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 l^atural History which 
do not come appropriately within the systematic collections of 
Group of the departmental series. One group already in place, shows 
Goldfinches ^^^^ 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 character. 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 outward garbs ; 
but from whatever point of view they are regarded, they 
illustrate the difficulty, continually increasing as knowledge 
increases, of defining and limiting the meaning of the term 
" species," of such constant use in biology. 
Another case is in preparation to show the great variation to 
which a species may become subject under the influence of 
domestication, as illustrated by choice examples of the best 
marked breeds of Pigeons. 
Bays or The bays or alcoves round the hall, five on each side, are 
the HalL^°^^ devoted to the Introductory or Elementary Morphological Col- 
lection, designed to teach the most important points in the 
structure of the principal types of animal and plant life, and 
the terms used in describing them, all which should be known 
before the systematic portion of the collection can be studied 
with advantage. This has been called the " Index Museum," 
as it was thought at one time that it would form a sort of 
