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

FICJ. 152. Head of a Brill (lUm/nlni* he via) having the dorsal fin separated from the head as described in the text. (From YAIUUXL.) 1 The literature relating to discontinuous variations consisting in the presence of bony tubercles upon the blind side of Khombi is extensive. See especially - I know no detailed description of a flat-fish wholly pigmented on the underside, having the dorsal fin normal, but numerous authors (Gottsche, Duhamel, &c.) make mention of such cases. Since this chapter was written 1 have seen two recent papers on the subject by GIARD (Comptt-s reml. Si><\ 1:/<>I. , Is;i2, S. (.t, iv. p. 31 and Nut. ,SV/., IS'.IH, p. 356) contributing furtla-r ovideuce on the subject and giving new cases in the Turbot. According to (liard, of tlounders (/'. //<.-•(/.-•) at Wimeruux 3 °/0 are fully coloured on the blind side, in addition to many that are piebald. This must be a very much higher proportion of abnormal specimens than is found in English fisheries.

head. STEEXSTRUP states that the variation has, he believes, been observed in all flat-fishes1 except the Halibut (Hippocjlossus). In several but not all cases of this abnormality the eye belonging to the lower side was not placed in its normal position on the upper surface, but stood in an intermediate position on the top of the head, so that it could be partially seen in profile looked at from the " blind " side. It seems possible that the pigmentation of the " blind " side is in some way correlated with some abnormal delay in the shifting of the eye and a consequent continuation of the power of receiving visual sensations from this side.

The abnormality of the dorsal fin is in accordance with this supposition. To understand the nature of this condition it must be remembered that the form of the flat-fish is derived from the usual " round " form by two principal changes. (1) By a twisting of the head the eye is brought over from the blind side to the upper side. (2) The dorsal fin is extended forwards above the eye thus shifted ; for as STEENSTRUP and TuAQUAiR2 have shewn, this anterior extension of the dorsal fin is not in the morphological middle line. It is in fact an anterior repetition of the series of dorsal fin-rays along the new contour-line of the body, and occurs irrespective of the fact that the tissues with which it is there associated are not median at all.

STEENSTRUP and TRAQUAIR shewed plainly that it is insufficient to suppose that there is a twisting of the head, for this does not explain the presence of the dorsal fin in the position in which it is found, curving along that which was once the side of the head. Traquair suggested that these relations could be attained by two processes; (1) a twisting of the head so as to bring over the eye from the future " blind " side, and (2) a forward growth of the dorsal fin along that which is then the upper contour-line of the head. These processes have now been actually seen by AGASSIZ* in several PleuronecticUe. The first observation of a specimen at the stage when the eye is on the top of the head and the dorsal tin is not yet extended, seems to be that of MALM* and there can be little doubt that the normal development proceeds in this way5. It has been pointed out by many writers that if the upper eye were to remain in an intermediate position on the top of the head, and the dorsal fin were then to grow forwards, arching over it, the condition of these abnormal forms would be reached. That this is what has actually occurred in them seems likely.

A number of difficult questions are thus raised as to the histological 5 Allusion should be made to the fact that in the genus " Plagusia " the dorsal fin acquires its forward extension at a time before the shifting of the eye occurs. When the time for this change comes the eye of the future blind side passes under the dorsal fin and above the skull, through the tissues from one side of the heml ti> the other. This was first observed by S'TEEXSTRUP, and afterwards by AGASSIZ in great detail and the fact can hardly now be questioned. This mode of development is peculiar to " Plagusia," though when Steenstrup wrote he expected that the same would be found to occur in other Pleuronectidfe.

processes by which the dorsal fin comes to stand where it does. \Ve are accustomed to think of the repetition of the fin-rays as being an expression of the fundamental segmentation of the trunk, accessory to it no doubt, but still of the same nature and histologically dependent upon it. The extension of this repetition along the morphological side of the face is thus an anomaly. Further comment on the nature of the variation will be made after the chief cases have been given.

