NESTLING BIRDS AND" WHAT THEY TEACH 251 



The interpretation so far submitted of the facts in question 

 differs materially from the conclusions of earlier writers. 



According to Dr. Gadow ' three deductions are possible : 

 (i) That the developmental period (embryonic + nestling stage) 

 stands in direct relation to the ultimate perfection in develop- 

 ment which is attained. That is to the relatively " high " or 

 "low" position they hold with the class. (2) That it is pro- 

 fitable because safer both for mother and -offspring for the 

 embryonic and brooding periods to be as short as possible. 

 (3) That the helpless types, and among these especially the 

 Passeres, are the most perfectly developed types, and these have 

 absolutely the shortest brooding and the longest nestling period. 



If it be admitted, he concludes, that the preponderance 

 of the nestling over the incubation period is a sign of higher 

 development, then the Passeres must be allowed the highest 

 position in the system, for these have, relatively, the longest 

 post-embryonic and shortest incubation period of all birds. 



Similar views have been expressed by later writers. Thus, 

 Messrs. Jordan and Kellogg, in their work on Animal Life, 

 assert that " among the lower or more coarsely organised birds, 

 such as the Chicken, the Duck, and the Auk as with reptiles, 

 the young animal is hatched with well-developed muscular 

 system and sense organs, and is capable of feeding itself," but 

 the offspring of the more highly organised forms, such as the 

 Thrushes, Doves and Song-birds generally, " are hatched in a 

 wholly helpless condition, with ineffective muscles, deficient 

 senses, and wholly dependent on the parent". 



We must pass now to a brief review of the facts appertain- 

 ing to the coloration of nestling birds. 



This subject falls under two different heads : (a) the colora- 

 tion of the body as a whole, and (/3) the coloration of definite 

 regions of the body. Under the first section we have all those 

 birds which are nidifugous, or active from the moment they 

 leave the shell, and some nidicolous or helpless birds. These 

 all agree in that they are downy, but they present different 

 types of coloration, all of >vhich, however, belong to the protec- 

 tive resemblance group. Under the second we have some of 

 the downy forms, and those nidicolous or helpless types, which 



' Gadow, Bronns Their, reichs. Vogel., p. 6g8. 



