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

It may be stated generally that if the tooth which is the last ..f a normal series is relatively a small tooth, as for example m? or •"- in the Dog, then in cases of an addition to the series, by which this terminal tooth becomes the penultimate, it will often (though not always) be found that this penultimate tooth is larger and I" M.I developed than the corresponding ultimate tooth of a normal animal of the same size. Of this phenomenon two striking examples (q. v.) have been given, ('(mix azane No. 249 and Dasyurns maculatus No. 385. I '.. sides these are several others of a less extreme kind e.g. Otocyon meyalotis No. 250, Mastiff No. 259, Dog No. 260. The same was also seen in the molars of Bettongia.

This phenomenon, of the enlargement of the terminal member of a series when it becomes the penultimate, is not by any means confined to teeth ; for the same is true in the case of ribs, digits, &c., and it is perhaps a regular property of the Variation of Meristic Series s.. graduated that the terminal member is comparatively .-mall. This fact will be found of great importance in any attempt to realize the physical process of the formation of Meristic Series, and it may be remarked that such a fact brings out the truth that the members of the Series are bound together into one common u hole, that the addition of a member to the series may be correlated with a change in the other members so that the general configuration of the whole series may be preserved. In this case the new member of the series seems, as it were, to have been reckoned for in the original constitution of the series.

Lastly there are a few cases, rare no doubt in higher forms but not \ery uncommon for example in the Sharks and Rays (see pp. 259, &c.), in which the members of the series seem to have been so far remodelled that the supposed individuality of the members is superseded. In the Selachians several such cases were given, but in Mammals the most manifest examples were seen in the Phalangers and Ateles marginatus No. 200 (q.v.). In the latter specimen there were four premolars on each side in the upper jaw, and there was nothing to indicate that any one of them was supernumerary rather than any other. In such a case I submit that the four premolars must be regarded as collectively equivalent to the three premolars of the normal. The epithelium which normally gives rise to three tooth-germs has here given rise to four, and I believe it is as impossible to analyze the four teeth and to apportion them out among the three teeth as it would be to homologize the sides of a triangle with the sides of a square of the same peripheral measurement.

Such a case at once suggests this question : if the four premolars of this varying Ateles cannot be analyzed into correspondence with the three premolars of the typical Ateles, can the three premolars of this type be made to correspond individually with the two premolars of Old World Primates ? In the case of Rhinoptera No. 396, for the reason given in describing the specimen, there is plainly no correspondence between the rows of plates of the variety and those of the type, and the rows are, in fact, not individual, but divisible.

Though cases so remarkable as that of Ateles marginatus are rare, there are many examples of supernumerary teeth, in the region of the anterior premolars of the Dog or Cat for instance, which cannot be clearly removed from this category. As indicated in the fourth section of this Chapter, it is impossible to distinguish cases of division of particular teeth from cases of the formation of a new number of teeth in the series. Finally, on the analogy of what may be seen in the case of Meristic Series having a wholly indefinite number of members, it is likely that the attempt thus to attribute individuality to members of series having normally a definite number of members should not be made.

Ix this chapter are given some miscellaneous examples. Most of them illustrate the Meristic Variation of parts standing in bilateral synnnrtry on either side of a median line. Here also are included certain cases of Variation concerning the series of apertures in the shell of Haliotis, though probably they are of a wholly different nature. Among animals possessing an exoskeleton composed of scales, the number of the scales or of the rows of scales found in particular regions is usually more or less definite. So constant are these numbers in their range of Variation that in both Reptiles and Fishes either actual numbers or certain ranges of numbers are made use of li>r purposes of classification,

Considerable Variation in these numbers is nevertheless well known, and many instances are given in works dealing with Keptiles or Fishes. The following cases are given as illustrations of soim of the larger changes which may occur. 403*. Clupea pilchardus (the common Pilchard). Among the Pilchards brought to the curing factories at Mevagissey, Cornwall, specimens have from time to time been found by Mr Mathias Dunn, the director, having the scales of one side very many more in number than those of the other side. Two specimens1 shewing this abnormality were given to me by .Mr Dunn in IMS!). Owing to the fact that the fresh Pilchards are shovelled wholesale into the brine-vats, it is not until the fish are picked over for packing after the salting process that any individual peculiarities are

noticed. This was the case with the present specimens, which were given to me as they came salted from the presses. Nevertheless when received they were in fairly good condition. The first specimen measured 8 in. to the base of the caudal fin. The head and opercula were normal on both sides. The number of scales along the lateral line or the left side is 32 and the number on the right side is 56 or 57. On the left side the scales have the size usually seen in Pilchards of this length, and on the right side for a distance of about an inch behind the operculum the scales are not much smaller than those of a normal Pilchard, but behind this point each scale is of about half the normal size.

