254 PROCELLARIIDiE. 



In every respect except colour this bird resembles OE. arminjoniana, the 

 dimensions agreeing very closely. This, coupled with the fact that the spe- 

 cimens of both these species were secured on the same day in the vicinity of 

 the same island, prompts the supposition that, notwithstanding the great 

 diiference in colour, they really belong to one and the same species. 



If this be so, the question arises. What is the meaning of the great 

 disparity in the colouring of the plumage observable in different individuals 

 of this species ? 



In answer to this question it may be urged that there is reason to 

 believe that in some species of Petrels, just as in some Skuas (^Stercorarius), 

 there prevails a dimorphism as regards the colour of the plumage, which is 

 not dependent on age or sex, and that the colouring of each individual is 

 assumed with the first plumage and retained through life. The evidence that 

 such is the case in some species of Petrels is at present, perhaps, hardly 

 sufficiently precise to justify the union of CE. trinitatis with GE. arminjoniana ; 

 and the question can only be decided by actual observation ; still it is a point 

 to be borne in mind by future explorers. The truth of the suggestion has 

 some support from analogy with what appears to prevail in other species of 

 this group. M acgillivray (Zoologist, 1860, p. 7133) speaks of some such 

 diversity of plumage being observable in a colony of CE. torquata observed 

 by him in Aneiteum Island, one of the New Hebrides. Moreover the 

 difference exhibited in the birds figured by Mr. Gould, in his ' Birds of 

 Australia,' as "young" and old of CE. mollis is another case in point. I 

 recently stated (Ibis, 1875, p. 376) that the young of CE. externa assumed 

 the full livery of the adult bird with its first feathers and exhibited no inter- 

 mediate or " young " stage ; so that, if analogy may be trusted, the " young " 

 of CE. mollis according to Gould is not really or necessarily the young, but 

 simply one of the two forms in which CE. mollis presents itself when adult. 

 Then, again, with the bird called CE. phillipi by Gray. The bird figured as 

 the Norfolk-Island Petrel in Phillip's 'Voyage,' upon which Gray founded 

 the species, is in ashy -fuliginous plumage ; whereas the specimens so called 



