Evolutionary irreversibility: once regarded as inevitable, but now considered to apply mainly in special cases. The potential for further useful mutation may well be very limited in highly specialized organisms, since only those mutations that will allow the organism to continue in its narrow niche will normally be functionally possible. In such cases there is therefore a self-perpetuating, almost irreversible, evolutionary trend, so much so that it is regarded virtually as a law, ‘Dollo’s law’ (after the palaeontologist Louis Dollo). The trend results from steady directional selective pressure, or orthoselection reinforced by specialization, or developmental canalization (see canalizing selection).