*727. Rhombus maxirnus (Turbot). Two specimens respectively 9 in. 9 lines and 7 in. G linos in length, 7 in. 6 lines, and 5 in. 6 lines broad. Both sides of a similar coffee-brown colour. The smaller had a yellowish white spot, about 1 in. square, on the operculum of the lower side. The colour was more uniform than usual and the dark spots normally found on the fins of the Turbot were absent. Both sides irregularly beset with horny tubercles, only slightly more developed on the upper than on the lower sides. Fine scales were also found deep in the skin of both sides. All fins except the dorsal were normal in form and position. The dorsal fin was anteriorly detached from the head, being borne on a horn-like projection. The separation between the head and dorsal fin was continued backwards as a semi-circular notch to a level behind the eyes. Upon many of the fin-rays of the dorsal, anal and caudal fins there were 1 — 7 small knotty elevations of the size of poppy-seed. In the smaller specimen these elevations were smaller, and on the caudal fin absent. The left eye had its normal position, but the right eye [of "blind" side] was placed on the top of the head, but in such a position that it could scarcely have seen any thing not directly over it. [See further details given.] SCHLEEP, I sis, 1829, p. 1049, PL in.

Similar specimen Coucn, Fishes Brit. IsL, in. p. 157. Dried specimens in Brit. Mus., Newcastle Mus., &c. *728. Very good figures of such a Turbot are given by DUHAMEL DU MONCEAU (Traite general des Pesches, 1777, in. Sect. IX. p. 262, PI. ill. figs. 3 and -i). The under side was of nearly the same colour as the upper and the tubercles generally found on the upper side only were present on the lower side also, though of smaller size. A slight notch separated the dorsal fin from the head; but the upper eye is figured as in its normal place, not being on the top of the head, and it would of course be invisible from the " blind " side. [This important case is referred to by STEENSTKUP, but seems to be unknown to others, who attribute the separation of the dorsal fin to the persistence of the eye on the top of the head.]

729. A young turbot, similarly coloured on both sides, having the eyes still symmetrical, swimming on edge, is figured by McIxTOSH, Fishes of St. Andrews, 1875, PI. vi. figs. 5 and G. Prof. Mclntosh kindly informed me that these "double" individuals swim on edge much longer than usual. 730. Rhombus lacvls (Brill). Specimen presenting similar characters. The lower (rt.) side of a uniform dark colour with exception of a white patch on operculum. The right pectoral fin was whitish. The under side was rather darker than the upper and the mottling present on the upper side was entirely absent from the uuder side, which was without marking or spot. This is very probably a post-

mortem change. Eight pelvic fin dark, but the left was whitish, speckled with black. Nostrils normal. The eye of the right (blind) side was placed almost entirely on the left side, but not completely so, for it could be seen to some extent in profile from the right side. The notch separating the dorsal fin from the head was rounded, and extended to about the level of the posterior margin of the left eye. There were about 6 chief fin-rays borne by the prominence above the eye. The fish seemed to be in all respects healthy and well grown. Paris Mus., numbered ^90^:310. [This specimen was kindly shewn to me by Prof. Vaillant.]

Similar specimen, also having white patch on operculum DUHAMEL DU MONCEAU, I. c. See also Fig. 152, from YARRELL, Brit. Fishes, 3rd ed., i. p. 643. The specimen described by DONOVAN (Brit. Fish., 1806, iv. PI. xc.) under the name "Pleuronectes cy clops" was in Steenstrup's opinion a young Brill having this variation. In this specimen the right eye is seated on the top of the head and is seen in profile from the right side. The right side was coloured like the left, but was not so dark. The dorsal fin began behind the right eye. This specimen was found in a rock-pool "iuveloped in a froth" said to have resembled cuckoo-spit.

731. Zeugopterus punctatus (Miiller's Topknot). This fish is very liable to malformations of the anterior end of the dorsal fin, causing it to form an arch over the eyes. YARRELL (quoting COUCH), Brit. Fish., 3rd ed., i. p. 648. 732. "Platessa oblonga" De Kay (American Turbot) ; specimen having both sides darkly coloured ; upper eye placed on the top of the head ; dorsal fin separated by a notch. STORER, Mem. Arner. Ac. Set., vin. p. 396, PI. xxxi. fig. 2 b.

