A study of mutations in evolution

Summary

1. Very extensive changes of form appear during foetal and post-natal development in the horse as quantitative functions of increase in total size.

2. These developmental changes of skull shape correspond precisely, if comparison be made between specimens of the same absolute magnitude, to those shape transformations arising during the evolution of the horse.

3. Since progressive pre-optic preponderance in the individual and in the race is algebraically the same function of total size, one may analyse the form of any horse’s skull, either ancient or modern, as the manifestation of the characteristic equine skull pattern modified only by the absolute extent of growth. The post-optic extension ofHyracotherium and the pre-optic preponderance ofEquus are believed to represent a single embryological pattern, of which diverse manifestations occur as a function of total size.

4. The appearance of an “orthogenetic” trend in the evolution of the equine skull is due to the attainment of successively greater adult sizes by the more recent genera.

5. One may interpret the progressive augmentation of racial size as a fortuitous occurrence, possibly aided by some natural selection, perhaps chemically determined by the mutation tendencies of certain size genes, but not inconsistent with the nature of probability.