A study of the merger time-scales of various types of interacting galaxies is conducted on the basis of the collisional theory. The results indicate that in the absence of halos, violently interacting galaxies merge in a time-scale of ~ 108 years; but the mildly interacting ones have merger time-scales from ~ 109 to 1010 years. However, in the presence of halos, all types of interacting galaxies are likely to merge in a time-scale of ≲ 108 years (as indicated by preliminary calculations). Galaxy evolution by mutual interactions is likely to have its reflection on the fundamental plane, as during the process the dynamical structures of the progenitors change and dissipation occurs.