XXX.

For an argument against the unity of the human species, drawn from the analogy of the lower animals, to be valid, it must be taken from a species co-extensive in its geographical distribution with man.

To be thus co-extensive, it must not only be spread over a large area, but it must be spread _continuously_.

To be thus co-extensive, it must be found at equally high and low sea-levels, as well as at equally distant degrees of latitude and longitude.