268 THE ANIMALS OF NEW ZEALAND 



and tliis points to the probability of D. nigripes, which remains 

 dark all through life, being- nearer to the prototype albatross than 

 any other now living. It seems probable, therefore, that 

 albatrosses originated in the Northern Hemisphere, and passed 

 south through the Pacific Ocean. 



No two species of albatross or mollymawk are knoAvn to breed 

 in the same locality. Even when two different kinds are found 

 on the same island, as D. exulans and D. regia, on Adams Island 

 of the Auckland group, they occupy widely separated sites, and, 

 as will presently be seen, there are no fewer than five months' 

 difference between the first to breed and the last. 



As these birds all live on the same food, and have the same 

 simple habits when they are at sea, it cannot be supposed that 

 their distinctive specific characters are due to natural selection, 

 for that which would favour one would favour all. Nor can it be 

 supposed that they are due to the action of external conditions, 

 because what would affect one would affect all. Nor, again, can 

 it be supposed that they are recognition-marks, for, when the 

 breeding-time is drawing near, each bird goes separately to its 

 old nest before courtship begins. The pink feathers on the sides 

 of the neck of D. chionoptera may possibly be due to sexual selec- 

 tion, but all the differences between the species do not have such 

 an origin. The birds appear to mate for life, so that there is very 

 little opportunity for choice. It cannot, therefore, be that the 

 species of albatrosses were formed by competition on the ocean, 

 and subsequently chose separate breeding-grounds. It may be 

 believed that isolation preceded the development of their specific 

 characters. 



It is not difficult to imagine that those birds to which the 

 breeding impulse came first should retire to their breeding- 

 grounds and there mate; while those in which the impulse was 

 delayed might find their old breeding-grounds fully occupied 

 and would have to choose others. Thus, owing to physiological 

 isolation, a small number of birds would become physically 

 isolated, and new specific characters might arise and be preserved. 

 This method of physiological isolation has often played an 

 important part in the origin of species without any help from 



