2. The stratified portion of the earth's crust, or what has been called the "geological record," can be subdivided into natural groups, or series of strata, characterized by distinctive organic remains and recognizable by these remains, in spite of great changes in lithological character from place to place. A bed, or a number of beds, linked together by containing one or more distinctive species or genera of fossils is termed a _zone_ or _horizon_, and usually bears the name of one of its more characteristic fossils, as the _Planorbis_-zone of the Lower Lias, which is so called from the prevalence in it of the ammonite _Psiloceras planorbis_. Two or more such zones related to each other by the possession of a number of the same characteristic species or genera have been designated _beds_ or an _assise_. Two or more sets of beds or assises similarly related form a _group_ or _stage_; a number of groups or stages make a _series_, _formation_ or _section_, and a succession of formations may be united into a _system_. 3. Some living species of plants and animals can be traced downwards through the more recent geological formations; but the number which can be so followed grows smaller as the examination is pursued into more ancient deposits. With their disappearance other species or genera present themselves which are no longer living. These in turn may be traced backward into earlier formations, till they too cease and their places are taken by yet older forms. It is thus shown that the stratified rocks contain the records of a gradual progression of organic forms. A species which has once died out does not seem ever to have reappeared. 4. When the order of succession of organic remains among the stratified rocks has been determined, they become an invaluable guide in the investigation of the relative age of rocks and the structure of the land. Each zone and formation, being characterized by its own species or genera, may be recognized by their means, and the true succession of strata may thus be confidently established even in a country wherein the rocks have been shattered by dislocation, folded, inverted or metamorphosed. 5. Though local differences exist in regard to the precise zone in which a given species of organism may make its first appearance, the general order of succession of the organic forms found in the rocks is never inverted. The record is nowhere complete in any region, but the portions represented, even though extremely imperfect, always follow each other in their proper chronological order, unless where disturbance of the crust has intervened to destroy the original sequence.