vr ORIGIN OF INCUBATION 263 



on the other, I can only explain to myself by the fact that 

 the eggs are taken away from the former, so that many of 

 them have completely lost the habit of providing in any way 

 for the maintenance of the species — lost it in consequence of 

 experience acquired during life, and inherited ; while every 

 cuckoo must remain in a certain relation to her offspring. 

 On the other hand, the cuckoo knows by inherited as well as 

 by continually renewed memory that she has been reared by 

 her foster-parents, whom she must recognise. She must 

 come, therefore, to the reasonable conclusion that her offspring 

 will be reared by birds like her foster-parents. It is to be 

 remarked that there are in fact only a very limited number 

 of birds to which the cuckoo entrusts its eggs. Our European 

 cuckoo confines its attentions principally to the reed- warblers, 

 the water- wagtails, the hedge-sparrows, and the pipits. 



It will of course be asked w^hy does the cuckoo take care 

 to provide for her eggs ? For she cannot know that she was 

 produced from an egg. But this objection applies also to the 

 care of all other birds for their eggs. Every bird must, from 

 the first time it hatches its eggs, draw the conclusion that 

 young will also be produced from the eggs which it lays 

 afterwards, and this experience must have been inherited as 

 instinct. Besides this, it is allowable to assume that physio- 

 logical causes are present which give rise to the habit of 

 incubation. At the period of reproduction an abundant fiow 

 of blood towards the sexual organs takes place, especially to 

 the oviducts and ovaries, which in birds whose eggs are 

 provided with a large yolk require abundant nutrition at 

 this period. The blood thus required, after the eggs are laid, 

 causes an increased sensation of warmth in the skin, whicli 

 the bird perhaps, as other naturalists have supposed, origin- 

 ally tried to alleviate by sitting on the cooler eggs. This may 

 have been the first inducement to sitting, which afterwards 

 gradually became instinctive. 



