248 ' Mr Black wall ow the Imttncts lyf Birds. 



deviation from their ordinary instinct is rare, and may, with 

 great probability, be ascribed to a change in their organization^ 

 occasioned by the artificial mode of life to which they have been 

 subjected. Now, as it is a maxim in physiology, that the exer- 

 cise of every animal function is dependent upon its appropriate 

 material organ, any display of new instinctive phenomena, in 

 birds which have long been under the control of man, may also 

 be attributed to the operation of the same physical cause. The 

 singular propensity of the cropper-pigeon to inflate its craw 

 with air, and the still more remarkable disposition of the tumb- 

 ler to turn itself over backwards when on wing, which are per- 

 manent characters in these varieties of the rock-dove, being 

 transmitted by generation, can be satisfactorily accounted for on 

 the foregoing supposition only. How unsafe it must always be, 

 to draw general conclusions from the habits and propensities of 

 domestic fowls alone, whose instincts are frequently changed al- 

 most as much as their plumage, by the unnatural way in which 

 they are kept, needs scarcely to be insisted on. 



Dr Darwin conjectures, that birds learn how to build their 

 nests from observing those in which they are educated, and from 

 their knowledge of such things as are most agreeable to their 

 touch in respect to warmth, cleanliness, and stability ; but the 

 undeniable fact, that birds, when taken very young, even before 

 they can see, and brought up in confinement, do sometimes con- 

 struct nests, is alone sufficient to refute this opinion. 



The sparrow-hawk and kestril often make use of the deserted 

 habitation of the magpie as a receptacle for their eggs, and the 

 sparrow frequently takes forcible possession of the rustic dwell- 

 ing of the house-martin for the same purpose. Why, then, are 

 they never known to build nests similar to those which they 

 thus appropriate to themselves ? and why does not the cuckoo, 

 which is always brought up in the nest of some other bird, con- 

 struct one itself*? The reason is obvious, the act of nidifica- 

 tion is not regulated by observation or instruction, but is under 

 the immediate direction of instinct. 



Guided by this mysterious power, individuals of the same 



• I have pointed out the errors into which Pr Darwin has fallen in his re- 

 marks on the cuckoo, in my observations on that bird, j>rinted in the fourth 

 volume of the new series of the Society *s Memoirs. 



