CORMORANTS 303 



as well as on the lower surface, and tliat it would be nnsym- 

 metrieal. As to the other theory. P. melanoleucns is not 

 sufficiently common in New Zealand to allow us to suppose that 

 the abnormal birds are hybrids. A simpler explanation of these 

 variations is found in the theory of occasional reversion to 

 ancestral eharactei's. Evidently, the surrounding conditions are 

 not sufficient to account for the origin of the variations in P. 

 trevirostris. If they were, other cormorants, with white breasts 

 and similar habits, like P. varius, would be affected in the same 

 manner. It seems more probable, therefore, that the variation, 

 as suggested before, is due to a partial reversion to P. pygmaeus, 

 and that the reversion is not yet complete, so that the plumage 

 of P. melanolcucus, the immediate ancestor, often appears. 



Supposing that the variation has arisen from reversion, it may 

 be asked, how has it been preserved 1 Cormorants had no 

 enemies in New Zealand, and a black abdomen cannot l)e better 

 suited than a white one for fishing, as many cormorants are 

 white on the lower surface. Natural selection, therefore, is out 

 of the question. The idea of sexual selection is not satisfactory, 

 and, before forming an opinion on this point, we nuist know a 

 great deal more about the breeding habits of these birds. It 

 should also be stated, before passing from P. hrevirosfris, that, 

 when young, it is entirely black, the white chin and throat 

 coming afterwards. Why should there be a partial advance in 

 the white throat after a commencement has been nuide with the 

 entirely black plumage of P. pygmaeusf No explanation of this 

 can be offered at present, though it should be pointed out that 

 a white throat is a feature of the young of other species. 



The second group of New Zealand cormorants, P. punctatus 

 and P. fcatlicrstoni, forms the sub-genus Sticticarho, in which the 

 South American species, P. gaimardi, is also included. The facts 

 are too scanty to justify an opinion as to which way this group 

 has travelled. It may be possible to learn something about the 

 migrations of the third group. It forms the sub-genus 

 Leucocarbo, and, besides the New Zealand species, contains four 

 from South America and one from Kerguelen Island. Our species 

 of the sub-genus may be divided into a carunculated section. 



