We discuss the evolution of the optical and mechanical properties of InP single crystals over a 40 year period. Systematic monitoring advances the claim that the occurrence of the net of uniformly distributed impurities instead of chaotically disposed aggregates of impurity atoms as the result of their long-term redistribution leads to an increase in microhardness and to a decrease in its anisotropy along the different directions on the crystal planes. This net creates a more serious obstacle to the dislocation movement than the system of the same impurities and of the same concentration disposed in the form of clusters with big intervals between them and enough for gliding dislocations. In this work, we reveal and discuss a tight connection between the long-term evolution of optical (Raman light scattering) and the mechanical properties (microhardness and movement of dislocations) in InP.