Materials for the Study of Variation Treated with Especial Regard to Discontinuity in the Origin of Species

In the conditions already described though the digits are not all clearly divided from each other yet no one whole digit can be supposed to be absent. Even in the specimen shewn in Fig. 104, II, from the presence of separate metaearpals III and IV the identity of the several digits is still easily recogni/.ed. These simplest cases however by no means exhibit all the phenomena. From a large group of cases the three following are chosen as each illustrating a distinct possibility.

1 Owing to the ambiguity of some records as to the similarity of the condition in tinriL'ht inul left hands I cannot give exact numbers. Upon the morphological questions arising out of these facts comment will be made when the whole subject of numerical Variation of digits is discussed. Representation of digits II and III of the pes by one digit. *529. Man having four digits in the right foot as shewn in Fig. 105. The calcaneum, astragalus, navicular, first (internal) cuneiform and cuboid were normal. The navicular had on its peripheral surface three facets as usual. The second and third cuneiforms were completely united to form one bone which bore no traces of its double nature as shewn in the figure (c- -f c3). The peripheral surfaces of both form one plane. Taking the four digits in order, the minimus has its normal form and tarsal relations. The digit next to it has the normal form and relations of a digit IV.

FIG. 105. Bones of the right foot of No. 529. I, hallux. 11 + III, digit apparently representing index and medius. IV, annularis. V, minimus, a, astragalus, sc, navicular. cb, cuboid, c1, internal cuneiform. c2 + c3, bone apparently representing the middle and external cuneiforms. (After GBCBER.) Internal to this is a nietatarsal of abnormal thickness articulating with the single bone presumably representing the external and middle cuneiforms. This metatarsus presented no trace of duplicity. It bore a digit of three phalanges of more than normal thickness but otherwise normal. The hallux was normal, having two phalanges. Each of the other digits had three phalanges, but the 2nd and 3rd phalanges of the minimus were ankylosed.

Of the muscles, the transversalis pedis, one of the lumbricales, one of the inter - ossei dorsales and one of the interossei plantares were absent. The extensor and flexor longus each had three tendons. [Detailed description of bones and soft parts given.] GRUBEE, W., Virch. Arcli. f. path. Anat. u. Phi/s., 1869, XLVIT. p. 30-4, PI. vm. Single digit articulating with the cuboid [probably a case of representation of digits IV and V by one digit]. *530. Man having four digits on the left foot as follows. The foot is well formed. The digits I, II and III are normal and have normal tarsal relations. The fourth digit has a well-formed nietatarsal and three phalanges. The bones are perhaps rather more robust than those of a normal fifth digit, but the nietatarsal has the normal tuberosity at the base strongly developed. This metatarsal articulates with a cuboid of somewhat reduced size having only one articular facet on its peripheral surface. The other parts were all normal, and even in the muscular system only a trifling abnormality was found. Parents normal. STEINTHAL. C. F.. Virch. Arch, f. path. Anat. u. Phys., 1887, cix. p. 347.

*5-'51. [This case is introduced here for comparison with the last.] A left foot having abnormalities as follows. Calcanenm, astragalus, internal cuneiform normal in size :uid shape. The second cuneiform i.- rather broader than usual, but the surface which it presents to the internal cuneiform has all the characters of a middle cuneiform. External t<> this middle cuneiform is only one large tarsal bone in the distal row. This bone presents no clear sign of duplicity, but from its form and relations it appealed that it represented both the cuboid and the ecto-cuneiforrn. Thehallux and ilii/it II have approximately normal relations. The large cuboid-like bone • \ternally a ruetatarsal a<.'m.'iuj_' in shape with a metatarsal V ; and internal to tliis the same tarsal bone bears another metatarsal which upon its external side !/ivr.s off yet another metatarsal of reduced size. Each of the five metatarsals bore a digit, but the digits of the minimus and of the slender IV were webbed together. [Full details given.] BRENNKK, A., 1'irch. Arch. f. path. Anat. it. Pliys., 1883, xciv. p. "23, PI. ii.

