by mutual sterility of varieties, ii. [185]-[189].

Spencer, Lord, on selection in breeding, ii. [195].

Spencer, Herbert, on the "survival of the fittest," i. 6;

increase of fertility by domestication, ii. [111];

on life, ii. [148], [177];

changes produced by external conditions, ii. [281];

effects of use on organs, ii. [295], [296];

ascent of the sap in trees, ii. [296];

correlation exemplified in the Irish elk, ii. [333]-[334];

on "physiological units," ii. [375];