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

downward prolongation of the ovisac. FIG. 144. Double poison-spine No further description.] PROVOST, B., of a Scorpion (Palamnaus borne- Mem, sur les Chirocephales, p. 232 : in e™1*)- L Fro1? ^orsal side- T , rj- . , \T j n II. From ventral side, n, the J urine s Hist, des Monocles, Geneva, spine which bore the openings of 714. Buccinnm undatum. A number of specimens were formerly obtained from Sandgate in Kent l, having the operculum double. Sometimes the two opercula were separate, sometimes united. Many specimens of this variation are in the collection of Dr A. M. Norman, who kindly shewed them to me. The shells and opercula alone remain and consequently it is not now possible to determine the position of the line of division relatively to the morphological planes of the animal; but, from the fact that in several instances the two opercula were related to each other as images, it seems likely that the division was in the longitudinal median plane, though this must be uncertain. Moreover in one of Dr Norman's specimens, from the fragment of dried flesh adhering, it appeared that the apex of the foot might have been bifid. Four cases are shewn in Fig. 145. In two of them (I and II) there is a fairly close relation of images, while in III this relation is less clear and in IV it is practically destroyed, though it is of course quite possible that this may be the result of unequal growth. Several of these opercula are much contorted and without any very definite shape.

FIG. 145. Cases of duplicity in operculum of Buccinnm iiiulutitni, from specimens in the collection of Dr A. M. Norman. I and II nat. size. Ill and IV enlarged. Ill and IV were kindly drawn for me by Mr J. J. Lister. It was intruded to liavc introduced here some account of the s and very rare cases in which, for a greater or less region of This phenomenon is the converse of that described above. Kxamples of median union are found in many organs of different kinds. In vertebrates such union is especially well known in the case of the eyes, the ears, and the posterior limbs, producing the cyclopic, synotic and symmelian conditions respectively.

Each of these is of some interest to the student of Variation by reason of the symmetry and perfection with which the union takes place. In the cyc-lopi.m the degree to which the two eyes are compounded presents all shades intermediate between the perfect duplicity of the normal and the state in which the eye-balls are united in the middle line of the forehead and have one circular cornea2. These variations are closely comparable with those of the eye-spots on feathers referred to on p. 449 ; for there also all stages are seen between a pair of eye-spots placed one on either side of a middle line and complete union to form one eye-spot bisected by the middle line. There is of course no normal vertebrate having the eyes thus united in the middle line, but as MECKEL has remarked, the case of the cyclopian is not essentially different from that of the Cladocera in which the compound eyes, paired in other Crustacea, are united to form a single median eye. The cases No. 718 and 719 of median union of the compound eyes of Bees may also be considered in this connexion.

A very similar series of variations occurs in regard to the ears of vertebrates, which in the synotic or cephalotic condition are compounded in the middle line to a varying degree3. Such union of the ears is especially common in the Sheep, cyclopia being most frequent in the 1'iir. I > \i:r:sTi-:4 states that the first beginning of the cyclopian condition appears in the Chick as a precocious union of the medullary folds in the region of the fore-brain, occurring before the optic vesicles are fully formed from it. The degree to whu-h the union of the eyes is complete then depends on the earliness with which the folds begin to meet relatively to the time of budding off of the optic vesicles. DARESTES also declares that the cephalotic state is similarly first indicated by a premature union of the folds in the region of the medulla, taking place

• For an extensive collection of cases illustrating the various degrees of cyclopia sec especially AIILFKLH, Mi.mh. <l. .!// nxcli., Abschn. n. Iss-j. before this part of the brain has widened out. In this way the auditory involutions are approximated. This account however cannot apply to all cases of union of ears; for the compounded ears are sometimes on the ventral side of the neck, as in Guerdan's case1. The body of the symmelian ends posteriorly in an elongated lobe made up of parts of the posterior limbs compounded together by homologous parts. The two femora are usually united to form a single bone, the tibise are separate and the two limbs are again compounded in the tarsal region. The axial parts posterior to the hind limbs are always greatly aborted2.

