264 MENTAL FACULTIES sec. 



Moreover, it is undecided how far all birds, even before 

 they begin to incubate themselves, obtain a knowledge of 

 their origin from eggs by observing others incubating. 



The cuckoo has no longer the impulse to incubate because 

 in her, in consequence of the slower rate at which her eggs 

 are produced, different physiological conditions occur than in 

 other birds, and this is true likewise of domestic hens and 

 ducks. The time must come, although it cannot be predicted, 

 when the latter will cease to incubate altogether, if the 

 present conditions continue. That the incubating instinct 

 in birds must have developed gradually follows from the 

 fact of their derivation from reptiles. It will be replied to 

 this, that the Pythonidse lie upon their eggs ; but I am 

 inclined to suppose that this is really a case of the protection 

 of the eggs by their parents, wdiich is observed in so many 

 other animals, — at the same time I by no means reject the 

 supposition that this protecting habit may have been among 

 the conditions from which the habit of incubation took its rise 

 in birds. When the gradual evolution of the high temperature 

 of the blood, either connected or not with the desire to cool 

 the skin, is added, the rest follows naturally. 



The instinct of taking care of offspring is one of the most 

 rooted, one of the most perfect of all which occur in animals, 

 and it is surprising enough that it can be lost even under the 

 influence of domestication, as in our hens and ducks. That . 

 the instinct has not entirely disappeared in the cuckoo I 

 have attributed partly to the possibility of the persistence of 

 a certain degree of relation between the old cuckoos and 

 their young, without, how^ever, laying much weight upon this 

 reason. I felt, however, obliged to refer to that possibility, 

 because the love of parents for their children and their care 

 for them obviously depends ultimately on the advantages of 

 society, or on the advantages which the members of a family 

 afford one another, and particularly which the children afford 



