PROF. ARTHUR DENDY'S presidential address, delivered in February last, before the members of the Quekett Microscopical Club appears in the journal of the club for April, and will probably be much discussed, inasmuch as it is devoted to an analysis of the relation of mutation to the evolution theory, the arguments being based on data drawn from the sponges. The phenomena of mutation, it is contended, is more a chemico-physical than a biological phenomenon. Mutations, such as are observable in sponge spicules, in his opinion, strongly suggest the existence of definite factors in the germ plasm. The factorial hypothesis, he considers, is further supported* by evidence which is accumulating as to the general course of evolution followed by the Tetraxonida. On the whole this evolution seems to have been progressive, accompanied by increasing complexity 01 siructure, manifested especially in the skeleton. Along certain lines of descent, however, it appears that the culminating point has been passed, and regressive evolution is taking place, resulting in simplification of structure, by the dropping out of certain types of spicule. This loss cannot be regarded as an adaptive modification, nor can it be explained as due to mechanical necessities. Prof. Dendy concludes, therefore, that it is due to some change in the germ-plasm, affecting the power of the sponge to produce the particular spicules in question.