26 
CENTRAL HALL. 
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. 
Bays or The bays or alcoves round the hall, five on each side, are 
the°Hali r0Und ^ evotec ^ to tne 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 
epitome or index of the main collections in the galleries ; but 
the name does not exactly express what it has developed into. 
It is really more like the general introduction, which almost 
always precedes the systematic portion of treatises on any 
branch of natural history. As mentioned before (p. 18), this 
collection is at present far from complete ; and as nothing 
exactly like it has been exhibited in any public museum 
before, it will, as its formation goes on, be subject to much 
modification and improvement; but it is hoped that it may 
ultimately serve as a guide for the formation of educational 
biological museums elsewhere. The space being strictly 
limited, the number of illustrative specimens is necessarily 
restricted, probably to the advantage of the student, at all 
events in the earlier part of his career. In examining this 
collection the visitor should follow each case in the usual order 
of reading a book, from left to right, and should carefully study 
all the printed explanatory labels, to which the specimens are 
intended to serve as illustrations. 
The bays on the west side (left-hand on entering the hall) are 
