We should also remember that many fresh-water productions are low in the scale of nature, and we have reason to believe that such beings become modified more slowly than the high; and this will give time for the migration of aquatic species.
Gogoratu behar dugu halaber ur gezatako ekoizkin asko behe-mailakoak direla natur eskalan, eta badugu arrazoirik uste izateko izaki horiek goi-mailakoak baino astiroago aldatzen direla; eta honek uretako espezieen migraziorako behar den denbora bermatuko du.