122 THE DESCENT OF MAK. 



or pain. We are ourselves conscious that some habits ar<3 

 much more difficult to cure or change than others. Hence 

 a struggle may often be observed in animals between dif- 

 ferent instincts, or between an instinct and some habitual 

 disposition; as when 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 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 t .ge 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 instinct is, leading even 

 timid birds to face great danger, though with hesitation, 

 and in opposition to the instinct of self-preservation. 

 Nevertheless, the migratory instinct is so powerful that 

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

 quently desert their tender young, leaving them to perish 

 (miserably in their nests.* 

 We can perceive that an instinctive impulse, if it be in 

 ally 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 num- 

 bers. Whether this is the case with the migratory in com- 

 parison with the maternal instinct, may be doubted. The 



* 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. Black wall. This latter care- 

 ful observer examined, late in the autumn, during two years, thirty- 

 six nests; he found that twelve contained young dead birds, five con- 

 tained 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, pp. 108, 118. For some additional evidence, although 

 this is not wanted, see Leroy, " Lett res Phil.," 1802, p. 217. For 

 swifts, Gould's "Introduction to the Birds of Great Britain," 1823, 

 p. 5. Similar cases have been observed in Canada by Mr. A-<apis; 

 "Pop. Science Review," July. 1873, p. 283. 



