine: her master. But tlie most curious instance known to 

 me of one instinct conquering another, is the migratory 

 instinct conquering the maternal instinct. The former is 

 ■wonderfully strong ; a confined bird will at the proper 

 season beat her breast against the wires of her cage, until 

 it is bare and bloody. It causes young salmon to leap 

 out of the fresh water, where they could still continue to 

 live, and thus imintcntionally to commit suicide. Every 

 one knows how strong the maternal instinct is, leading 

 even timid birds to face great danger, though with hesita- 

 tion and in opposition to the instinct of self-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 miser- 

 ably in their nests.^*' 



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 



'* This fact, the Rev. L. Jenyns states (see his edition of ' 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. Blackwall. This latter careful ob- 

 server 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 Blackwall, ' Kcscarches in Zoology,' 1834, pp. 108, 118. 

 For some additional evidence, although this is not wanted, see Lcrov, 

 'Ijbttres Phil' 1802, p. 217. 



