Although numerous elaborate experiments, directed chiefly towards practical ends, have been made to determine the physiological effects of the parotid secretion of those Colubrine snakes whose bite is fatal to man, yet very little seems to have been done, by studying the effects of the saliva of the non-poisonous Colubrines, to assist us in forming some opinion as to how, on the theory of gradual modification by means of natural selection, the efficient lethal mechanism of the poisonous Colubrines may be supposed to have originated and become gradually perfected in all its parts.