The second specimen has a very similar length. It differs from the first in having the reduplication on the left side instead of on the right. Furthermore the scales are normal in size as far as the level of the anterior end of the dorsal fin, behind which place they are of about half the normal size. The transition in this specimen is quite abrupt. The scales had been somewhat rubbed, and the counting could not be very accurately made, but the total number along the left lateral line was approximately 48.

As these abnormal individuals were taken with the shoal there can be little doubt that they were swimming with it. In P. Z. S., 1887, p. 129, PI. xv. DAY described a specimen, also obtained from Mr Dunn, exhibiting characters similar to those above described. The number of scales along the lateral line is given as 32 on the right side and 51 on the left. In the figure no transition from normal to abnormal scales is shewn, but there is a general appearance of uniformity.

Mr Day regarded this specimen as a hybrid between the Herring (C. harenyus) and the Pilchard, and before adopting the view that the case is one of Variation this suggestion must be discussed. This view was chiefly based on the presence of the small scales on one side, but it is added that the ridges on the operculum, which are characteristic of the Pilchard as compared with the Herring, were better marked on the right side than on the left, though they are stated to have been very distinct on the left side also. In the specimen described, the gill-rakers were 61 in the "lower branch of the outer branchial arch" (viz. the bar consisting of the first hypobranchial and ceratobranchial), and it is mentioned that this number is intermediate between that found in a Pilchard (71) and in a Herring (48); but whether this

intermediate number was found on the side shewing the " Herring characters, or on the other, or on both, is not stated. These gill-rakers are also said to have been intermediate in length between those of a Pilchard and those of a Herring. From these points of structure Mr Day concludes that the specimen was a hybrid between the Herring and the Pilchard. As against the theory that these specimens are hybrids it may be remarked that no direct evidence is adduced which points to hybrid parentage. The suggestion is derived from (1) the condition of the

scales, (2) the number of the gill-rakers, (3) the alleged difference in tinopercida of the two sides. In view of the first point, viz. that the number of the scales mi one side is intermediate between that of the I'il.-lianl and that of tin- Herring, it seemed desirable to know whether the rc-endilance extended to the minute structure of the scales or was r-.-ti-i.-ted to their mnnlier only. < >n comparing microscopically the scales of tin- Pilchard and the Herring, I find that those, of the H'-ning bear eonceiitric lines which are almost always smooth and without serration^. while those of the Pilchard are marked with lines whii-h are waved into very characteristic crenelated serrations. On comparing the -rales which are repeated, it was found that they also shew these characteristic serrations and that in pattern they differ in nowise from the scales of the Pilchard. This evidence appears to tell very vtrungly against the theory that the small scales are derived from a 1 lei ring parent.

The evidence from the gill-rakers seems to be also unreliable. In a normal Pilchard Mr Day found 71 on the hypo- and cerato-branchials • if the first gill-liar, and in a specimen examined by me 72 were present and in normal Herrings 48. But in my two specimens shewing the repeated scales tin-re were present, on the normal sides 79 and 67 respectively, and on the abnormal sides 78 in the one fish and 67 in the other. In si/.e and shape the gill-rakers were like those of the Pilchard, being smooth, and unlike those of the Herring, which bear well-marked teeth.

AJB it is stated that the serrations characteristic of the operculum of the Pilchard wenvery distinct on the abnormal side, it is impossible to lay much stress on the circumstance that they were less distinct than those of the other side. In addition to the considerations given above, there are several a /iriori objections to the hypothesis of the hybrid origin of these forms ; as, for example, that unilateral division of parental characters is certainlv not a common phenomenon in hybrids, if it occurs at all, and so on. l>ut since the evidence advanced for the theory of hybrid parentage is already open to criticism, it is perhaps unnecessary to discuss these further ditliculties.