Union of the kidneys in the middle line (Fig. 146), forming the " horse-shoe kidney " of human anatomists, is a similar phenomenon. As to the mode of development of this variation I know no evidence. Usually the kidneys together form a single horse-shoe shaped mass of glandular tissue, the union being posterior3 ; very FIG. 146. Kidneys united in the condition known as "horse-shoe" kidney (Man). In this specimen there were three renal arteries on each side.

2 See especially, MECKEL. Arch. Anat. Phys., 1826, p. 273 ; GEOFFROY ST HILAIRE, Hist, des Anom., ed. 1837, n. p. 23; GEBHARD, Arch. Anat. Phys., 1888, Anat. Abth., p. 164 (good fig.). To the determination of the morphology of the hind limb the structure of the symmelian monster is of unique importance, but I do not know that it has had the notice it deserves from comparative zoologists. From the manner of union of the parts of the two limbs may be obtained a positive proof of the morphological relations of the surfaces of the two limbs to each other. In a symmelian the feet are united by their fibular borders, the minimi being adjacent, the halluces exterior, and the combined plantar surfaces ventral. The great trochanters are dorsal, being often united into one in the dorsal middle line, and the patella are also dorsal, being also not rarely partly compounded. From these facts, even were other indications wanting, we have a proof that if the hind limbs were laid out in their original morphological relations to each other (as the tail-fins of a Crayfish may be supposed to be) the halluces would be external and anterior, the minimi internal and posterior, the flexor surfaces of the thigh and crus and the plantar surface of the (human) foot would be I'entral and the extensor surfaces of the thigh and crus and the dorsum of the (human) foot would be dorsal. This is of course affirmed without prejudice to any question of phylogeny ; but that these must be the ontogenetic relations of the parts is clearly proved by the symmelian.

ran-ly the posterior ends of the kidneys are joined by a bridge of ligamentous t issue1. A ivmarkablr case, iii which the union of the two kidneys was very rmiij,],-!.- ami only indications of duplicity remained, is given by PlCHANCOURT, Gaz. hebd., 1879, p. 514. To these familiar instances are added a few less generally known. *71~>. Capreolus caprea (Roebuck): specimen having the two horns compounded in tinmiddle line, forming a common beam for almost

tinlower half of tin' hum (Fig. 147). This specimen was exhibited uniting a large series of abnormal horns in the German Exhibition hfld in London 1891. Casts of it are in the Brit. Mus. and Camb. Univ. Mus.'. 7 1 <;. Limax agrestis : specimen having the upper tentacles united into one in the middle line. The eyes were paired as usual. KOUBES and HAXLKY, Hist. Brit, Moll, iv. p. 288 and I. PI. JJJ, fig. 4. 717. Helix hispida : specimen in which the tentacles were united together. They were adherent throughout, excepting for a slight cleft at the end, about one line in length. A shallow longitudinal suture was visible between the two. The animal and shell were otherwise normally formed. ROBERTS, Gv Science Gossip, I88(j, xxii. p. 259.

*71S. Apis mellifica (Honey-bee): a worker having the two compound eyes continued up so as to unite on the top of the head (Fig. 148). The union between the eyes of the two sides was complete. There was no trace of any groove or division between them and the resulting structure was perfectly symmetrical. In a normal the three simple eyes are arranged in a triangle between FIG. 148. A worker Bee (Apis mellifica) No. 708, having the two compound eyes united across the middle line, seen from in front, and from the side. CE, the united compound eyes. Oc, a single structure representing the three simple eyes of the normal. (After STANNIUS.)

the upper edges of the compound eyes, but in this specimen they were united into a single structure which was symmetrically placed in the middle line in front of the united compound eyes (Fig. 14S, Oc). The body thus formed by the union of the simple eyes was a round projection beset with long yellowish hairs. In a normal male the compound eyes are much larger and are in contact with each other at the top of the head, but the division between them is sharply defined. In a normal worker, however, the compound eyes are widely separated.

