Parasitology, owing to its economic importance, has received very great attention during the last 50 years and several good text-books have been devoted to this subject. In almost all recently published books, however, stress has been laid chiefly upon the economic side of the problem: medical, veterinary, agricultural, etc., but the scope of Parasitology is by no means restricted to the problems economically connected with man. We know now that there is hardly any group in the animal or vegetable kingdom which does not harbour parasites. The latter themselves do not escape this rule and are often parasitised in their turn. Moreover, the interrelations between living organisms are not restricted to parasitism only; there are, in fact, all shades of transition between parasitism on the one hand, and commensalism and symbiosis on the other. These interrelations have never been adequately dealt with in text-books of biology or parasitology and all the information concerning them is scattered in a great number of papers dealing with various biological or parasitological problems. This gap has been recently filled by a small volume issued by Prof. M. Caullery [of the “Laboratoire d'Evolution des Êtres Organisés de la Sorbonne, Paris” under the title Le Parasitisme et la Symbiose in which the author has incorporated a series of lectures delivered by him on this subject during the academic year 1919–1920. The book deals with the most interesting cases of commensalism, parasitism, and symbiosis and with various important cases of transition between these three great ethological groups.