more rapid than would at first sight appear, for it is probable that but a few seminal generations intervene between the most divergent cultivated plants and their wild progenitors. But now suppose we chose any one of these highly divergent varieties, and without using any selection, bred from seed alone, what would happen? There is ample evidence leading us to believe that in the vast majority of instances the variety would swiftly (i.e. in a very few generations) revert to something very like the wild stock from which they originally descended;—but not to the wild stock precisely, for no doubt while the cultivated species was undergoing evolution in one direction, it was under the changed conditions undergoing retrogression in other particulars, and in these the reverted variety would differ from the original stock.
The truth therefore appears to be, that while there is a limit within "assigned" time to evolution, there is practically none to retrogression.
The above considerations may afford an explanation of another set of facts, viz. that cultivated plants and domesticated animals are much more variable than the wild varieties of the same species. In a state of nature plants and animals exist under conditions which, normally, are uniform during long ages; owing to which, and to the circumstance that in a state of nature many traits are essential to survival, evolution is slow, and therefore the traits of wild animals and plants acquire a certain fixity, because if any recent ancestor be reverted to in any particular, the change is not great; moreover, any retrogression must generally cause elimination; therefore since evolution is slow, since any reversion can seldom be great, and since reversion tends to cause elimination, there can seldom be any great or observable change of form. With cultivated plants and domesticated animals fewer
