A study of mutations in evolution

Summary

Two modes of evolution are involved in the history of the foot of the horse, continuous and discontinuous.

Continuous variation is in accord with the principles governing the relative growth of parts, whereby any augmentation of total size favours unequally its component parts. Given that digits ii and iv are initially shorter than digit iii in the primitiveEohippus, and granted that the apex of the longest digit will support the body weight, it is obvious that progressive elongation of the limb observed inMesohippus andMerychippus tends to elevate the paramedian toes and eventually deprives them of an opportunity for weight-bearing.

Discontinuous variation is observed in the abrupt transformation from a three-toed to a one-toed foot. The course of relative growth in the latter case is still algebraic, but a new equation is required.

Whereas the occurrence of “continuous” evolution requires a succession of general size mutations, the “discontinuous” mode of evolution may be attributed to the intervention of a specific form mutation.