3.  The larger zoological and botanical groupings survive longer than the smaller.  Species are so short-lived that a single geological epoch may be marked by several more or less complete extinctions of the species of its fauna-flora and their replacement by other species.  A genus continues with new species after all the species with which it began have become extinct.  Families survive genera, and orders families.  Classes are so long-lived that most of those which are known from the earliest formations are represented by living forms, and no sub-kingdom has ever become extinct.

Thus, to take an example from the stony corals,—­the Zoantharia,—­ the particular characters—­which constituted a certain species—­ Facosites niagarensis—­of the order are confined to the Niagara series.  Its generic characters appeared in other species earlier in the Silurian and continued through the Devonian.  Its family characters, represented in different genera and species, range from the Ordovician to the close of the Paleozoic; while the characters which it shares with all its order, the Zoantharia, began in the Cambrian and are found in living species.

4.  The change in organisms has been gradual.  The fossils of each life zone and of each formation of a conformable series closely resemble, with some explainable exceptions, those of the beds immediately above and below.  The animals and plants which tenanted the earth during any geological epoch are so closely related to those of the preceding and the succeeding epochs that we may consider them to be the descendants of the one and the ancestors of the other, thus accounting for the resemblance by heredity.  It is therefore believed that the species of animals and plants now living on the earth are the descendants of the species whose remains we find entombed in the rocks, and that the chain of life has been unbroken since its beginning.

5.  The change in species has been A gradual differentiation.  Tracing the lines of descent of various animals and plants of the present backward through the divisions of geologic time, we find that these lines of descent converge and unite in simpler and still simpler types.  The development of life may be represented by a tree whose trunk is found in the earliest ages and whose branches spread and subdivide to the growing twigs of present species.