C4 THE VOYAGE OF H.M.S. CHALLENGER. 



No living Petrel has this combination of characters ; the Oceanitidse having lost their 

 colic caeca, the Procellariidas the accessory semi-temlinosus (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. 