733. Pleuronectes platessa (Plaice) : specimen completely and similarly pigmented on both sides far from rare. In a specimen thus coloured the ' tubercula capitis ' were as strongly marked on the one side as on the other. In several examples the anterior end of the dorsal fin was separated from the head, GOTTSCHE, Arch. f. Naturg., 1835, n. 1, p. 139. 734. Pleuronectes flesus (Flounder) : several specimens found at Birkenhead, having a deep notch of this kind above the eyes. These fishes were 'very dark brown (almost black) on both sides.' In the length of the fins these examples differed somewhat from the Flounder, HIGOINS, Zoologist, 1855, p. 4596, fig. Specimen of this kind figured by TRAQUAIR, Trans. Linn. Soc., 1865, xxv. p. 288, PI. xxxi. figs. 8 and 9. See also NILSSON, Skandin. Fauna: Fiskarna, Lund, 1855, p. 621 ; COUCH, Brit. Fishes, 1864, in. p. 198.

/35. Solea vulgaris. Many authors mention Soles coloured on both sides, but I know no good description of one. YARRELL (1. c., p. 669) says "we have not seen the Solea Trevelyani of Ireland (Sander's News-letter, 16th April, 1850). It is dark-bellied and is described as bearing a projection on the head like the monstrosity figured on p. 643." DUHAMEL DU MONCEAU (I. c., PL i. figs. 3 and 4) represents a sole darkly coloured on both sides. The dorsal fin is shewn in its normal state, not separated from the head. No special description is given, and as the author does not state that he had himself seen such a sole the figure was perhaps not drawn from an actual specimen. A sole with the under side piebald is described in Zool. x. p. 3660.

In connexion with this evidence STEENSTRUP refers to a small flatfish, Hippoglossus pinguis, found in a few localities in Scandinavian waters, having a form almost intermediate between a "flat" and a "round" fish. The eye of the "blind" side is exactly on the top of the head and can be seen in profile from the blind side. The blind side is nearly as muscular as the upper side, and its skin is yellowish-brown in colour and is only slightly paler than that of the upper side. The dorsal fin begins behind the eye, not arching over it. Steenstrup looked on this creature as representing in a normal form the "double" condition presented as a variation in the cases we have been speaking of. See description and figures in SMIT'S edition of FRIES, EKSTROM and SUKDEVALL'S Hist, of Scand. Fishes, 1893, pp. 416 and 417. SMIT makes a new genus, Platysomatichtliys, for this animal.

In tincases preceding many will no doubt see manifest examples of !:• version. Tli'i'- U . in which this view must be true, for it can s.-.-uv.-ly be questioned tli;it if we had before us the phylogenetic series through \\hi.-h ill.- Flat ti>li.-s, the Narwhal, ic. are descended, it would be seen ili.it each did at some time have a bilaterally symmetrical an. -.-tor. Hut. f, ,r all that, in an unqualified description of the change at i reversion tinsiirniticance of the facts is missed. By the statement that a LMV.-II variation is a r>-\ crsion it is meant that in the varying individual a form, once the normal, n -appears. The statement moreover is e.-p.-ciallv intended to imply that the definiteness and magnitude of the .--t.-p t'min normal to variety is due to the circumstance that thivarietv was mice a normal. It is meant, in fact, that the greattie,, of the modern change can be explained away by the suggestion that in the pa>t, the form now presented as a variation, was once built up I'V a gradual evolution, and that though in its modern appearance there is Discontinuity, yet it was once evolved gradually.

Now the attempt to apply this reasoning, especially to the case of the "double" Flat-fishe-, leads to difficulty. We may admit that in so far as the varietio are bilaterally symmetrical they represent a normal. Their bilateral symmetry, as a quality apart, may be an ancestral character, if any one is pleased so to call it. But that in the contemporary resumption of a I >ilateral symmetry we have in any further sense a reappearance of an ancestral form is very unlikely.

