But according to the view that all the species of a genus, though distributed to the most remote points of the world, are descended from a single progenitor, we ought to find, and I believe as a general rule we do find, that some at least of the species range very widely.
Bada arrazoirik uste izateko, geologi frogetan oinarrituz, klase handi bakoitzaren barnean beheragoko mailako organismoak goragokoak baino astiroago aldatzen direla eta ahalbide gehiago izanen zutela, beraz, zabal hedatzeko eta jatorrizko izaera berari atxekitzeko ere.