16 
CENTRAL HALL. 
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. 
Group ilius- Another group shows that two forms of Crows which 
mediate ^PP^^^ quite distinct, and, judged by their external 
Forms in characters, might be regarded as different species, may in a 
Nature. 
Fig. 5. — Somali Tsetse-Fly {Gloadna lonfjipennis). Enlarged 4 diameters. 
Shows the complete closure of the wings, and the tliick, straight proboscis, 
characteristic of the genus. 
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- 
