Flight of Albatrosses. 83 



selection, for that which would favour one would favour all. 

 Nor can we suppose that they are due to the action of external 

 conditions, because what would affect one would affect all. 

 Nor can we suppose them to be 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 selection, but we cannot suppose that all the 

 differences between the species have such an origin ; for 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. 

 We must believe that isolation preceded the development of 

 their specific characters. 



Now 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. I believe that this method of physiological 

 isolation has often played an important part in the origin of 

 species without any help from natural selection, not only in 

 birds, but also in insects. It is evident that in an equable 

 climate, where the exact time of breeding was not very 

 important, many variations might be preserved by this 

 means j while in more rigorous climates, where the breeding- 

 season must necessarily be short, this kind of physiological 

 isolation could not occur. And this may account for the 

 greater number of species in tropical countries, especiallv 

 on islands, as contrasted with the enormous number of 

 individuals belonging to very few species which is charac- 

 teristic of temperate regions with continental climates. 



One of the most peculiar and characteristic habits of the 

 Albatrosses — as well as of all the larger Petrels — is their so- 

 called ''sailing" method of flight, which enables the birds to 



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