BLEXDED OEIGIN, OR PLASTICITY OF INSTINCT. 201 



Although for the sake of clearness I have so far kept 

 separate these two factors in the formation of instinct, it has 

 now to be shown that instincts are not necessarily confined 

 to one or other of these two modes of origin exclusively ; but, 

 on the contrary, that instincts may have, as it were, a double 

 root — the principle of selection combining with that of 

 lapsing intelligence to the formation of a joint result. Thus, 

 hereditary proclivities or habitual actions, which were never 

 intelligent but, being useful, were originally fixed by natural 

 selection, may come to furnish material for furtlier improve- 

 ment, or be put to improved uses, by intelligence ; and, con- 

 versely, adjustments originally due to lapsed intelligence 

 may come to be greatly improved, or put to improved uses, 

 by natural selection. 



As an example of the first of these complementary cases 

 — or that of a primary instinct modified and improved by 

 intelligence — let us regard the case of the caterpillar which, 

 before changing into a crysalis, crosses a small space with a 

 web of silk (to which the crysalis can be firmly suspended), 

 but which when placed in a box covered with a muslin lid 

 perceives that this preparatory web is unnecessary, and 

 therefore attaches its crysalis to the already woven surface 

 supplied by the muslin ;* or let us regard the case of the 

 bird described by Knight, which observed that, having placed 

 her nest ujDon a forcing house, she did not require to visit it 

 during the day when the heat of the house was sufficient to 

 incubate the eggs, but always returned to sit upon the eggs 

 at night when the temperature of the house fell.t In. 

 both these cases of primary instincts modified by intelligent 

 adaptation to particular circumstances — and hundreds of 

 others might be added — it is evident that if the particular 

 circumstances were to become general, the adaptation to them, 

 becoming likewise general, would in time become instinctive 

 by lapsed intelligence : if muslin and forcing houses 

 were to become normal additions to the environment of 

 the caterpillar or the bird, the former would now cease to 

 build its web, and the latter cease to incubate her eggs by 



* See Kirby and Spence, Entomoloqif, rol. ii, p, 476. It is evident that 

 the -wearing of a web by a caterpillar adapted to the needs of its future con- 

 dition as a crysalis, must be due to instinct of the primary kind, inasmuch as 

 no individual caterpillar prior to the formation of such a structure can have 

 known by experience what it is to be a crysalis. 



t Loc. cit. 



