224 



KNOWLEDGE 



[October, 1903. 



their eggs upon the ground. It proves that the arboreal 

 nursery was forsaken after the adaptation to the tree- 

 dwelling life had taken place. 



Some birds, it should be noted, like the cormorants, 

 herons, and certain of the gull tribe, to take a few 

 examples, build as occasion demands, either on the ground 

 or in trees. It is probable that we should be correct in 

 approximately estimating the relative length of time which 

 has elapsed since this specialization by the amount of 

 down which the 3'oung develop. Thus it is probable that 

 the storks and birds of prey, for example, came under the 

 influence of this liberty-curtailing factor later in time 

 than the song-birds, or Parrots and Kingfisher for example. 

 In the storks, and Inrds of prey — and herein we may for 

 the nonce include the owls — the young are invested in a 

 coat of woolly down immediately after birth. In the 

 song-birds, only tufts of extremely weak down make their 

 appearance, and these are not numerous enough to 

 effectually cover the body. lu the Kingfisher even these 

 are wanting. By way of lending still further support to 

 this view, we have evidence that the youug of some birds 

 are at the present day slowly undergoing this process of 

 "hobbling" — for such it amounts to — and for reasons quite 

 different to those which we have just been reviewing. 

 Those are the gulls and petrels. The young of the former 

 are, as yet, only reduced to a condition of partial helpless- 

 ness ; those of the latter have become completely so. 

 T?oth, it is to be noted, have a very thick clothing of down, 

 and both are reared in nurseries built upon the ground, 

 and appear to have been so reared from time immemorial. 

 Breeding in large colonies, however, or on the leilges of 

 precipitous cliffs, the reduction of the food-yolk and 

 helplessness of the young are obviously advantageous, 

 since it would be impossible for the parents to recognise 

 their own offspring if fully active and running about among 

 those of their neighbours. In consequence, a large number 

 would almost certainly go imfed and soon starve, whilst great 

 activity among the young of the cliff-breeding species would 

 be accompanied hy an enormous mortality owing to falls 

 from the cliffs. They have in consequence been starved 

 into sedentary habits, by cutting off the embryonic food- 

 supply exactly as in the case of arboreal birds. The life- 

 history of the young albatross is sufiiciently remarkable to 

 deserve special mention here. Whether on account of the 

 nature of the food, or of a constitutional habit, is not 

 known, but by the time it is six weeks old it has become 

 literally a mass of fat. This condition attained it is for- 

 saken by the parents and remains on the nest unattended 

 for, it is said, twelve months ; during which time it is 

 nourished only by the absorption of this reserve store ! 



Whilst a large number of birds have adopted the expe- 

 dient of ciu-tailing the activity of the young, and thereby 

 have increased the burden of family cares, there are a few 

 species of Game-birds known as Megapodes, or mound- 

 builders, which have succeeded in reducing the ties of 

 offspring to the smallest possible limits — without descend- 

 ing to parasitism — by enormously increasing the size of 

 the egg so as to include a proportionately large amount of 

 food-material for the developing embryo. As a conse- 

 quence, the whole of the normal nestling period is passed 

 within the shell, and the young bird emerges fully feathered 

 and able to fly. The parental instinct seems, in conse- 

 quence of this habit, to have become well nigh extinguished, 

 for there is no brooding of the eggs, and little or no care 

 displayed for the chicks. The eggs are deposited either in 

 a huge mound of decaying vegetable matter scraped 

 together by a number of birds, or in sand warmed by 

 hot-water springs, and here they are left to hatch. Some 

 species, it is said, hover around in the neighbourhood of 

 the nest and assist the young birds to escape by scratching 



away the earth, but in the majority of cases no such 

 solicitude appears to be displayed. 



That the Megapodes were originally hatched in trees, like 

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

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

 an 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 {>lace until some time after the 

 descent to the ground for nesting 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 resembled 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 im- 

 possible, 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 that of their 

 bodies. 



This remarkable habit of burying the eggs has received 

 a quite different explanation to that adopted here, and 

 one which seems to be founded on a confusion of the 

 difference between cause and effect. 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 l>y the sitting bird. Consequently, each is 

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

 chance, just as obtains among the reptiles. They hold, in 

 short, that the Megapodes 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 natural incubator, the parent being 

 unable to undertake this duty. 



It may seem to some that we have drifted a long way 

 from our starting-point. A moment's reflection, however, 

 will show that this is not really the case. We set out by 

 showing that the vestiges of claws present in the wings of 

 modem birds are something more than mere survivals of a 

 reptilian phase of development. That they are indeed 

 remnants of organs once of extreme importance; primarily 

 during the nestling period, and, secondarily, during the 

 moulting periods of the life-history of the earliest birds. 

 Later, their use became restricted to the nestling period 

 only, whilst to-day they are functional onlv in a single 

 species — the Hoatzin. The degeneration of these claws 

 followed in consequence of the introduction of changes in 

 the nursing of the young, whereby the use of the claws 

 was abolished, and infant mortality became at the same 

 time reduced. Thus, then, these vestiges of once useful 

 organs prove to 1)e indices of the highest importance, 

 throwing a flood of light on what would otherwise have 

 remained an impenetrable mystery. 



CYCLES OF ECLIPSES. 



By A. C. D. Crommelin. 

 Part IH. 

 The 300" Cycle. — It seems to me that tliis is a cycle of 

 considerable utility. It is easily remembered, being an 



