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SOUTH AFRICAN TORTOISES.
By J. E. DUERDEN, Ph.D., A.R.C.S. Professor of Zoology, Rhodes University College; Keeper of
Zoological Department, Albany Museum.
Coloration of Carapace.
Axillary and Inguinal Shields.
k. Frontal and Prefrontal Shields.
a. Type of Variation.
Introduction of New Characters.
i. Testudo boettgeri.
Number of Species represented. b. Geographical Distribution.
Original Type of the Geometrica-group. III. SUMMARY.
In South Africa there occurs an extremely well-defined group of tortoises, all the species of which are referred to by Prof. G. A. Boulenger, in the British Museum Catalogue of Chelonians, as
allied to Testudo geometrica." They are readily distinguished from other tortoises by having a black or dark brown carapace, each shield of which is conical with bright yellow rays extending from
the central areola. The original type, T'estudo geometrica, is a wellknown Linnean species, found in the Cape District.
With the extended exploration of South Africa, more and more of the geometrica-like tortoises have been discovered, so that at the present time the group comprises ten described species :-1, Testudo geometrica, Linneus, 1766 ; 2, T. oculifera, Kuhl, 1820; 3, T. tentoria, Bell, 1828 ; 4, T. verre auxii, Smith, 1839; 5, T. trimeni, Boulenger, 1886; 6, T. smithii, Boulenger, 1886; 7, T. fiskii, Boulenger, 1886; 8, T. strauchi, Lidth de Jeude, 1893; 9, T. seimundi, Boulenger, 1903 ; 10, T. boettgeri, Siebenrock, 1904. It is a significant fact that five of the species have been founded on single specimens, and in not more than two or three instances has a large number of specimens been available for determining the limits if variation or the relationship of the species to others previously described.
The fact that so many species of a clearly defined group occur in one region, all acknowledged to be very closely allied, suggested that a thorough study of the geometrica-tortoises on the spot might yield results of importance as regards the origin of the variations, their degree of distinctness, and their relationships ; in other words, might illustrate in some way the method of evolution of the different species. For the prosecution of such a study large numbers of speci
are required, obtained from as many different sources possible. From efforts already made, about 300 examples have been procured from various localities in South Africa. The collection can probably be regarded as fairly representative of the different types of the geometrica-group, though a perfect study of this kind would require that every individual specimen, living and dead, should be compared, and, where possible, the results stated in statistical terms. For specimens received acknowledgments are due to the Director of the South African Museum, Cape Town, of the Natal Government Museum, and of the King William's Town Museum, as well as to numerous contributors throughout South Africa, among whom Mr. S. C. Cronwright Schreiner, M.L.A., deserves special mention.
During the progress of the work, as more and more specimens became available for study, the more difficult became the task of arranging them systematically among the recognized species, and the more evidence was accumulated as to the close relationship of the species so-called, thereby confirming the idea of their common genetic nature. Moreover, it soon became manifest that the different series include some remarkable transitional forms, and also that characters incipient or fluctuating in one group of specimens were well developed or fixed in other groups. In every direction evidence was afforded of determinate variation or variation along definite lines (orthogenesis) in contrast with indeterminate or discontinuous variation (mutation).
Though the occurrence of transitional series of organisms is by no means unfamiliar to the zoologist, yet it is very desirable that they should all be thoroughly worked out with the object of discovering what particular new light they may shed upon the all-important subject of genetics, or even to strengthen and further illustrate
, conclusions already accepted. Such studies naturally follow in the train of the systematist. The systematist is the pioneer ; he discovers and describes his species, he makes known what material is available, and then the evolutionist attempts to show from a study of relationships and environmental conditions how the different forms have come to be, that is, their method of transformation.
The object of the present paper is to discuss in a preliminary manner the various characters which are relied upon by systematists for diagnostic purposes within the geometrica-group, to endeavour to trace how the characters have been evolved, and how by their combinations they have given rise to types which are, or may ultimately become, distinct species ; we have to consider, first, the variations of the individual specific characters, and, second, the origin of the specific types. With the acquisition of still further material, it is hoped that a more complete account will be possible, illustrated by coloured drawings.
The claim for specific recognition of the ten forms hitherto described will be discussed later. For present needs the following may be allowed to be sufficiently well differentiated to warrant their acceptance as distinct species or, perhaps better, as sub-groups, namely: geometrica, oculifera, tentoria, and trimeni. As regards the other species described, it is, to my mind, very doubtful whether they can be considered as representing more than one complex assemblage, the members of which are as yet very fluctuating in nearly all their characteristics. The evidence for this conclusion will be presented later, and in the meantime I shall consider them as grouped around verreauxii.
The following are the principal taxonomic characters which are relied upon for distinguishing the various specific forms : (a) Coloration of carapace and plastron ; (b) axillary and inguinal shields; (c) margin of carapace; (d) margino-costal angle ; (e) nuchal shield; (f) antebrachial shields; (g) femoral tubercle ; (h) form of shields of carapace and plastron ; (j) beak; (k) frontal and prefrontal shields.
I. INDIVIDUAL SPECIFIC CHARACTERS.
COLORATION OF CARAPACE.