I irst it might be fairly argued that it is improbable that there was ever a typical Ilat-tish ha\in.L,r <>n finfh sides the peculiar pigmentation of the PI-CM -nt upper sides of the Pleuronectida? of our day. Such a creature would !>«• highly anomalous. But even if in strictness we forego the assumption that since the evolution of Flat-fishes there has never been an ancestor fully pigmented on both sides, there still remains the dilliculty that each species may in the "double" state have upon its lower side the >•/».•///.- colour proper to its own upper side. A notable instance of this has been mentioned in the Plaice (p. 467); and here not only was the pigmentation of the lower side, as far as it went, like that of the upper, but the spots were even almost bilaterally symmetrical. It is true that the lower side does not in every case copy the upper in colour, but it HHII/ do so; and, in proportion as it does so in different species, so far at least are the changes not simply reversions; fur the several patterns of Turbot, Plaice «fcc. are mutually exclusive and it can hardly be supposed that each species had separately a "double"' ancestor having the present specific pattern on both sides.

The outcome of this reasoning is to shew that the hypothesis of Reversion in the strict sense is an insufficient account of the actual variation in these Flat-fishes, and in the production of these varying forms there is thus a Discontinuity over and above that which can be ascribed to Reversion. The facts stated in connexion with the Plaice (p. 4G7), especially the symmetry of the spots, probably indicate the real nature of this Discontinuity, and raise a presumption that in the new resemblance of the lower side to the upper we have a phenomenon of Symmetry resembling that Homoeosis shewn to occur between parts in

Linear Series. In the Flat-fish the right side and the left have been differentiated on different lines, as the several appendages of an Ar- thropod have been, but on occasion the one may suddenly take up all or some of the characters, whether colour, tubercles or otherwise, in the state to which they have been separately evolved in the other. "What may be the cause leading to this discontinuous change we do not know. That it is often associated with a dchiy in the change <>t' position of the eye of the "blind" side seems clear from the frequent detachment of the dorsal fin in these cases. But it should be borne in mind that even in such examples the eye may still eventually get to its normal place, though probably it was delayed in the process and so led to detachment of the fin. Taken with the fact that the young " double " turbots swim on edge longer than the normals it must be concluded that the bilateral symmetry of colour is associated with reluctance or delay in the assumption of the asymmetrical state, but more than this cannot be affirmed.

I do not urge that the same reasoning should be applied in other cases, but the possibility must be remembered. In the Narwhal, for instance, it is perhaps unlikely that there was ever an ancestor which had two tusks developed to the extent now reached by the left tusk of the male ; but if there ever were any such form, it is hard indeed to suppose that it could have been connected with the present species by a series of successive normals in which the right tusk gradually diminished while the left was of its present size. On the whole it seems more likely that when the right tusk now develops to be as long as the left, it is taking up at one step the state to which the left has been separately evolved.

However this may be, the fact that such Homoeosis is possible should be kept in view in considering the meaning of such cases as that of a Tornaria with two water-pores. For while on the one hand we may suppose that Balanoglossus kupfferi with its normal pair of waterpores is the primitive state and that the varying Tornaria is a reversion, on the other hand B. kupfferi may be a form that has arisen by a Homceotic variation from the one-pored form, and of this variation Bakmoglossus No. 725 may be a contemporary illustration1.

1 The following interesting example of a similar Variation has appeared since these pages were set up. Eledone cirrhosa : specimen having not only the third left arm developed as a hectocotylus, as usual, but the third right arm also. The right had 57, the left 66 suckers, but otherwise they were alike. APPELLOF. A.. Bergens Museums Aarbog, 1893, p. 14. i >F nil classes of Meristic variations those consisting in repetition ,,r division of appendages are by far the most complex and the most difficult to bring into system. There is besides no animal which normally presents the condition seen in the variationabout to be described, though there may be a true analogy between them and phenomena found in colonial forms. It has nevertheless seemed well to introduce some part of this evidence In iv t<»r two reasons. First the subject is a necessary continuation of in.' evidence as to digits, which would otherwise be left incomplete ; secondly it will be shewn that though many of the cases are irregular and follow no system that can be detected, there remain .i large number of cases (being, indeed, the great majority of those thai have been well studied) whose form-relations can be put in terms of a simple system of Symmetry. Thus not only are we introduced to a very remarkable property of living bodies, but also the way of future students of Variation may be cleared of a mass of tangled facts that have long been an obstacle ; for on apprehen- -ion lit' i he system referred to it will be seen that cases of repetition in Secondary Symmetry are distinct from those of true Variation within the Primary Symmetry and may thus be set apart.

