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.

According to our theory, these several relations throughout time and space are intelligible; for whether we look to the allied forms of life which have changed during successive ages, or to those which have changed after having migrated into distant quarters, in both cases they are connected by the same bond of ordinary generation; in both cases the laws of variation have been the same, and modifications have been accumulated by the same means of natural selection.

Classification, groups subordinate to groups -- Natural system -- Rules and difficulties in classification, explained on the theory of descent with modification -- Classification of varieties -- Descent always used in classification -- Analogical or adaptive characters -- Affinities, general, complex and radiating -- Extinction separates and defines groups -- Morphology, between members of the same class, between parts of the same individual -- Embryology, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age -- Rudimentary organs; their origin explained -- Summary.