MELANISM — INTERMEDIATE FORMS. 
13 
in which the pigment, or colouring matter, usually present in 
the tissues constituting the external covering of the body, and 
giving them their characteristic hue, is absent. Individuals 
in this condition occur among many animals of various kinds, 
and are called " albinos!' In some of the specimens shown in 
the case the albinism is complete, but in many it is partial, 
the absence of colouring matter being limited to portions of 
the surface only. — 
The adjacent case shows examples of the opposite condition. Group illus- 
called melanism, depending upon an excess of dark-coloured ^ei^gm 
pigment in the skin and its appendages the hair, feathers, etc., 
beyond what is normally met with in the species. This is by 
no means so frequent as albinism. A black Leopard in the 
middle of the case is a good illustration. It is not a distinct 
species, but an individual variety of the common Leopard, born 
from parents of the normal colour. A black Bullfinch is intro- 
duced as an example of acquired melanism, the bird having 
turned black in captivity. 
Another group shows that two forms of Crows which Group iiius- 
appear quite distinct, and, judged by their external Mediate 
characters, might be regarded as different species, may in a Forms in 
state of nature unite, and produce offspring of an inter- 
mediate character. In the same case are a series of Goldfinches, 
showing a complete gradation between birds of different color- 
ation, which have been regarded as different species. Both 
these examples may by some naturalists be considered instances, 
not of crossing of distinct species, but of " dimorphism," or the 
occurrence of a single species in nature under two different out- 
ward garbs ; but from whatever point of view they may be 
regarded, tliey illustrate the difficulty, augmenting as know- 
ledge increases, of defining and limiting the meaning of the 
term " species," of such constant use in natural history. 
In the middle line of the hall is placed a case containing Tsetse-fly and 
greatly enlarged models of the African Tsetse-fly, and of 
Mosquitoes or Gnats ; also still more enlarged gelatine models 
of mammalian blood-corpuscles, showing the parasites by 
which they are infested in the diseases respectively communi- 
cated by means of Tsetse and Mosquitoes. Models of the 
parasites themselves are also shown. 
