The trajectory of enzyme evolution is estimated from the relationship between the overall enthalpy of activation and overall entropy of activation. As evolution proceeds, the trajectory passes a turning point and then go toward diffusion-controlled region. Thus, the following two possibilities are put forth as candidates of a goal of evolution. One is that evolution will stop at a turning point, where specificity of a reaction is highest. The other is that the ultimate goal of evolution is the diffusion-controlled reaction, where catalytic efficiency is highest.