.'i.'!2. Besides the-e simpler cases there are very many recorded instances of reduction in number of digits in which the identification of the parts is quite uncertain. From the point of view of the naturalist it is worthy of remark that even in some of the cases departing most widely from the normal form the limb though having only thiee or perhaps two digits still presents an approach to a symmetry. Examples of this kin. I :uv given by GuYOT-DAUBES (Rev. d'Antliropol., 1888, xvn. p. 541, figs.) and by Fm 111.1:1. v (I'.rit. M>'<1. Jour., 1886 (1), p. 97o/f.'/-s'. ) and many more. Fotherby's record is interesting as relating to a family among whose members feet bearing only two opposable claw-like digits of irregular form recurred for five generations. Evidence relating to limbs of this kind is so obscure that it is not possible as yet to make deductions from it, but there seems to be a general agreement among anatomists that when two digits only remain one of them has the characters of a minimus.

Reference must be made also to the fact that in cases of absence of radius the pollex is almost always absent. This seems to be established in very many cases. The only examples of a pollex present in the absence of a radius known to me are that of GRCBER, Virch. Arc-it, f. path. Anat. u. PInj*. INI;."), xxxn. p. 211, and that of GEISSEXDORFER, Zur Casuistik d. comjvn. Radiusdefectes, Munch. 1890. V.-i riation in the number of digits in the Horse1 has been repeatedly observed from the earliest times. The mode of occurrence of tin.- change is by no means always the same, but on the contrary -rvt-ral distinct forms of Variation may be recognized. ( )n inspection the cases may be divided into two groups.

A. Cases in which the extra digit (or digits) possesses a distinct metacarpal or metatarsal. B. < iasea in which the large metacarpal or metatarsal (III) gives articulation to more than one digit. llesides these I have placed together in a third group (C) two very remarkable cases which cannot be clearly assigned to either of the other groups. These instances are of exceptional interest from 'he tact that in them is exhibited a condition intermediate between those of the other two groups. We have seen repeated 1}* that

1 In the Mule two cases have been recorded, but in the Ass I know no instance of polydactylism. Describing a polydactyle horse seen on a journey in Rio Grande VON .IHKKIN.; (Ko.-oniis, iss-t, xiv. p. li'.t) states that he belies.'- polydactyle horses to be lunch more common in S. America than in Europe, aii.l that most prisons who have tra\elled much in that country have met with cases. Mules between the jackass and mare are bred in great numbers, but he had heard of no case in a mule.

Meristic Variation may take place by division of single members of Series, a phenomenon well seen in the B group ; and we have also seen many cases of numerical Variation by addition to the Series associated with a reconstitution, or more strictly a redistribution of differentiation amongst the members of the series thus newly constituted ; but here in these rare examples of the C group the nature of the parts is such that it cannot be predicated that the change is accomplished by either of these methods exclusively. From such cases it follows that the two processes are not really separable, but that they merge into each other. (Compare the similar facts seen in regard to teeth p. 269, and mammae p. 193.)

(1) Two digits, one being formed by the development of the digit II. (2) Two digits, one being formed by development of the digit IV. (3) Three digits; the digits II and IV both developed. Rare. It will appear from the evidence that though the same variation is often present in the limbs of both sides this is not always so. The fore and hind limbs also sometimes vary similarly and simultaneously, but in other cases they do not. Different forms of numerical Variation are also sometimes found on the two sides, and not rarely the variation in the fore limb is different fi'om that in the hind limb.

(1) Two digits, one formed by development of the digit II. a. Three metacarpals or metatarsals only. To this division and to the next, (1) b, belong the great majority of cases of polydactylism in the Horse. Unfortunately most of the records have been made from living animals and contain no anatomical description : in the absence of such particulars it is not possible to know whether a given case belongs to this division or to the next, and it thus is impossible to determine the relative frequency with which the two forms occur.

