ібо Geese of Europe and Asia 



Novaia Zemlia, and Taimyr, and polymorphous (only in its light -bellied form) in North 

 America and Greenland. 



How, then, are we to explain the phenomenon that on Kolguev and other northern 

 localities the two forms of the species occur together, while only one of them— the light- 

 bellied goose — is peculiar to North-East America and Greenland? 



It seems to me the explanation of this fact is not so difficult as may at first appear ; 

 and I think that at one time in both the Old and the New World the two forms were met 

 with, and that while in America and Greenland, by means of crossing, the light-bellied 

 gradually got the better of the dark-bellied form (absorbed or amalgamated it), just the 

 opposite condition took place over the greater part of the range of this species in the Old 

 World, where undoubtedly the dark-bellied form now predominates. In Kolguev, however, 

 and probably in Novaia Zemlia, the struggle between the differently coloured representatives 

 of the one species still continues, and probably one will ultimately absorb the other. 



At one time I explained the geographical distribution of dimorphous forms in the 

 Lepidoptera somewhat differently. 1 I considered, for instance, that a species, the home or 

 centre of distribution of which was a particular locality, became dimorphous or even poly- 

 morphous, and afterwards only spread gradually to new localities in one or other of its forms, 

 according as this was better adapted to exist in the new surroundings. The explanation I 

 here give of the fact of the occurrence of one form only of B. bernicla in America and 

 Greenland, and of two 'forms of that species in the Old World, seems, however, to be 

 perfectly applicable to the case cited of the range of various forms of one species among the 

 Lepidoptera, or, in other words, that the particular species of the genus Lyccena was 

 dimorphous or trimorphous over the whole extent of its range, but that little by little one or 

 other form, according to its greater fitness, has absorbed the other, and that in the localities 

 where dimorphous forms are now met with, the struggle between them still continues, and 

 will terminate in favour of the one best adapted for continuous existence. 



If it should eventually prove that the light-bellied brent is not simply a dimorphous 

 form of the common brent, but an independent species (which I consider unlikely), even 

 then, in consequence of the occurrence of both together and their interbreeding (as shown 

 by the intergrading forms in Kolguev), it would be impossible to doubt the existence of a 

 struggle between them of long standing, and destined inevitably to end in the triumph of 

 one of the two, just as a struggle has been and is still maintained locally between grey and 

 black crows. 



The appearance on the coasts of England of the light-bellied form of the brent, 

 formerly explained by its coming from Greenland to winter, may now be more simply 

 accounted for by its breeding in Spitzbergen, Kolguev, Novaia Zemlia, and farther east- 

 wards to the Taimyr peninsula inclusive. This question is not yet matured, however, for 

 final decision, as it is first of all necessary to ascertain whether the Russian are identical 

 with the North American and Greenland birds, and if this should prove not to be the case, 

 whether the British birds can be identified with one or the other. 



Notwithstanding, then, the probability that it is the Russian light-bellied brent that 

 winters in British waters, this question cannot be regarded as strictly answered, and 

 therefore still remains open. 



1 See article in the Transactions of the Entom. Soc. Lond. 1901, pt. iv., "On some Cases of Dimorphism and Polymorphism 

 among Palaearctic Lepidoptera," by S. Alpheraky. 



