212 MENTAL EVOLUTION IN ANIMALS. 



thus adequate, not only to make a bird transport her young 

 upon her back, or, as in the case of the woodcock, between 

 her legs, but even to make a web-footed w T ater-fowl build 

 her nest on a high tree, I think we can have no doubt that, 

 if the need of such adjustment were of sufficiently long 

 continuance, the intelligence which leads to it would eventu- 

 ally produce a remarkable modification in the ancestral 

 instinct of nest-buildino-. 



Lastly, " a curious example of a recent change of habits 

 has occurred in Jamaica. Previous to 1854, the palm swift 

 (Tachornis phcenicohca) inhabited exclusively the palm trees 

 in a few districts of the island. A colony then established 

 themselves in two cocoa-nut palms in Spanish Town, and 

 remained there till 1857, when one tree was blown down, and 

 the other stripped of its foliage. Instead of now seeking out 

 other palm trees, the swifts drove out the swallows who built 

 in the piazza of the House of Assembly, and took possession 

 of it, building their nests on the tops of the end walls and at 

 the angles formed by the beams and joists, a place which 

 they continue to occupy in considerable numbers. It is re- 

 marked that here they form their nests with much less 

 elaboration than when built in the palms, probably from being- 

 less exposed."* 



Turning now from the instinct of nidification to that of 

 incubation, I shall give the results of some observations and 

 experiments which I made several years ago and published in 

 " Nature," from which I quote the account. In these cases 

 the plasticity of the maternal instinct was shown by the fact 

 that the instinct was directed in all its force to the young of 

 other animals, although there is ample evidence to prove that 

 the foster-mothers perceived the unnatural character of their 

 brood. Indeed, it is just because of this evidence that I 

 quote these cases in the present connection, for otherwise they 

 might rather be taken to exemplify non-intelligent variations 

 of instinct with which we were concerned in the last chapter. 

 But inasmuch as the intelligence of the animals was displayed 

 by the manner in which they adapted their ancestral instincts 

 to the requirements of their adopted progeny, the cases become 

 available rather as proof of the intelligent variation of instinct.f 



* Wallace, Natural Selection, Chapter VI, where see for some of the pre- 

 ceding and also for other instances. 



+ The yearning for progeny which arises from the parental instinct being 



