Page - 367 - in The Origin of Species

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 367 inherit different characters from their distinct progenitors; and organisms already differing would vary in a different manner. For instance, it is possible, if all our fantail pigeons were destroyed, that fanciers might make a new breed hardly distinguishable from the present breed ; but if the parent rock-pigeon were likewise destroyed, and under nature we have every reason to believe that parent-forms are generally supplanted and exterminated by their improved offspring, it is incredible that a fantail, identical with the existing breed, could be raised from any other species of pigeon, or even from any other well-established race of the domestic pigeon, for the successive variations would almost certainly be in some degree different, and the newly-formed variety would probably inherit from its progenitor some char- acteristic differences. Groups of species, that is, genera and families, follow the same general rules in their appearance and disappearance as do single species, changing more or less quickly, and in a greater or lesser degree. A group, when it has once dis- appeared, never reappears ; that is, its existence, as long as it lasts, is continuous. I am aware that there are some ap- parent exceptions to this rule, but the exceptions are surpris- ingly few, so few that E. Forbes, Pictet, and Woodward (though all strongly opposed to such views as I maintain) admit its truth ; and the rule strictly accords with the theory. For all the species of the same group, however long it may have lasted, are the modified descendants one from the other, and all from a common progenitor. In the genus Lingula, for instance, thespecieswhichhave successively appeared at all agesmust have been connected by an unbroken series of gen- erations, from the lowest Silurian stratum to the present day. We have seen in the last chapter that whole groups of species sometimes falsely appear to have been abruptly devel- oped; and I have attempted to give an explanation of this fact, which if true would be fatal to my views. But such cases are certainly exceptional ; the general rule being a gradual increase in number, until the group reaches its maxi- mum, and then, sooner or later, a gradual decrease. If the number of the species included within a genus, or the number of the genera within a family, be represented by a vertical