200 Evolution and Adaptation 



" Now with the tree-sparrow {P. montanus) both sexes and 

 the young closely resemble the male of the house-sparrow ; 

 so that they have all been modified in the same manner, and 

 all depart from the typical coloring of their early progenitor. 

 This may have been effected by a male ancestor of the tree- 

 sparrow having varied, firstly, when nearly mature ; or sec- 

 ondly, whilst quite young, and by having in either case 

 transmitted his modified plumage to the females and the 

 young ; or, thirdly, he may have varied when adult and trans- 

 mitted his plumage to both adult sexes, and, owing to the 

 failure of the law of inheritance at corresponding ages, at 

 some subsequent period to his young." 



The further admissions made in the following quotation are 

 also significant : — 



" The plumage of certain birds goes on increasing in 

 beauty during many years after they are fully mature ; this 

 is the case with the train of the peacock, with some of the 

 birds of paradise, and with the crest and plumes of certain 

 herons, for instance, the Ardea ludovicana. But it is doubt- 

 ful whether the continued development of such feathers is 

 the result of the selection of successive beneficial variations 

 (though this is the most probable view with birds of para- 

 dise) or merely of continuous growth. Most fishes continue 

 increasing in size, as long as they are in good health and 

 have plenty of food ; and a somewhat similar law may pre- 

 vail with the plumes of birds." 



We need not follow Darwin through his discussion of 

 those cases in which the adults have a winter and a summer 

 dress and the young resemble the one or the other in plu- 

 mage, or are different from either. The discussion of these 

 cases, confessedly very complex, adds nothing to our under- 

 standing of the theory, and little but conjecture is offered 

 to account for the facts. 



The extreme to which even conjecture can be carried may 

 be gathered from the following quotation, taken from the 



