180 NATURAL HISTORY OF BIRDS. 



"good and sufficient to unite tlio Stcganopodal group together, and at the same time 

 mark it off from all other groups of birds." 



Another tiling is, that Mivart has shown that the four supergenera, included in 

 brackets al)ove, arc more intimately related inter xe than to the two other ones. 

 These two, on the other hand, chiefly agree to differ from the former four in negative 

 ]i<)ints, and hence their exclusion from these does not indicate any |)articular mutual 

 intimacy. On the contrary, the tropic-birds and the frigate-birds are as different 

 between themselves as each of them is from the rest. We therefore i)roi)ose to dis- 

 member the order in three su])erfamilics, IVlecanoidea", Fregatoidea', and Phaethon- 

 toidea-, an arrangement which is essentially the same as that ]>r()i)osed by Professor 

 IJrandt forty years ago. 



This arrangement needs a short explanation. There will be found, later on, a few 

 more details concerning the jieculiar arrangement of the neck vertebraj of the firet- 

 mentioned sujierfamily. In the last two the neck is normal, and, consequently, they 

 have not develo|)ed as off-shots from the stem of the Pelccan<)i<lcic. The extreme 

 specialization of Fref/nla in regard to the thigh muscles, A+, can, tiierefore, not be 

 derived fnmi the Pelecanoiiloas notwithstanding that the niyological formula of the 

 latter, AXH-, otherwise would allow of such an iiUcrpretalion. On the other hand, it 

 is even more jdain that the myological formula, AXY +, of the tropic-bird cannot 

 directly or indirectly be derived from the j)elicans or the frigate bird, nor, indeed, 

 the latter two from the former. We are, consequently, compelled to assume a 

 common ancestor with normal arrangement of the cervical vertebne and a myological 

 formula consisting of, at least, AXY+. 



We have occasionally had oj)portuiiity to hear people ridicule the stress laid u]ion 

 the presence or absence of such a trifling thing as a small muscle of the leg seems to 

 be. In some instances the presence or absence, considered alone, throws no light upon 

 the maimer in which two forms have developed, and in other cases it seems to the 

 sni).erficial observer to have no systematic importance, — for instance, when a s])ecies has 

 a certain muscle which is wanting in a closely allied form of the same genus; but 

 even then it is of considerable interest, since it shows that the latter has devolo))ed 

 out of the former, and not vice versa. The above exainj)le, however, derived from 

 the present order, should convince even the most superficial observer that there are 

 cases in which these tiny muscular slips play a most im])ortaiit role. 



We have discussed the distinctness of the three groups here projiosed only on the 

 basis of a few characters, since want of space j)rcvents us from going further into 

 details; but in order to show that the iliffercnces are r.ather deep-rooted, it will be suf- 

 ficient to remind one of the fact that they are apparently n(»t due to direct telcoloijical 

 causes. In all three groujis there are excellent flyers, with long wings; but one of 

 them also comprises rather short-winged divers. Similarity in habits and manner 

 of life may account for the external and sujterfici.al resemblance between a gannet and 

 a tropic-bird, but we know of no difference in their habits sufficient to explain the 

 anatomical diversities alluded to above. 



In additiitn to the diaracters common to all the members of the grou]), as "-iven 

 at the beginning of this chapter, the double condition of the pectoral muscle is here 

 described in Professor Garrod's words : — 



''The great ])cctoral muscle is composed of two independent Layers, — a superficial 

 large one, arising from the inferior border of the sternum, its carina, and from the 

 oiiter border of the furcula; and a deep one from the ujiper two thirds of the deeper 