*533. Horse of common breed, having a supernumerary digit on the inner side of the right fore foot (Fig. 100 . Humerus and radius: no noticeable variation. Ulna a littlemore developed than usual; lower end slightly broken, having probably reached to lower fourth of radius. The part of the inferior and external tuberosity of the radius which is usually supposed to represent the ulna is larger than in the normal form. B. The carpal bones enlarged. M, magnum, sc, scaphoid, u, unci-

forrn. t-, trapezoid. tl, supernumerary bone not found in normal, representing trapezium. IV, the nietacarpal representing digit IV. Ill and II, rnetacarpals bearing those digits respectively. (After ARLOING.) Carpus consisted of eight bones, instead of seven as usual. iil much larger than normal; lunar, cuneiform and pisiform normal. In the lo\\rr row the magnum and uncit'orm have normal relations, but in the place of the normally single trapezoid are two bones, one anterior (?-), the other posterior (tl). These together bear the enlarged inner mrtaearpal (II). The posterior of these bones had a short pyramidal pnu-os lying beside the inner metacarpal. This process was partially constricted off and is regarded by

ARLOING as a representative of the metacarpal I, the carpal portion of the bone being the trapezium. The outer metacarpal (IV) was perhaps slightly larger than usual. The inner metacarpal (II) was greatly enlarged at its central end, articulating with the two bones t1 and t-, and partly with the magnum. In its central part this metacarpal was fused with the large metacarpal (III) and above is united to it by ligameutous fibres. Below it again separates from the large metacarpal and is enlarged, bearing an additional digit of three phalanges, the lowest bearing a hoof. [This hoof is not curved towards the large hoof as in many specimens described, but is convex on both sides, resembling the hoof of an ass.] The large central metacarpal was flattened on the side adjacent to the enlarged metacarpal II. The muscles, nerves and vessels are fully described (q.v.). ARLOIXG, M. S., Ann. Sci. Nat., Zool., Ser. V. f. vm. pp. 61— G7, PL

534. Foal having two toes on each fore foot. The father and mother of this foal were both of the "variete chev aline comtoise." The foal in question was the only one which this mare dropped and she died two months afterwards. The foal was in nowise abnormal excepting for the peculiarity of the fore feet. The carpus was normal and the external metacarpal was rudimentary as usual and ends in a small knob. The internal metacarpal is thicker than the external one and bears a digit of three phalanges, the terminal phalanx bearing a small hoof. This hoof is curved outwards towards the normal hoof. The ligaments and tendons of the foot did not suffice to keep it stiff, and as the animal walked, it not only touched the ground with the hoof but also with the posterior surface of the phalanges. This led to inflammation of the foot, in consequence of which the foal was killed. CORXEVIN, Nouveaux cas de didactylie c/tez le cheval, Lyons (18821). [Note that this case differs from the last in the fact that the carpus was normal.] A similar case in the right fore foot is given by KITT, Dent. Ztsch. f. Thiermed., 1886, xn. Jahresb., 1884—5, p. 57, fig.

Among the many accounts of polydactyle horses I know none which gives an anatomical description of a case of a fully developed digit II in the hind foot. The following case, indeed, is the only one known to me in which any facts respecting the condition of the tai'sus of a polydactyle horse have been ascertained. In it, as will be seen, the digit II was not fully developed. 535 Horse having the metatarsal II enlarged and bearing a rudimentary digit (Fig. 107 B and C). In the left hind foot the arrangement was as shewn in Figs. B and C. The metatarsal II was enlarged and articulated with "two united cuneiform bones " [presumably one bone with indications of duplicity]. Internal to this digit was a " first cuneiform bone," but the digit I was not developed. The metatarsal II bore peripherally a rudiment of a digit as shewrn in the figure. The right hind foot was similar to the left but it is stated that the "three small cuneiform

*;. navicular. ch, cuboid. 4, ecto-cuneiform. 1, 2, 3, three bones placed as cuneiforms. t<l, trapezoid. tin, trapezium. it, iinciform. m, rnagnum. I. II, III. IV, numerals affixed to the mctacarpals on the hypothesis that these are tin ir limnologies. Cp. Fig. 10*, which is lettered on a different hypothesis. This condition is a higher manifestation of the variation seen in the eases just given. In Xo. 533 the digit II was developed and in addition the trapezium had appeared ; in the cases now to be

