text: " Now as they are here stated the two opinions are not necessarily contradictory; differences in external condition may effect remarkable changes in tribes of human beings and yet the collective body may be made up of different races: and to set before the reader a clear and distinct notion is to prove nothing although indeed as we shall see in the sequel the author has a very strong conviction and believes that he succeeds in proving as far as a matter incapable of mathematical demonstration can be proved the negative of the latter proposition