On the Differences between Nestling Birds. 4Ó9 



That the ancestral Megapodes were originally hatched in 

 trees, like the young Hoatzin, there can be no doubt, since, 

 like the latter, the wing of the young bird shows a free finger- 

 tip and an arrested development of the outer quill-feathers, 

 characters which, as we have seen already, are direct adapta- 

 tions to the peculiar locomotion of tree-climbing nestlings. 

 We may be almost certain that the increase in food-yolk, just 

 referred to, did not take place until some time after the 

 descent to the ground for breeding purposes,, since the wing 

 of the young Megapode forms an exact counterpart of that of 

 the young Fowl and Turkey, and their allies, whilst, had the 

 increase taken place earlier, the wing would have resembled 

 that of the Hoatzin in the possession of large claws. The 

 latter are present now only during embryonic life. 



The increase in the food-yolk, allowing the earlier nestling 

 stages to be passed within the shell, must be accounted for by 

 supposing the adult Megapode to have been obliged to adopt 

 this expedient to avoid perils attendant on normal incubation, 

 perils which may since have passed away leaving no record of 

 their nature. A return to the normal method of incubation 

 is now impossible, the instinct therefore having been replaced 

 by that which induces the birds to bury their eggs, and leave 

 them to be hatched by artificial heat. 



That too much stress has been laid by systematists on the 

 condition of young birds at birth is admitted. I further 

 maintain that its significance has been misunderstood, and 

 that the facts here brought forward are strong enough on 

 the one hand to refute the older views, and on the other 

 to justify the theory, firstly that birds were originally 

 arboreal, and their young niclifugous, secondly that niclicolous 

 habits and helplessness of young birds are specialized adapta- 

 tions to an arboreal or gregarious mode of life ; and, thirdly, 

 that the young of Gallinaceous birds form a link in the chain 

 of the evolution of niclifugous habits, the free finger-tip and 

 arrested development of the outer quills pointing to a prior 

 arboreal habit, and the accelerated deve] opment of the inner 

 quills indicating an adaptation for the escape of enemies, 

 which, proving disadvantageous, has either not been adopted 

 or has been discarded by other niclifugous forms. 