1 MAKSII introduces this case in support of a contention that these variations are "f the nature of lo .< rsion. Upon the same page appears the statement that "in speeimen examined, where the carpal or tarsal series of bones were preserved and open to inspeetion, the extra digits were supported in the usual manner," /. c., p. 345: thi- a--ertion i* hardly in agreement with the previously stated fact that the mi tatarsal II i- -upported by tiro cuneiform bones. On p. 349 Marsh comments "ii the presence of five bones in the di>tal row of the tarsus, and from the expres- -ions iiM'd it is implied that five such bones had been met with in other polydactyle hind feet. A number «\ alternative explanations are proposed; (1) that the five tai>alcorrespond "'" those of the reptilian foot"; (2) that the lirst may be a amoid " ; >'•'•} that the t;r>t may he a remnant of the first metatarsal, for such a rudiment "apparently exists ill some fo^il hoi>es." With conjectures of this class mnrphi'loji>ts are familiar. Into their several merits it is impossible to inquire, but it may lie mentioned that the real ditliciilty is not the presence of the cuneiform marked 1, but the fact that the tarsal element of the digit II seems to have been double, and that the dibits in reality are not supported in the usual manner.

given the digit II is extensively developed and the trapezium bears a splint bone representing the metacarpal I, like that which in the normal represents the digit II. This is a phenomenon illustrating the principle seen in the case of teeth and other p.-irtin series (see p. 272), namely, increase in the degree of development of the normally last member of a series correlated with the appearance of a new member beyond it, Nevertheless the same cases have sometimes been described (e.g. Gated. Mus. Coll. Surg.) on a different hypothesis. This is illustrated by the lettering of Fig. 108. On this other view the innermost carpal is considered to be the trapezoid and its splint-bone is regarded as the original metacarpal II. The second digit, ac, and its tarsal bone are supposed to be " accessory " or "intercalated." To these terms it is difficult to attach any definite meaning. The proposal that some digits are to be reckoned in estimating homologies and that others are to be omitted is arbitrary, and, if allowed, would make nomenclature dependent on personal choice. It is, as has been often pointed out in foregoing chapters, simpler to number the parts in order as they occur and to accept the visible phenomena as the safest index of the methods and possibilities of Variation. Nevertheless, to illustrate the point at issue I have introduced two cases of the same Variation, the one, No. 536, lettered on the view advocated by the Catalogue of the College of Surgeons, &c., the other, No. 537,

FIG. 108. Eight fore foot of Horse No. 53G from behind. The upper surfaces of the carpal bones of the distal row are separately shewn above. Specimen in Coll. Surg. Mus., Ter. Cat., 304. T, trapezoid. J/, magnum. U, unciform. ac, accessory carpal bone. II, III, IV, metacarpals. acm, accessory metacarpal. This figure is lettered to illustrate the hypothesis adopted in the Catalogue, which is alternative to that adopted in Fig. 107, A. lettered "ii the other and in the case of polydactyle horses, more usual nit-thud.

*o3G. Hoi-*,-: right inanus with extra digit (Fig. 108). The distal row of the carpus is present. It consists of four bones, the uncifbrni, magnum and two other bones. Of these that lettered T on the \ iew of tin- ('iitii/nffue must be supposed to be the normal tra]>f/oid, while nr \< considered to be an intercalated bone, perhaps an additional os magnum. The unciform bears a splint-bone, namely UK-JI. 1 V. Tinmagnum bears a fully-formed mcp. and digit III. With the bone ac articulates a large and substantial ni'-tacur]>al with a digit of three phalanges and a hoof, while the IMHH- T hearanother splint-bone, marked II in the figure on the hypothesis that the digit ac is not to be reckoned. Cat. Mus. Coll. Surg., Ti-riit. Series, 1872, No. 304. As mentioned above, it would lie more consistent with fact to count the bone ac as trapezoid with mcp. II and the hone T as trapezium with mcp. I.

Probably the feet of a large number of polydactyle horses would be found to be in this condition if examined. MARSH, I.e., mentions three other cases known to him in Yale Museum. *539. Horse having five metacarpals and one supernumerary digit in the left manus, and/cw metacarpals with a similar supernumerary digit in t IK- ri-'ln manus. In the left manus with the trapezoid there articulated a welldeveloprd nietararpal II bearing the extra digit. Internal to this wa- a trape/iiun bearing a splint-bone, 6 cm. long, To wide at proximal end, representing metacarpal I [as in Section (1)6] coalescing peripherally with III. On the external side of III the splint-bone IV was present as usual. The case is remarkable from the fact that external to the metacarpal IV there was another rudimentary nieiacarpal, presumably representing V. This bone was distinctly separated from IV at the central end, but was for the most part united with it. PUTZ, Dent. Ztschr. f. Thierm., 1889, \v. p. 224, fifjs. [The figures illustrating this paper are carefully drawn. The representation of mcp. I is quite clear, but the condition of the mcp. V cannot be well seen, as the whole foot is represented with its ligaments, &c., which partly conceal the structure. The whole account is very minute and gives confidence in the statements.]