The facetting and the hairs on these eyes were normal and the animal was in all other respects properly formed. STANNIUS, Mutter's Arch, Anat. Phys., 1835, p. 297, PI. '719. Apis mellifica having the compound eyes completely and symmetrically fused. This individual was either a young and abnormally developed queen, or else a worker. Its struct mv was in several respects abnormal. The third pair of legs are like those of the workers, as is shewn by the structure of the first joint of the tarsus, the brush of hairs on the outside of the leg is not so

much develop^ I as in tin- \v<>ik.-ix and this feature suggested that perhaps the .-pecinien mav !>•• a \oimg and abnormal queen. The abdomen i- -mall and seems to have been arrested in its development, but its -hape i< that of the abdomen of the workers. The Las1 segment «>f the abdomen is elongated, triangular, and slightly '_M'oo\ed in the middle of the posterior border, so as to permit the pa-sage of the stiir_r. The wings are more like those of a queen or worker than iho-e of a male; for in the latter they generally greatly exceed the abdomen in length. The thorax is Hnall, narrow, and cont raeted more than in the normal form, being also !' sfi convex. The -pare between the wings is less than in a fully de\ doped bee. The antenna? are mutilated, but seem to have been normal: but th--ir last joints are slightly reddish brown as they are in females, whether workers or queens, and not black as tiny are in drones. The two compound eyes were completely fu>ed together in the middle line, across the place in which the -imple eyes ought to be found. The simple eyes are not present at all. LUCAS, H., Ann. Soc. Entom. France, S. 4, vm. 1868, p. 7:i7, PI.

The following facts, taken from WATASE1, are introduced in further illustration of the mode of occurrence of bilaterally symmetrical M eristic Variation. *720. Loligo pealei. In the blastoderm the nucleus is placed eccentrically, being rather nearer to the posterior pole, as shewn in Fig. FIG. 149. Diagrams illustrating variations in segmentation of a Squid. (Loligo pealei). I. Normal uusegmented ovum. ?;, the nucleus eccentrically placed. A, anterior. P, posterior. L, left. E, right. II, III, and IV. The shaded portions shew areas in which in some specimens nuclear division was precocious. V. In the two shaded areas triasters occurred in one specimen. VI. The hlastomeres of the shaded areas in one specimen were not divided from each other. 1, 2, 3, successive planes of division, ar, anterior right quadrant, pr, posterior right quadrant. ar', pr', etc. areas separated off by the third segmentation-furrow. (After WATASE.)

I, I. The first furrow, 1, 1, divides the blastoderm into two halves and corresponds with the future longitudinal middle line. The second furrow, '2, 2, is at right angles to this, dividing the blastoderm into anterior and posterior halves, and the third furrow, 3, 3, pa>se> ae .-hewn in Fig. 14!), V. In the subsequent segmentations various irregularities were seen in single eggs, some of the variations being bilaterally symmetrical while others were confined to a particular half or to a particular quadrant. For example, in some ova the nuclei of the <-ells formed from the left half of the blastoderm, excepting those next the median axis posteriorly (Figs. 149, II and 150, I), began to divide before those of the right side and reached an advanced stage of karyokinesis while the nuclei of the right half were still resting. The nuclei of each half kept time very nearly (for details see original figures). This curious variation was seen in three ( perhaps four) ova all taken from one mother.

In another the nuclei of the two anterior quadrants al, ur, in their divisions kept ahead of those of the posterior quadrants. Fig. 149, IV. represents an ovum in which the nuclei of the right posterior quadrant on the contrary divided before those of the 3 other quadrants. Another variation is shewn in Figs. 149, VI and 150, II. There the four blastomeres shaded had either been never fully divided from each other or had subsequently fused together symmetrically on each side.

