In this dictionary the authors' aim is to explain biological terms which a layman may meet when reading scientific literature; to define the terms which a student of biology has to master at the beginning of his career -the thousand or so words which so grimly guard the approaches to the science; and to provide a reminder for the professional biologist reading outside his own narrow field. The entries are not restricted to a bare definition: some information about most of the things named is given, so as to convey something of their significance in biological discussion.