The right manus of the same animal came into the possession of the University of Graz and was described independently. In it also the metacarpal II was developed and bore a well-formed digit. There was also a rudimentary metacarpal I beside it, having a length of 5'7 cm., and a breadth of 1*5 cm. at the central end. [The description is brief and makes no mention of a mcp. V : further account promised.] MOJSISOVICS, Anat. Am., 1889, iv. p. 255. (2) Two digits, one being formed by development of the digit IV.

Cases of this variation are exceedingly rare. No. 540 is the only instance known to me in which a proper account exists. Most writers on the subject make a general statement that such cases exist, but give no references. FIG. 109. Eight fore foot of Horse, No. 540, the being developed, bearing a digit. (3) Three digits [? the diyits II and IV lining both developed]. Examples of this variation are alluded to by many authors but I kno\v of no anatomical description. The following are all very impert'''<-t ly described.

541. Foal (foetus): left manus having tliree sub-equal digits; ri^ln manii> two digits and rudiments oi' a third more developed than usual. Hind feet normal. GEOFFROY ST HILAIRE, Ann. Sci. Nat., xi. 1827, 542. Horse from Texas, having extra digit on inside of each manus, and an extra digit l>oth on the outside and on the inside of each pes [external view only]. MARSH, Am. Jour, tici., XLIII. 1892, p. 34-1, (4) Tiro till/it* ; tindiyitii II and IV both developed, III aborted wholly or in part.

Mention of these cases must be made in illustration of the possibilities of Mi ristic Variation, but the parts were in all three instances so misshapen that the animals could not have walked. 544. Foal having two toes on each foot, the developed toes belonging to the metacarpals and metatarsals II and IV, while the normally large III was not developed at all in the fore feet and was in the hind feet represented by a wedge of bone only. Hindft'tt. I./ ft. Bones of leg and tarsus said to have been normal. Metatarsal III represented by a wedge of bone fixed between the greatly developed metatarsals

II and IV. The wedge-like bone 5 cm. wide at upper end, having usual tarsal relations. Its length about the same as its width. Laterally it is united to the metatarsals II and IV which curved round it till they met, and then curve away from each other again. Each was about 20cm. long and bears a misshapen digit consisting of a proximal phalanx and a hoof-bearing distal phalanx. A small nodule of hour attached to the proximal phalanx may or may not represent part of a middle phalanx. Right. Very similar to left, but the wedge-like III was rather broader — [for details see original].

For,1 fi'i-t. More misshapen and less symmetrical than hind feet: metacarpal III not developed at all. The metacarpals II and IV curved towards each other and crossed, giving an unnatural appearance to the feet. Ri<iht foot. Cuneiform and lunar united, and upon the surface of the bone formed by tlreir union there was a groove occupied by two parts of the tendon of the anterior extensor metacarpi passing to mcp. II and IV respectively. Pisiform and scaphoid normal [this is not clear from the figure]. Magnum absent. Unciform and trapezoid abnormal only in respect of their relations, for whereas they should articulate with the magnum they do not do so, for both magnum and mcp. Ill are not represented. Metacarpai II wall cm. long, mcp. IV being I'.tcm. long. Each bore a digit with a hoof; the digit

IV having a proximal and a distal phalanx connected by a fibrous cord instead of a middle phalanx. The digit II had a rudimentary distal phalanx only. Left foot like ;he right, but with the mcp. and digit II more fully developed. [Muscles fully described. It may perhaps be thought that there is not sufficient proof that the ili '\ eloped digits are actually those normally represented by the splint-bones II and IV. hut tin eoii.liiion of the hind feet is practically conclusive that this is the right interpretation.] VvV.in \KI i.. /../ /w//(/,/, •/////, <•/„ : /,.-• x, ///;,,•, /,•.-;, from •/. de la soc. r. di-x .-•(•/. uti'd. (/<• /;/i/.r,//,.v. is;-.*. Plate.