FIG. 150. Variations in sefjmrnkitinn of ovum of Loligo pealci. I. Case in which the nuclei of cells of the left half of the blastoderm began to divide precociously. II. Case in which the blastomeres of the areas ar' and al' were uot distinct from each other. (After WATASE.) Fig. 149, V, illustrates another remarkable Meristic variation which symmetrically affected the portions shaded. In both of these shaded segments the nuclei divided into three by triple karyokinesis, forming " triasters."

In proportion as an animal is bilaterally symmetrical the right side is an image of the left. Nevertheless in many substantially symmetrical forms there is asymmetry in the condition of some one or more organs present on both sides. (This asymmetry, in the cases to be considered, is of course distinct from that due to asymmetrical disposition of unpaired viscera, such as the heart and liver of vertebrates, &c.) In several of these cases there is evidence that both sides may on occasion assume the form normally proper to one only.

Some one will no doubt be prepared with the suggestion that these variations are reversions : with this suggestion I shall deal after the facts have been recited. 7-1. Pelobates fuscus : a tadpole, 7 cm. long, having two spiracles symmetrically placed (Fig. 151), one on the right side and the other on the left1. [No details given.] HERON-ROVER, Bull. soc. zool. Fr,, ix. Fiu. 151. A tadpole of Pelobates fuscus, having, as a variation, two spiracular openings, No. 721. (After HERON-KOYER.)

722. Monodon monoceros (Narwhal). In normal males the left tusk alone is developed while the right remains abortive in its alveolus. In the female both tusks are in this rudimentary condition. No reliable record (1871) of a specimen having the right tusk only developed, but in eleven cases from various sources the two tusks were both developed, and in several of these the two were of about equal length. The normal asymmetry of the skull is not affected by the presence or absence of the teeth. CLARK, J. W., P. Z. S., 1871, p. 42, Jigs, (full literature); see also TURXER, W., Jour. Anat. Phys., 1871, p. 133 and 1874, p. 516.

It might be anticipated that development of the right ovary and oviduct in birds would be a frequent form of Variation, but as a matter of fact very few such cases are recorded. In consideration of 1 In the same place is recorded a case of a tadpole of this species having the spiracle on the right side instead of the left, perhaps a case of situs inversus. the large numbers of birds, wild and domesticated, annually dissected in laboratories it may perhaps be concluded that these variations are exceedingly rare1.

723. Hen having a small right ovary in addition to the left ovary. The left oviduct was normal, but the left ovary was partially transformed into sacculated tissue. [Full histological details of the structure of both ovaries given.] The hen had partly assumed the plumage of the cock, having four sickle-feathers and other characters proper to the male. BRANDT, Z.f. w. Z., XLVIII. 1889, p. 134, Pis. 724. Hen li;i \ ing a normal left oviduct and in addition a partially developed right oviduct which formed a large thin-walled cyst distended with gas. C. S. M., Ter. Cat., 1872, 455.

Proboscis-pore of Balanoglossus and water-pores of larvcv of Asterias. *725. Balanoglossus kowalevskii. The anterior or proboscis-bodycavity is continued backwards into the proboscis-stalk as two hollow horns. In this and most other species the left of these alone acquires an opening to the exterior at the proboscis-pore. In B. kupfferi alone there are two such pores, one opening into each of the two horns2. A specimen of B. kowalevskii in which both horns thus opened to the exterior was seen by MORGAN, T. H., Jour, of Morph., 1891, v. p. 442. 726 Asterias vulgaris. The Bipinnaria larva as commonly seen ' resembles the usual Tornaria in having a left water-pore only. In several larvse 3i to 4 days old the presence of two such water-pores, a right and a left," symmetrically placed, has been observed by FIELD and BROOKS. The right pore subsequently closes. This condition is believed by Field to represent not a variation but a normal phase of development [though further confirmation is needed]. FIELD, G. W., Q. J. M.S., 1893, xxxiv. p. 110, PL xiv. figs. 22 and 23.

