NESTLING BIRDS AND WHAT THEY TEACH 249 



That the Megapodes were originally hatched in trees, like 

 the young Hoatzin, there can be little doubt, since, like the 

 latter, the wing of the young shows a free finger-tip, and the 

 arrested development of the outer quills, characters which, as 

 we have already seen, are direct adaptations to the peculiar 

 locomotion of tree-climbing nestlings. Further, we may feel 

 sure that the increase in the amount of food-yolk did not take 

 place until some time after the descent to the ground for 

 nestling purposes, since the wing of the young Megapode forms 

 an exact counterpart to that of the young Fowl or Turkey. 

 Had the increase taken place earlier, the wing would have re- 

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

 These are now present only during embryonic life. 



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

 stages to be passed within the egg, 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 now impossible, the instinct therefor having been replaced by 

 that which induces the birds to bury their eggs and leave them 

 to be hatched by heat other than their bodies. 



This remarkable habit of burying the eggs has received 

 a very different explanation to that adopted here, one which 

 seems to show that the effect has been mistaken for the cause. 

 The great size of the egg, say the supporters of this hypothesis, 

 takes up so much room within the body cavity that only one 

 can ripen at a time, and consequently, long intervals must 

 elapse between the deposition of each egg.. To wait till all 

 were laid would be dangerous, and furthermore, they could 

 not all be covered by the sitting bird. Consequently, each is 

 deposited as it is laid, in an incubator, and left to take its chance 

 just as among reptiles. They hold, in short, that the Mega- 

 podes lay their eggs in mounds because of their size, whilst 

 the converse appears to be the case — the large egg has 

 been produced because of the need of depositing it in some 



chick leaves the egg these are of considerable length, they are still enveloped 

 in the sheath common to growing feathers. In a few hours this sheath crumbles 

 away, and when this process of disintegration is complete, the quills are ready 

 for use. Then, and not until then, the young nestling makes its way out of its 

 premature grave 1 



