i6o DISCOVERY REPORTS 



DISCUSSION 



If the main features of the development of these penguins are reviewed certain points 

 are outstanding. The first is the close similarity between the early stages of their develop- 

 ment and those of other birds. Compared with the fowl the only conspicuous difi^erences 

 are due to the greater absolute bulk of the penguin embryo in some of the stages inde- 

 pendent of the yolk. 



The embryos are obviously penguins after probably one-third of the incubation 

 period. The fore-limb, the webbed hind-limb, the powerful beak and the development 

 of feather papillae all over the surface of the skin, are all characteristic. 



There are not many special peculiarities in the development. The pineal body is de- 

 veloped in the early stages to an exceptional degree, and the thymus gland is dis- 

 proportionately large at the time when feather filaments first make their appearance. 

 The history of the feathers seems to show that they are of a very primitive type. The 

 skin itself is very scale-like, and the heart at one stage has a reptilian flexure. But in 

 more fundamental features the development follows the normal course and does not 

 provide evidence of the sort that will settle the debatable problem of the true position 

 of penguins in the class Aves. Are they specialized for their very isolated manner of 

 life, or are they a primitive stock? On this question it was hoped that the embryology 

 would supply a final answer. It has in fact failed to produce irrefutable evidence one 

 way or the other, but it has supplied some new and suggestive hints. The embryos fail 

 to produce any sign of teeth, for example, but they do develop a most peculiar pair of 

 fore-limbs. The buds when they first arise are not exceptional, but as soon as the 

 cartilages begin to appear in them they grow at a prodigious pace. Long before the 

 embryo is ready to hatch, before even it assumes its coat of pre-natal down, the limbs 

 have completed their development as far as the skeleton is concerned. It is surely not 

 an accident that this should occur. The appearance of the trunk, with the limbs hanging 

 vertically and the back arched, as in Plate V, fig. 2, is extraordinarily reminiscent of the 

 aquatic reptiles of the Mesozoic Age. The whole body of the embryo at this stage is that 

 of a creature specially adapted, with long paddles, for an aquatic habit of life, and if this 

 adaptation — so early in development — is not merely a coincidence, it may be regarded 

 as supporting the opinions of those who do not agree that it is reasonable to regard the 

 penguin as simply a flying bird that took to water. 



The opposite view, that the avian stock actually arose from aquatic fish-eating reptiles, 

 and that penguins were probably an early oftshoot of them that never succeeded in 

 getting into the free air (Kerr, 1919, p. 453), has not as yet attracted many followers. 

 It is nevertheless an idea admirably suited to the record of the primitive characteristics 

 of both adult and embryo penguins. The adult characters of (i) a generalized type of 

 skeleton, especially in the hind-limbs, and (ii) an unvaried type of plumage, have been 

 discussed often enough. The suggestive points that have arisen in the course of the 

 study of their embryology have been mentioned. There is, however, one special feature 

 of the development that oflFers new light on the problem. It is concerned with the small 



