APPENDIX. o77 



pit-fall in loose sand, lying motionless at the bottom, and if 

 the prey is about to escape, casting jets of sand all round.* 



It has been asserted that animals are endowed with 

 instincts, not for their own individual good, or for that of 

 their own social bodies, but for the good of other species, 

 thouiih leadinii' to their own destruction : it has been said 

 that fishes migrate that birds and other animals may prey on 

 them : f this is impossible on our theory of natural selection 

 of self-profitable modification of instinct. Ikit I have met 

 with no facts in support of tliis belief worthy of considera- 

 tion. Mistakes of instinct, as we shall presently see, may in 

 some cases do injury to a species and profit another; one 

 species may be compelled, or even apparently induced by 

 persuasion, to yield up its food or secretion to another species ; 

 but that any animal has been specially endowed with an 

 instinct leading to its own destruction or harm, I cannot 

 believe without better evidence than has hitherto been 

 adduced. 



An instinct performed only once during the life of an 

 animal appears at first sight a great difficulty on our theory ; 

 but if indispensable to the animal's existence, there is no 

 valid reason why it should not have been acquired through 

 natural selection, like corporeal structures used only on one 

 occasion, like the hard tip to the chicken's beak, or like the 

 temporary jaws of the pupa of the Caddis-fly or Phryganea, 

 which are exclusively used for cutting open the silken doors 

 of its curious case, and which are then thrown oft' for ever.f 

 Nevertheless it is impossible not to feel unbounded astonish- 

 ment, when one reads of such cases as that of a caterpillar 

 first suspending itself by its tail to a little hillock of silk 

 attached to some object, and then undergoing its meta- 

 morphosis ; then after a time splitting open one side and 

 exposing the pupa, destitute of limbs or organs of sense and 

 lying loose within the Joicer part of the old bag-like split 

 skin of the caterpillar : this skin serves as a ladder which tlie 

 pupa ascends by seizing on portions between the creases of 

 its abdominal segments, and then searching with its tail, 

 which is provided with little hooks, thus attaches itself, and 



* Kirby and Spence, Entomology, vol. i, pp. 429-435. 

 t Linnaeus in Amoenitates Academicce, vol. ii; and Prof. Alison on 

 " Instinct" in TodcVs Ci/cI. of Anat. and Physiol., p. 15. 

 X Kirby and Spence, Entomology, vol. iii, p. 287. 



