1586 THE STORY OF THE UNIVERSE 



investigated. Moulting obviously presents some 

 analogies with skin-casting and hair-shedding in 

 other animals, and must be associated with some 

 deep-seated constitutional change, such as its con- 

 nection with the end of the breeding season suggests. 

 Besides this annual growth of new feathers, many 

 birds exhibit double and some triple moulting. The 

 ptarmigan, for instance, changes its suit three times 

 in the year, moulting after breeding into gray, chang- 

 ing this for white as the winter sets in, and acquir- 

 ing in spring a third and most attractive set of 

 feathers. In association with sexual attraction many 

 male birds seem to undergo a partial moult, as the 

 result of which they acquire those special decorations 

 which are the index of a reproductive climax. 



Birds usually pair in springtime, but to this rule 

 there are many exceptions. Fertilization is internal, 

 and all birds are oviparous. The number of eggs 

 is often in inverse proportion to the size of the bird. 

 Several, such as the apteryx, lay only one; the doves 

 and birds of prey lay two or three, but the majority 

 of birds many more. 



It is important to notice that the higher develop- 

 ment of birds, as compared with reptiles, is asso- 

 ciated with the production of fewer offspring, but at 

 the same time with the enormous increase of parental 

 care and sacrifice. If the young are to be devel- 

 oped within the eggs, the latter must be kept at an 

 approximately constant warmth. In almost all cases 

 this is effected by brooding, the frequent helplessness 

 of the young, the very common arboreal habit, the not 

 infrequent enemies, have necessitated a most varied 



