THE DESCENT OB ORIGIN OF MAN 147 



cases strength, must have been previously acquired, proba- 

 bly through- natural selection, y' 



Of the various instincts and habits, some are much 

 stronger than others; that is, some either give more pleas- 

 ure in their performance, and more distress in their preven- 

 tion, than others ; or, which is probably quite as important, 

 they are, through inheritance, more persistently followed, 

 without exciting any special feeling of pleasure or pain. 

 We are ourselves conscious that some habits are much more 

 difficult 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 disposi- 

 tion ; 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 pup- 

 pies and for her master — for she may be seen to slink away 

 to them, as if half -ashamed of not accompanying her mas- 

 ter. But the most curious instance known to me of one in- 

 stinct getting the better of another is the migratory instinct 

 conquering the maternal instinct. The former is wonder- 

 fully 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 unin- 

 tentionally 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 migra- 

 tory instinct is so powerful, that late in the autumn swal- 

 lows, house-martins, and swifts frequently desert their ten- 

 der young, leaving them to perish miserably in their nests. "' 



" This fact, the Rev. L. Jenyns states (see his edition of "White's Nat. 

 Hist, of Selhome," 1853, p. 204), was first recorded hy the illustrious Jenner, 

 in "Phil. Transact.," 1824, and has since heen confirmed by several observers, 

 especially by Mr. Blackwall. 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 



