REPORT ON THE SPHENISCID.4;. 231 



So far as the metatarsal bones are concerned, it appears from the observations of 

 Gegenbaur/ that even in those birds in which the metatarsal bones ultimately fuse to 

 form a single undivided mass, these bones originally present the form of four distinct 

 and separate elements. It seems, therefore, if conclusions based upon embryology are of 

 any value, that we must conclude that birds as we now know them were derived from an 

 ancestral group, the members of which, along with other peculiarities, were possessed of 

 at least four distinct and separable metatarsal bones. These four bones were originally 

 separate and distinct, but subsequently became more or less completely fused together 

 to form the single metatarsal bone which is characteristic of the majority of l:)irds. Inas- 

 much as the Penguins retain the individuality of the separate metatarsal bones to a 

 greater extent than other birds, it would appear that they are the modern representatives 

 of a group which had diverged from the primitive avian stem at a time when as yet the 

 metatarsal bones had neither lost their individuality nor had become fused together to 

 form the single bone which is one of the characteristics of the majority of birds of the 

 present day. 



This conclusion can only be denied on the supposition that the earliest members of 

 the group of the Spheniscidae were derivatives from the avian stem, at a period when the 

 separate metatarsal bones had been already fused to form a single mass, as in modern 

 birds; a supposition which appears to the last degree improbable, when we consider that 

 in accepting it we must suppose that the avian metatarsal bones must in the first instance 

 have undergone coalescence, and thereafter became differentiated from one another in the 

 members of one particular group, and in one only. It would therefore appear that the 

 group Spheniscidse is one of considerable antiquity, and that it must have diverged from 

 the avian stem at a time when as yet the metatarsal bones formed distinct and independent 

 entities in the members of the entire class of birds. 



At this time, moreover, birds had so far become differentiated from their reptilian 

 ancestors, that their anterior extremity, instead of forming organs of support adapted to 

 terrestrial progression, had become modified to form wings adapted to aerial progression. 

 This conclusion is foi-ced upon us by an examination of the wing of the Penguin of the 

 present day, in which we find nearly every muscle which is characteristic of the wing of 

 the ordinary bird represented, but represented not liy muscular 1jut by tendinous bands, 

 which have attachments similar to those of the muscles in question. These tendons, inas- 

 much as they are functionally useless, could only have been derived through a process of 

 structural degeneration from muscular bands whicli had at one time been functionally 

 active in the ancestral Penguin, but which, in accordance with the law that when highly- 

 organised tissues become functionally useless, they gradually degenerate and assume a 

 lower form of organisation, and were consequently converted into tendon. 



In the Penguins apparently the muscles of flight originally present had proved to be 



' Uutersuchunsen, Heft i., 1864. 



