OP THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 51 



Now, if it be true that ontogeny is an abbreviated recapitulation of phylogeny we 

 are forced to conclude that the fore limb of the penguin is a progressive structure, the 

 fore limb of the duck, partially at least, a regressive. The reason for this, doubtless, 

 is that the mesoblastic portion of the duck's wing has, with the advancing specialisation 

 and adaptation of the epiblastic structures, become less valuable. To all flying birds 

 feathers, not bone and muscle, are of prime importance in securing wing spread for 

 aerial flight : for the penguin, on the other hand, a rigid bony paddle has been evolved, 

 adapted to resist the pressure of the water to which it is subjected when the bird's 

 great pectoral muscles are in strenuous action. 



And we thus are led to the interesting conclusions : 



1. That the mesoblastic structures in the penguin's "wing" are progressive, the 

 epiblastic regressive. 



2. That the mesoblastic structures in the duck's wing are regressive, the epiblastic 

 progressive : for in the development of the duck we find that the developmental energy 

 is suddenly on the 1 7th day switched off, as it were, from the mesoblastic structures on 

 to the epiblastic ; for from that day onward the down rapidly develops and the meso- 

 blastic framework loses ground. 



With regard to these developmental facts the question arises : 

 Is the duck's or the penguin's wing the more direct descendant of the common 

 ancestor ; or have they both diverged from the common stock approximately equally, 

 but in opposite directions ? 



Embryology alone cannot answer this question, but the evidence is clear in this, 

 that the fore limb of the penguin in its development goes through a progressive and 

 continuous series of stages along one unbroken line. The mesoblastic portion of the 

 fore limb elongates, but its characters do not alter. It elongates, however, with a 

 relatively greater rapidity towards the end of development, whereas the duck's fore limb, 

 after being relatively longer than the penguin's ever is, regresses rapidly. So that the 

 answer to our question, so far as the embryological evidence is concerned, must be that 

 the wings of both these birds are different from the ancestral wing, and that the 

 differentiation has been in opposite directions and that the common ancestor was a 

 flying bird of a somewhat primitive type depending in large measure for the spread of 

 its wing upon bone and muscle. 



SECTION III. EXTERNAL CONFIGURATION OF THE EMBRYOS FROM THE 



TWELFTH DAY ONWARDS. 



1. Gentoo Penguin Embryo, 12 Days. (Fig. 10.) 



The embryo was not fully formed, the brain was covered by the thinnest of mem- 

 branes, the beak was short and soft, the eyes were very prominent, with dense white 

 opaque pupils. The fore limb well developed ; the elbow flexure completely marked. 



(BOY. soc. EDIN. TRANS., VOL. XLVII., 237.) 



