64 THE VOYAGE OF H.M.S. CHALLENGER. 
No living Petrel has this combination of characters ; the Oceanitidz having lost their 
colic cseca, the Procellariidee the accessory semi-tendinosus (Y) muscle, and both groups 
having become specialised in other ways. 
Such an ancestral form as here indicated may be supposed to be an early, and in some 
respects—as shown by the large vomer, schizognathous palate, large third pectoral 
muscle and formula AB, XY—more primitive form, that diverged from the common stock 
of the Ciconiiform birds very early, when the latter had only acquired the most prevalent 
of the characters now existing in the various groups of that sub-order. One branch of 
this stock has since become greatly modified in the Tubinarial direction, whilst the other 
branch, loosing ‘‘ B” and the large vomer, and becoming desmognathous, split up and gave 
origin, at different times and in different ways, to the remaining families of the group. 
The definiteness of the characters of these, and the amount of specialisation they 
show, indicate not only a great antiquity for the whole group, but also the great amount 
of extinction that has gone on amongst its members in the past, in the process of which 
nearly all the intermediate and less specialised forms have disappeared. 
