104 THE DESCENT OP MAN. 



sympathy, reason, experience, and a tendency to imitation; or 

 again, whether they are simply the result of long-continued hahit. 

 So remarkable an instinct as the placing sentinels to warn the 

 community of danger, can hardly have been the indirect result 

 of any of these faculties; it must, therefore, have been directly 

 acctuired. On the other hand, the habit followed by the males of 

 some social animals of defending the community, and of attacking 

 their enemies or their prey in concert, may perhaps have orig- 

 inated from mutual sympathy; but courage, and in most cases 

 strength, must have been previously acquired, probably through 

 natural selection. 



Of the various instincts and habits, some are much stronger 

 than others; that is, some either give more pleasure in their per- 

 formance, and more distress in their prevention, than others, or, 

 which is probably quite as important, they are, through inherit- 

 ance, more persistently followed, without exciting any special feel- 

 ing of pleasure or pain. We are ourselves conscious that some hab- 

 its are much more diflScult to cure or change than others. Hence a 

 struggle may often be observed in animals between different in- 

 stincts, or between an instinct and some habitual disposition; as 

 when a dog rushes after a hare, is rebuked, pauses, hesitates, pur- 

 sues 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 accompanying her master. But the most curious instance 

 known to me of one instinct getting the better of 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, in 

 which they could continue to exist, and thus unintentionally to 

 commit suicide. Every one knows how strong the maternal in- 

 stinct is, leading even timid birds to face great danger, though 

 with hesitation, and in opposition to the instinct of self-preserva- 

 tion. Nevertheless, the migratory instinct is so powerful, that 

 late in the autumn swallows, house-martins, and swifts frequently 

 desert their tender young, leaving them to perish miserably in 

 their nests.^^ 



22 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 illustrious 

 Jenner, in 'Phil. Transact.' 1824, and has since been confirmed by sev- 

 eral 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, 'Researches in Zoology,' 1834, 



