It seems highly probable that chemotropism is also the explanation of the restless and persistent energy with which for days together the mammalian spermatozoa seek the entrance to the uterus, although the natural current produced from the mucous membrane of the uterus is from within outwards. The spermatozoon, in spite of all mechanical and other hindrances, makes for the egg-cell with an almost incredible certainty. In this connection we may call to mind the prodigious journeys made by many fish; salmon travel for months together, practically without taking any food, from the open sea to the sources of the Rhine, against the current of the river, in order to spawn in localities that are safe and well provided with food. I have recently been looking at the beautiful sketches which P. Falkenberg has made of the processes of fertilisation in some of the Mediterranean seaweeds. When we speak of the lines of force between the opposite poles of magnets we are dealing with a force no more natural than that which irresistibly attracts the spermatozoon and the egg-cell. The chief difference seems to be that in the case of the attraction between the inorganic substances, strains are set up in the media between the two poles, whilst in the living matter the forces seem confined to the organisms themselves. According to Falkenberg’s observations, the spermatozoa, in moving towards the egg-cells, are able to overcome the force which otherwise would be exercised upon them by a source of light. The sexual attraction, the chemotactic force, is stronger than the phototactic force. When a union has taken place between two individuals who, according to my formula, are not adapted to each other, if later, the natural complement of either appears the inclination to desert the makeshift at once asserts itself in accordance with an inevitable law of nature. A divorce takes place, as much constitutional, depending on the nature of things, as when, if iron sulphate and caustic potash are brought together, the SO₄ ions leave the iron to unite with the potassium. When in nature an adjustment of such differences of potential is about to take place, he who would approve or disapprove of the process from the moral point of view would appear to most to play a ridiculous part.