Of course much information about limited topics is to be found in more
general works; the very best description of a single plant or of a
family of plants may perhaps be contained in a botanical encyclopædia.
This fact, however, must be impressed upon the inquirer in the preface
of the catalogue or in a printed card giving directions for its use;
it is out of the question to make all possible references of the
ascending kind. From «Cathedrals», for example, one would naturally
refer to «Christian art» and to «Ecclesiastical architecture», because
works on those subjects will contain more or less on cathedrals. But
so will histories of architecture and {59} histories of English,
French, German, or Spanish architecture; so will travels in England,
France, Germany, Italy, Spain. And anyone who desired to take an
absolutely complete survey of the subject, or who was willing to spend
unlimited time in getting information on some detail, would have to
consult such books. Yet the cataloguer may very excusably not think
of referring to those subjects, or if he thinks of it may deem the
connection too remote to justify reference, and that he should be
overloading the catalogue with what would be generally useless.
There are many things that are seldom used, and then perhaps but for
an instant, and yet their existence is justified because when wanted
they are indispensable, or because they make useful what is otherwise
useless: a policy of insurance, life-preservers in a steamer, the
index of a book, large parts of the catalogue of a library, among
others the cross-references. Of such a nature, but much less useful,
more easily dispensed with, is a