DIMORPHISM 149 



DIMORPHISM, a term originally used by botanists to express 

 the fact that in certain plants a ditference, whether in form or colour, 

 more or less considerable, exists between individuals belonging to 

 the same species, this difference not being attributed to local influ- 

 ence or of the kind called accidental, but yQt one that is constantly 

 exhi1)ited. As analogous cases are observable in animals, the term 

 has been adopted by zoologists, and, disregarding other classes, it 

 will be at once perceived that among Birds there are two kinds of 

 Dimorphism — -one depending upon sex, in which the secondary 

 sexual characters of the male and female may differ in very many 

 ways, and the other which is apparently quite independent of 

 sexual distinction. Of this last kind, which seems to approach 

 most nearly to the Dimorphism of botanists, there are not many 

 undisputed instances. The best known is that of some species of 

 Skua, in which a parti- coloured bird may be frequently found 

 mated with one that is (so to speak) whole-coloui'ed — in some cases 

 the former being the male, the latter the female, and in others just 

 the contrary, it rarely happening that both partners are alike in 

 plumage. A similar state of things occurs on the confines of the 

 districts respectively occupied by the Black and Grey Crows of 

 the Old World, but here we are met by the difficulty that some 

 ornithologists consider these two forms to be distinct species, and 

 the produce of their union to be hybrids. The White-eyed and 

 Dark-eyed Crows of Australia present a phase intermediate between 

 that last mentioned and the first ; for, though some writers have 

 regai'ded them as distinct species, locality seems to have no influ- 

 ence on the difference, comparatively slight as it is, observable 

 between them. Another case more resembling the first is that 

 afforded by the Guillemot, for at nearly every one of its breeding- 

 resorts a portion of the tenants (perhaps one in a score) will be 

 found to have a white circle round the eye and a white line stretch- 

 ing backward from it — these Ringed or Bridled Guillemots being 

 of either sex and apparently paired with birds of normal plumage, 

 while no example is known which shews any intermediate condi- 

 tion.^ All these are instances in which Dimorphism is confined 

 to colour, but it may well be regarded as extending also to size, 

 though here we again meet with the objection that numerous 

 wi'iters regard the smaller or larger forms as cons'tituting two local 

 races if not species. The DuNLiN furnishes us with an instance of 

 this kind. Ranging throughout the Old World, but in far fewer 



^ At one time these Ringed or Bridled Guillemots were looked upon as a 

 distinct species, called Uria lacrymans, but that view has of late been wholly 

 ■abandoned. Similarly the dark, whole-coloured examples of the common species 

 of Skua were originally described as forming a separate species, Lestris richard- 

 soni, but though the name has by many writers been mistakenly retained none 

 now believe the birds to be distinct. 



