Summary
1. A quantitative study has been made of the successive changes of skull proportion associated with evolution in the horse.
2. The ratio of face length to cranium length has been doubled, increasing by insensible gradations from 0·85 in the cat-sizedHyracotherium to 1·70 or more in the largest living horses. This trend, in contrast to the progressive post-optic preponderance of titanothere evolution, has been described as a progressive pre-optic preponderance.
3. This aspect of evolution is strictly related to increase in total body size. Within a narrow range of variability, all specimens of the same size exhibit the same facial index, irrespective of phylogenetic status.
4. Although it is permissible to assume the occurrence, from time to time, of mutations affecting general body size, which may be in large measure responsible for differences of stature characterising the several genera, there is yet no evidence for the belief that specific mutations governing differences of form (in animals of equal stature) have played any significant part in the facial evolution of the equine skull.
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Cumming Robb, R. A study of mutations in evolution. Journ. of Genetics 31, 39–46 (1935). https://doi.org/10.1007/BF02982278
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DOI: https://doi.org/10.1007/BF02982278