ON THE INSTINCTS OF BIRDS. 125 
It must be admitted that an intermixture of di- 
stinct species does sometimes occur among our domes- 
ticated birds; but this 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 exercise 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 attri- 
buted 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 remark- 
able disposition of the Tumbler to turn itself over 
backwards when on wing, which are permanent 
characters in those varieties of the Rock-Dove, being 
transmitted by generation, can be satisfactorily ac- 
counted 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 frequently undergo changes as 
marked as those of their plumage, by the unnatural 
state im 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 
