Next to the visual act itself, convergence is admittedly one of the most interesting of the functions of human eyes. That it is evolutionally the last oculomuscular function to appear seems now equally well admitted. The gradual evolution of the invertebrate eye is scientifically satisfactory. The transition from the invertebrate to the vertebrate eye is involved in some doubt. From that point upward, however, the evolution is resumed and continues very regularly. In the lowest class of vertebrates (the fishes) the eye is probably no better than a squid's. Their eyes are situated on the side of the head with such widely divergent axes that there is no overlapping of the visual fields at all. There is no consensual movement (each eye being absolutely independent of the other). There is no common field of view, no common point of sight, no corresponding points in the two retinas; hence no stereoscopic