Both in time and space species and groups of species have their points of maximum development.

Groups of species, living during the same period of time, or living within the same area, are often characterised by trifling features in common, as of sculpture or colour.

In looking to the long succession of past ages, as in looking to distant provinces throughout the world, we find that species in certain classes differ little from each other, whilst those in another class, or only in a different section of the same order, differ greatly from each other.

In both time and space the lowly organised members of each class generally change less than the highly organised; but there are in both cases marked exceptions to the rule.