• R. Cumming Robb

      Articles written in Journal of Genetics

    • A study of mutations in evolution - I. Evolution in the equine skull

      R. Cumming Robb

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      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.

    • A study of mutations in evolution - II. Ontogeny in the equine skull

      R. Cumming Robb

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      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.

    • A study of mutations in evolution - III. The evolution of the equine foot

      R. Cumming Robb

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      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.

    • A study of mutations in evolution - IV. Ontogeny of the equine foot

      R. Cumming Robb

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      1. Three toes are present on each foot of the embryo of the modern horse. Ontogeny here seems to reflect phylogeny.

      2. The dimensions of the lateral toes of the embryo are quantitatively distinguishable from those of the primitive foot as early as the crown-rump 20 mm. stage, being (by extrapolation) then half as large (see Fig. 1).

      3. Subsequent to this stage the exponential growth-rate of the lateral digits (with reference to the central digit) is substantially identical in the tridactyl and monodactyl foot.

      4. In these modern cases of polydactyly which are described as atavistic we find the lateral digits to have reverted to ancestral dimensions. The genetic basis of this reversal is not clear.

      5. In the ontogeny of the equine foot there is no evidence of recapitulation, since at the earliest known stage the analgen present a size difference. Moreover, this difference is algebraically constant, although qualitatively less conspicuous in the younger individuals than in the older.

      6. Ontogeny may repeat or overstep phylogeny in some one structure or region (as in the dimensions of the skull), yet diverge completely in some other organ (such as the foot) of the same organism.

      7. It is inferred that certain basic genes present in the ancestor may continue to act unchanged in the modern embryo, whereas other genes which have undergone mutation will subsequently affect embryogenesis in a novel fashion. For this reason the relationship between ontogeny and phylogeny will require as many definitions as there are organs or genes involved.

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