Not all readaptation, however, is progress, for ideal identity must not be lost. The Latin language did not progress when it passed into Italian. It died. Its amiable heirs may console us for its departure, but do not remove the fact that their parent is extinct. So every individual, nation, and religion has its limit of adaptation; so long as the increment it receives is digestible, so long as the organisation already attained is extended and elaborated without being surrendered, growth goes on; but when the foundation itself shifts, when what is gained at the periphery is lost at the centre, the flux appears again and progress is not real. Thus a succession of generations or languages or religions constitutes no progress unless some ideal present at the beginning is transmitted to the end and reaches a better expression there; without this stability at the core no common standard exists and all comparison of value with value must be external and arbitrary. Retentiveness, we must repeat, is the condition of progress. The variation human nature is open to is not, then, variation in any direction. There are transformations that would destroy it. So long as it endures it must retain all that constitutes it now, all that it has so far gathered and worked into its substance. The genealogy of progress is like that of man, who can never repudiate a single ancestor. It starts, so to speak, from a single point, free as yet to take any direction. When once, however, evolution has taken a single step, say in the direction of vertebrates, that step cannot be retraced without extinction of the species. Such extinction may take place while progress in other lines is continued. All that preceded the forking of the dead and the living branch will be as well represented and as legitimately continued by the surviving radiates as it could have been by the vertebrates that are no more; but the vertebrate ideal is lost for ever, and no more progress is possible along that line. [Sidenote: Perfectibility.]