On the whole, therefore, it seems simpler to look on these abnormalities as instances of the phenomenon of Meristic Variation1. In Ophidia the number of scales occurring in different parts of the body is constant in smutgenera and specie*, and variable in others. Variation in the number of rows of scales on the body may be specially referred to as an instance of a change in number occurring at right angles to that JUM described. The number of such n.\\s in Trt>in'<li>iinfnx, for example, is generally 19, but Mr

405. Tropidonotus natrix is remarkably constant in the possesis described by STUDER, Mitth. natur. Ges. Bern, 1869, p. 24, as having 20 rows. This specimen was unusually dark in colour. [The presence of an even number of rows is in itself remarkable, but it is not stated whether this total was reached by duplicity in the median dorsal row or by inequality on the two sides.] 406. A specimen of Snake from Morocco closely resembled Macroprotodun inauritanicus Guichenot (= Lycognathus cucullatus Dum. Bibr.), but differed from it in having 23 rows of body-scales instead of 19, being 4 rows in excess of the normal number. PETERS, W., Sitzb. Ges. naturf. Fr. Berlin, 1882, p. 27.

For particulars as to the range of variation in these numbers in different species, see numerous examples given by BouLENGER, G. A., Fauna of Brit. India : Reptilia and Batrachia, 1890. Meristic Variation in these organs is well known and the principal forms found are described in most text-books of anatomy. Some information as to these is given below. The examples are all from the human subject. 407. Kidneys. Male having three kidneys. The left kidney was normal in shape, position and consistency but was abnormally large. The right kidney was placed opposite to it and weighed only half as much as the left. From it a ureter with a small lumen arose and passed in a normal course so far as the division of the aorta. At this point its course lay along the surface of the third kidney. This third kidney lay over the whole right iliac artery, a portion of the right crural artery for the space of 9 lines, the right crural vein and the psoas major muscle. It was larger than the upper right kidney and had the form of an oval with its ends cut off. The anterior and posterior surfaces were convex. The anterior surface was grooved for the passage of the ureter mentioned above, which received the ureter of the second kidney and passed normally into the bladder. The man was a sailor and died of enteritis at the age of 39. THIELMANN, C. H., Mutter's Arch, f. Anat. u. Phys., 1835, p. 511.

408. Renal Arteries. The number of the renal arteries in Man is liable to great variation. In specimens in which the kidneys are normal in position the arteries may be (a) diminished or (6) increased in number. The latter is much more common. Multiple renal arteries may be threefold, (a) Most commonly the additional branches spring from the aorta, (6) they may come from other sources ; or (c) there may be a co-existence of additional vessels from both sources.

In tincommonest t'..i-m. next to the normal condition of one on each side, t here are two on the rightside and one on the left. In the -.•.•Mild commoin-t condition there are two on the left and one on the right; but aiming the forms with larger numbers, the greatest iiuiiili.-r is more frequently seen on the left than on the right side. In all these cases one vessel arises in the position of the normal renal; a second commonly springs from the aorta much lower down, ^fin-rally on the level of, or below the inferior mesenteric ; tintlnnl \\lii-n present, is at a very short distance above the normal i-fiial, \< r\ close to the supra-renal and on the level of the -ii]>crior mesenteric. Cases of five on the right are described by OTTO and .Mi:< KI;L, nnd other multiple forms are recorded by the older anatomists. MACALISTER, A., Proc. Roy. Irish Ac., 1883, p.

Three renal arteries on each side, symmetrically placed (Fig. 7:1). In this case the posterior ends of the kidneys were united across the middle line in the condition known as "horse-shoe kidney" [see evidence as to Bilateral Series]. Guy's Hosn. Rep., 1883, p. 4,s, % Ureters. Male. Four ureters emerging from the hilum of each kidney. After proceeding about tour inches they became united, forming a pel\ is from which sprang the proper ureter. The hilum of the kidney was found to lie occupied by a quantity of

fat and connective tissue, imbedded in which the ureters could be traced to the infundibula, communicating with the calices and pyramids : thus there was no pelvis within the hilum, but the calices united to form infundibula of which these ureters seemed to be the continuation, and they became united in a pelvis some distance removed from the kidney. There were other signs of abnormal urino-genital development and the author believes that it is almost certain that the abnormality described was congenital and not a sequel of disease. RICHMOND, W. S., Jour. Anat. Phys., xix. p. 120.