As already indicated, the coloration of the shell is the most distinguishing and conspicuous character of the geometrica-group, and, from its general similarity throughout, is the one feature which at first sight suggests the close relationship of the different members of the group. It consists essentially of a series of yellow rays on a black or dark brown background. The rays on each shield radiate all the way from the central areola to the margin of the plate, though sometimes they extend only a part of the distance.
The yellow rays vary much in number, width, length, arrangement, and tint in different forms. Sometimes they are so broad that the lighter colour may be regarded as constituting the background on which narrow black bands are displayed. The following tints are found to occur : Pale yellow or straw colour, lemon, light and dark orange, reddish yellow, yellowish brown, and liver red. Usually any one tint is characteristic of an individual, but occasionally the distal part of the ray is darker than the part near the areola. The colours are rarely very clear and bright on the natural shell, but when the latter is wetted or rubbed with oil they are displayed in a brilliant manner.
Though the yellow rays present very varied appearances in the different sub-groups, a general underlying plan can be traced throughout, and followed in its greater and greater elaboration through the series. The simplest condition is that found in tentoria, and the most complex in oculifera. In the former a number of simple yellow
. bands radiate from the areola, which is also yellow; some of the rays extend all the way from the areola to the margin, while others are shorter, due to incompletion either proximally or distally. The width of the ray also varies much, and may be the same throughout or increased distally; frequently a broad ray is bifid distally. In general, the rays of one shield show no correspondence in position with those of the adjacent shields, so that no complex geometrical pattern for the whole shell is possible.
The neural and costal shields of tentoria are roughly quadrangular in their basal outline, and in some individuals there is a tendency for the rays passing to the four angles of the shields to be more conspicuous than those intermediate, and also for those of adjacent shields to correspond, thereby giving a more definite geometrical plan. All the types of the geometrica-group show this to be the fundamental disposition of the rays on both the neural and costal plates. The principal rays extend from the areola to the angles of the shields, and these four are the most conspicuous rays; midway between two adjacent principal rays may be another ray, and alternating with these two series may be others of lesser rank. The four principal rays will be spoken of as primary; the four intermediate, one or more of which may be wanting, will be secondary rays; and the third series, not always present, will be tertiary rays. On all the shields the four primary rays are always conspicuous, and, in the more complex patterns, such as those of oculifera, they are bifurcated distally. The secondary rays alternate with the primary, and vary much in the extent of their development; rarely they are all wanting on individual shields. Frequently the two longitudinal secondary rays of one shield correspond with those of the two adjacent shields, one in front and one behind, and thus give rise to a continuous yellow band, which is median along the neural shields and lateral along the costals on each side. The tertiary rays are generally imperfectly developed, and are rarely numerous except in certain tentoria. By the continuity of the two primary rays of one shield with the two primary rays of an adjacent shield there is produced a " diamond pattern which, combined with a median hand, gives an ocellated character to the carapace. This is very well shown in some specimens of the verreauxii sub-group (see Boulenger's
figure of T'. fiskii, P.Z.S., 1886, pl. LVIII.), and more especially in oculifera. The middle of the ocellus is constituted by the lateral secondaries, and, if these are bifurcated, still more distinctive ocelli are produced.
When the simple ray type of coloration found in the tentoria sub-group is compared with the complex ocellated pattern of some of the oculifera sub-group, the two would seem to be very divergent and unrelated, and it is only by studying intermediate stages that the pattern of oculifera admits of comparison with that of tentoria. The members of the verreauxii sub-group are particularly instructive for this purpose. They include all stages in the prominence of the primary rays and their correspondence with those of adjacent shields so as to strengthen the geometrical plan. Ocelli are also very clearly indicated in some of the extreme members. The geometrica subgroup also shows the same tendency towards the regularity and correspondence of the rays, but to a much less degree than most verre au xii; indeed the colour pattern of some of its members cannot be distinguished from that of tentoria. In trimeni the pattern seems to have become more fixed and regular, and is transitional in character ; rarely any of the tertiary rays are present, and usually not all the four secondaries.
Taking all the representatives of the geometrica-group into account, it becomes an easy matter to arrange a continuous series showing all the transitional stages from the simple ray pattern of tentoria to the very complex, ocellated display of oculifera. No one can question that all the colour variations are genetically related in a direct line from such a form as tentoria to that of oculifera, and that they represent a strictly continuous type of transformation. Corresponding rays can be traced all through the series, the modifications consisting in the closer relationship of those of one shield with those of adjacent shields, and in minor changes such as bifurcations. In certain of the sub-groups one transitional stage appears as if becoming fixed, while in other sub-groups another stage predominates, but probably nowhere do we get two individuals exactly alike. Even in specimens of any sub-group from the same locality there are great differences in the number of complete and incomplete rays present, and in the regularity of the pattern. While conforming closely to the general plan of the sub-group, the minor variations are innumerable.
b. COLORATION OF PLASTRON.
The plastron also affords a very complete gradational series in the production of an elaborate colour pattern. The coloration of certain members of the tentoria sub-group may be regarded as the extreme type of simplicity, while that of oculifera represents the extreme of complexity. Between these two there is a complete varietal series, showing that all are genetically related.
In what may be considered as a typical tentoria, the whole of the central part of the plastron is uniformly dark brown or nearly