22 
CENTRAL HALL. 
huge skull has undergone, and will show in the clearest manner 
to the least instructed visitor that the so-called fin or flipper of 
the whale is composed of all the same parts — shoulder, elbow, 
wrist, and fingers — as his own arm and hand. The hind limbs 
are entirely absent ; but two bones are seen suspended at some 
distance from the spinal column, which represent the pelvic or 
hip bones of other animals. In some species of whales there 
are even traces of the thigh, knee-joint, and leg attached to 
this, and like it deeply buried within the body of the animal. 
It is intended to place cases on the floor of the hall illustrat- 
ing general laws or points of interest in i^atural History which 
do not come appropriately within the systematic collections of 
the departmental series. 
Group of One case contains a series of specimens illustrating albinism, 
GoidSiches. ^ condition in which the pigment or colouring matter, usually 
present in the tissues constituting the external covering of the 
body, and which gives 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 speci- 
mens 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. 
A second group shows that two forms of Crows which appear 
quite distinct, and which, judged by their external characters, 
might be held to be different species, may in a state of nature 
unite, and produce offspring of a perfectly intermediate cha- 
racter. In the same case is also a series of Goldfinches, showing 
a complete gradation between birds of different coloration, and 
which have naturally been held to be 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 are regarded, 
they illustrate the difficulty, continually increasing as know- 
ledge increases, of defining and limiting the meaning of the 
term " species," of such constant use in biology. 
Another group, in a case placed at present near the entrance 
to the hall, shows the great variation to which a species may 
become subject under the influence of domestication, as illus- 
