Species of different genera and classes have not changed at the same rate, or in the same degree.  In the oldest tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms.  Falconer has given a striking instance of a similar fact, in an existing crocodile associated with many strange and lost mammals and reptiles in the sub-Himalayan deposits.  The Silurian Lingula differs but little from the living species of this genus; whereas most of the other Silurian Molluscs and all the Crustaceans have changed greatly.  The productions of the land seem to change at a quicker rate than those of the sea, of which a striking instance has lately been observed in Switzerland.  There is some reason to believe that organisms, considered high in the scale of nature, change more quickly than those that are low:  though there are exceptions to this rule.  The amount of organic change, as Pictet has remarked, does not strictly correspond with the succession of our geological formations; so that between each two consecutive formations, the forms of life have seldom changed in exactly the same degree.  Yet if we compare any but the most closely related formations, all the species will be found to have undergone some change.  When a species has once disappeared from the face of the earth, we have reason to believe that the same identical form never reappears.  The strongest apparent exception to this latter rule, is that of the so-called “colonies” of M. Barrande, which intrude for a period in the midst of an older formation, and then allow the pre-existing fauna to reappear; but Lyell’s explanation, namely, that it is a case of temporary migration from a distinct geographical province, seems to me satisfactory.

These several facts accord well with my theory.  I believe in no fixed law of development, causing all the inhabitants of a country to change abruptly, or simultaneously, or to an equal degree.  The process of modification must be extremely slow.  The variability of each species is quite independent of that of all others.  Whether such variability be taken advantage of by natural selection, and whether the variations be accumulated to a greater or lesser amount, thus causing a greater or lesser amount of modification in the varying species, depends on many complex contingencies,—­on the variability being of a beneficial nature, on the power of intercrossing, on the rate of breeding, on the slowly changing physical conditions of the country, and more especially on the nature of the other inhabitants with which the varying species comes into competition.  Hence it is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, that it should change less.  We see the same fact in geographical distribution; for instance, in the land-shells and coleopterous insects of Madeira having come to differ considerably from their nearest allies on the continent of Europe, whereas