DIMORPHISM 149 
DIMORPHISM, a term originally used by botanists to express 
the fact that in certain plants a difference, 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 yet one that is constantly 
exhibited. 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-coloured—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 
regarded 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 
writers regard the smaller or larger forms as constituting 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 
1 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 
uow believe the birds to be distinct. 
