55. On the Skeleton of a Bauriamorph Reptile.

Authors

  • D. M. S. Watson F.R.S., F.Z.S.


Summary.

The study of Ericiolacerta which constitutes this paper thus adds another to the very few Theriodonts in which the entire skeleton is known, and brings nearer the time when the great suborders Gorgonopsia and Therocephalia can usefully be subdivided into smaller groups whose evolutionary story can then be disentangled.

Ericiolacerta proves to be a member of a group of reptiles represented at all periods from the “Middle” Permian Tapinocephalus-zone to the Lower Triassic Cynognathus-zone. Throughout this long history the members of the group retain in characteristic form certain details of the architecture of the basicranial and otic regions and of the palate, in which they differ from all other known reptiles; but in the appearance of a secondary palate at a certain period of their existence they afford a parallel to the series leading from Gorgonopsians to Cynodonts and to the Anomodontia. Indeed, the most interesting general result which comes from the whole investigation is the further confirmation it affords to the view which I have long held, that the mammal-like reptiles, like many other groups of animals, show an astonishingly great amount of parallel evolution. Such features as the reduction of the hinder part of the lower jaw and of the quadrate, the attainment of a secondary palate, the reduction of the phalangeal formula to 2. 3. 3. 3. 3. the appearance of an obturator foramen, which are all changes in a mammalian direction, have occurred independently in many different groups of Therapsids.

The independent appearance of such identical, or at least similar, changes at once recalls the familiar fact of the widespread appearance of identical mutations in closely allied animals, and even in animals which are not extremely near to one another in a systematic sense.

The “gene” theory of heredity offers the possibility of an explanation of this remarkable phenomenon. If the things which determine the course of development of an animal form a fertilized egg be particulate, and the particles so small that many hundreds of them, each discrete and differing from its neighbours, can be arranged along the length of a chromosome, then its must be so small that it can contain only a relatively small number, at most a few hundreds, of molecules as large as those of proteins. Each gene is capable of multiplication by fission without change of its character or potentialities. It is therefore probable that it possesses a definite organization, just as a cell equally capable of reproduction by fission is organized.

It is quite certainly extraordinarily stable, surviving unchanged through the many cell divisions which in a single animal intervene between the fertilized egg and a ripe gamete. It is found still, in general, unchanged in most or all the individuals which compose a species and must in most cases have come down to them from ancestors which belonged to a different species or genus. It may be expected to pass on still unchanged into the bodies of members of descendent species.

But the “gene” may suffer, though rarely, that change which causes the fertilized egg of which it is a part to develop into a “mutant,” and this change may occur time after time at a rate which is constant under uniform conditions, but may perhaps be altered by external influences, such as X-rays.

Usually only one mutation of a gene is known, but rarely several may occur, leading to the formation of “multiple allelomorphs.”

Finally, the mutant gene, though stable, may itself mutate, reproducing the type of which it was a mutant.

Thus it is suggested that the organization or structure of a gene represents a stable condition, perhaps in some cases the only one possible with the materials of which it is composed, but often one of two, the other being that which, when present, produces the mutation which is allelomorphic to the original structure. Multiplc allelomorphs would imply the possibility of several stable patterns.

I fancy that this conception would imply that in a population all the possible stable positions should occur in definite proportional numbers representing an equilibrium determined by the chances of one or other pattern developing after a disturbance of the organized arrangement; but this could only be if the different allelomorphs formed parts of animals equally viable under the conditions of life to which the population was exposed.

Such a conception of organized particles with few possible positions of stability is a parallel to the current view of the nature of an atom.

It would afford the beginning of an explanation of the remarkable phenomenon of parallel evolution.

Ancillary