In the first instance I shall give the evidence as to Secondary S\ nmiet i ies in Insects and Crustacea, prefacing it with a preliminary account of the system of Symmetry obeyed by those cases which I shall call re<inl<n\ and explaining the scheme of nomenclature adopted. Besides the regular cases of extra parts in Secondary Symmetry there are many irregular examples which cannot be shewn to conform to the system set forth. Of all but a few of these, details are not accessible, and of the rest many are

mutilated or so amorphous that the morphological relations of the surfaces cannot be determined. Over and above these there remain a very few cases of Repetition of parts of appendages where the arrangement is certainly not in Secondary Symmetry, but is of a wholly different nature, exemplifying in Arthropods that duplicity of limbs already seen in the human double-hands (p. 331) and in the double-feet of Artiodactyles (p. 378). Genuine cases of this kind are excessively rare ; but owing to hasty examination great numbers of cases have been described as instances of duplicity, though in reality the supernumerary parts in them can be shewn to be of paired structure. To emphasize the distinctness of these cases they will be made the subject of a separate consideration. Logically they should of course be treated before the Secondary Symmetries ; but their essential features may be understood so much more readily if the latter are taken first that I have decided to change the natural order.

In continuation of the evidence as to Secondary Symmetry in Arthropods will be given a brief notice of similar phenomena in vertebrates. This evidence is comparatively well known and accessible and I shall attempt no detailed account of it, referring to the facts chiefly with the object of shewing how the principles found in Arthropods bear on the vertebrate cases. It will then be necessary to consider how repetitions in Secondary Symmetry are related to other phenomena of Repetition. Lastly something must be said with regard to the bearing of these facts on the general problems of Natural History.

Supernumerary appendages in Insects are not very uncommon, perhaps 120 cases of this kind being recorded1. Nearly all known examples are in beetles, but this may be due to the greater attention paid to the appendages in that order. They do not seem to appear more often in one family than in another, but perhaps the rarity of instances in Curculionidse is worth noting. They are found in both sexes, in all parts of the world, and in species of most diverse habits.

Supernumerary parts may be antennae, palpi or legs. (Extra wings are probably in some respects distinct. They have already been considered. See p. 281.) Extra appendages may be either outgrowths from the body in the neighbourhood ..t the part repeated, or, as in the great majority of cases, they occur as outgrowths from an appendage, extra legs growing from normal legs, extra antennie from antennae, &c. In every case there are two essentials to be determined: first the constitution i*Not including some 110 cases of alleged duplicity of appendages given later.

of tli.- extra parts, and secondly the symmetry or relation of form Mili-i-iin;^ on tli.- one hand between the extra parts themselves, and on tne oth.-r between the extra parts and the normal parts. In few cases of extra appendages arising from the body i !<>•!( ha\e these essentials been adequately ascertained. For brevity I >hall il. -scribe the phenomena as seen in extra leg-. Tli.- same description will apply generally to the antennae. Recorded cases «l' .-xtra palpi are very few, but probably are not materially different.

"I'lnpartcomposing extra legs do not as a rule greatly differ from those of tinnormal legs which bear them. Though in many instances extra legs are partially deformed, they are more often fairly good copies of the true leg. Not rarely the extra parts are more slender or a little shorter than the normal appendage, but in form and texture they are real appendages, presenting as a rule the hairs, spurs, &c. characteristic of the species to which they belong.