412. Subemarginula : specimen having a supernumerary eye on each eye-stalk (Fig. 74, II.). Author remarks that supernumerary eyes are common in forms having eyes borne on tentacles, but are rare in forms in which the tentacle is reduced as it is in Subemarginula. FISCHER, P., Jour, de Conch., S. 2, I. p. 330, PI. xi. fig. 4. 413. Patella vulgata : tentacle and eye repeated on left side (Fig. 74, I.). Right side normal. Supernumerary eye and tentacle of normal size. Ibid., S. 3, iv. p. 89, PL vm. fig. 8.

FIG. 74. Repetitions of eyes and tentacles in Molluscs. (After Fischer and Moquin-Tandon.) 414. Triopa clavigera <a Nudibranch): adult of the usual size, having tinlamellar rhinophore of the right side formed of three branches, "t which tin- t\vo anterior were lamellar, borne on a common peduncle, and the posterior was simple, of regular shape and )>i-ol>al>lv representing the normal rhinophore of the right side. The rhinophore of the left ride was normal Ibid., S. 3, xxvin. p. 131.

41 G. Helix kermorvani : a second eye present, close to, but Ibid., PI. xi. fig. 10. 417 Clausilia bidens : supernumerary eye on the right tentacle as shewn in Fig. 74, IV. Ibid., PI. xxm. fig. 24. 41, S Littorina : supernumerary eye on one tentacle. PELSENEER, In examining large numbers of Pecten of several species, Mr BRINDLEY occftsiuiially fouiiil one of the eyes imperfectly divided into two, the division being at right angles to the mantle-edge.

The following are examples of supernumerary eyes in Insects. They are mentioned as examples of the development of tissues of tinsame nature as those of the normal eye in abnormal situations. All the cases known to me occur in Coleoptera. 419. Toxotus ( = Pachyta) 4 — maculatus: a normal female. On the vertex of i lie margin of the right eye and abutting against it i> a small third eye. This third eye is round-oblong in shape. It is separated from the large eye only by the outermost margin of the eye, and though it is more convex than the latter there is nevertheless a considerable depression between the upper surfaces of the two eyes. This supernumerary eye is of a brighter colour than tlie normal e\e. being brownish-yellow, while the latter is of a pitchy black. It is facetted in the same way as the normal eye is. LETZNER, K., Jahresb. d. Schles. Gesell.fiirvaterl. Oultur., 1881,

42<>. Calathus fuscus : having a third eye. On the left side of the \ertex was placed a supernumerary eye. This structure was smaller and less projecting than the normal eye and was separated from it by the usual groove. It did not appear to be a part of the normal eye which had separated from it, for the normal eyes of the left and right sides were exactly alike. The integument of the head was slightly wrinkled around the supernumerary eye. DE i \ I '.in I.KKIK, 1'., Ann. de la Soc. Knt. <!<• France, S. 5, v., 1875, p. 426', note.

421. Vesperus luridus $ : head abnormal' and bearing a third 1 For cases of i\> i'"inp<>unded in the middle line (Bees), see evidence as to Bilateral Serif-. facetted eye. The consistency of the chitinous covering of the head, its sculpture and hairs, colour, &c. are all normal and of the usual structure. The left side of the head however is rather less developed than the right, and the left eye seems to be smaller and somewhat less convex, but there is no special deformity or alteration in the facetting.

At the left side of the head arises an irregular chitinous loop of unequal thickness and having a diameter of about 2'5 mm. This loop is attached to the substance of the head before and behind and these two attachments are distant from each other about 1 mm. The height of this loop from the surface of the head is about 1 mm. in the highest part. Upon the upper surface of the loop is a small, irregularly rounded eye. The diameter of this eye is about 2'5 mm. and its convexity is considerable. It is facetted, but its facetting is not quite regular and is finer and slighter than that of the normal eyes. VON KIESENWETTER, Berl. Ent. Ztschr., 1873, xvn. p. 435, Plate.

[A case is recorded by REITTER (Wiener Ent. Ztg., iv., 1885, p. 276) of a Rliyttirhinus deformis, having a "complete and fully formed facetted eye placed on the left side of the thorax." Upon the request of Dr Sharp, this specimen was most kindly forwarded by Dr Reitter for our examination, when it was found that upon the application of a drop of water, the supposed abnormal eye came off. The eye appeared to be that of a fly, and had no doubt become accidentally attached to the beetle either in the collectingbox or before its capture.]