INTRODUCTORY COLLECTION. 
25 
not be surpassed. A further stage of the same condition, called 
" Mimicry," is where the object resembled, or mimicked, is an- 
other living animal, belonging to a different species, family, or 
even order. The resemblance in these cases is also believed to 
be for protection, or to be in some way advantageous to the 
animal possessing it. We know, however, so little of the habits 
and life-history of animals in a state of nature that many of the 
purposes supposed to be served by particular colours or appear- 
ances can only be regarded at present as conjectural. Whatever 
be the real explanation, the facts shown by the specimens in this 
case are very curious, and well worthy of careful consideration. 
The last group shows that two forms of Crows which appear Group illus- 
quite distinct, and which, judged by their external characters, ^gj^ate^^^^^ 
might be held to be different species, may in a state of nature Forms in 
unite, and produce offspring of a perfectly intermediate cha- N^^^^®- 
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. 
The bays or alcoves round the hall, five on each side, are Bays or 
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 
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 
I collection is at present far from complete ; in fact, only in its 