A curious series of variations bearing on the relations of the right side to the left occur in Pleuronectidse. The evidence on this subject was collected by SxEENSTRUP3 in 1863. Flat-fishes are normally coloured on the upper side and are without chromatophores in the skin of the lower side4. Variations in colour occur in two ways : the upper side may be white like the lower, or on the contrary the lower side may be coloured like the upper. The former change cannot well be distinguished from other cases of albinism5 and does not call for special notice here.

1 In view of the cases of the Crayfish and the Cockroach mentioned in the Preface, much stress cannot be laid on this consideration. 4 In some species the coloured side is normally the right, in others the left, re versed specimens being common in some species (P. jli'xus), rare in others. The reversed condition concerns only the head, skin, muscles, &c., and there is no transposition of the internal viscera. '•> Evidence collected by STEENSTUCP. GOTTSCHE (Arch. f. Naturg., 1835, n. p. 139) states that P. plntt'*.«t is uot rarely wholly white on both sides. I have never

The converse variation, by which the lower side assumes the colour of the upper side is important in several aspects. Interest has of late been drawn to this subject especially through an experiment recently made by CUNNINGHAM l, who found that of a number of young flat-fishes reared in a vessel illuminated by mirrors from below, some became partially marked with pigmented patches on the lower side. The suggestion was made that this pigmentation was induced by the direct action of light. It is of course impossible here to enter into the theoretical questions raised in connexion with this subject and this account will be confined to description of the colour-variation as seen in nature and of the singular variation in structure commonly associated with it. Mr Cunningham has obligingly advised me in connexion with this subject.

Pigmentation of the lower side has been seen in Rhombus maximus, R. Icevis, Pleuronectes flesus, P. platessa, P. oblongus, Solea vulgaris [?] and probably other forms. Attention is drawn to one feature in these changes which from our standpoint has an important bearing. When the underside of a flat-fish is pigmented, it is often not merely pigmented in an indefinite way but it is coloured and marked just as the upper side is. There are, I know, many specimens upon whose undersides a brownish yellow tint is either generally diffused or restricted to patches, but when there is pigment of a deeper shade, as in all the well marked cases of the variation, the colour and markings are closely like those of the upper side. For example, a Plaice (P. platessa) sent to me by Mr Dunn of Mevagissey is fully coloured over the posterior half of the lower side ; but there is not merely a general pigmentation, for the coloured part of the lower side is marked with orange markings exactly like those of the upper side.

More than this : it was found by passing pins vertically through the body that there was in the case of most of the spots a close correspondence in position between those of the upper and those of the lower side. There were 13 spots on the coloured part of the lower side, which extended slightly beyond the line of greatest width. Of these, 13 spots on body and fins coincided exactly with those of the upper side ; 2 coincided nearly ; 2 were not represented on the upper side ; and 2 spots of the upper side were not represented on the lower. From these facts it is clear that in " double " flat-fishes we have an instance of symmetrical variation of one half of the body into more or less complete likeness of the other half, resembling other cases of Homoeosis in Bilateral Series already noticed.

This is made the more evident by the fact that in the two best described specimens of "double" Turbot (No. 727) not merely did the lower side resemble the upper side in point of colour, but upon it were also present the bony tubercles normally proper to the dark side, being only slightly less well developed on the lower side than on the upper. succeeded in seeing an entirely white specimen, though individuals partially white on the upper side are not rare. See also Zool, pp. 4596, 4914. Zeugopterus punctatus white on both sides, DAY, Brit. Fishes, n. p. 19. 1 CUNNINGHAM, J. T., Proc. Roy. Soc., 1893.

(Such a development of tuln-rcles1 on the lower side may however occur without any con-flati-d rliange of colour.) It is also stated that in tin- "douhle" turl.nts tinmuscles of the lower side are thicker than tln-y normally a iv, thus approximating to the upper side, a feature that may be taken as an indication that the manner of swimming is dillerent from that of normals. A tlat-tish having pigmentation on the lower side does not necessarily j>iv>ent any other abnormality2. The Plaice, for instance, just mentioned, was, colour apart, quite normal. But some specimen^ of Hat-fishes darkly coloured below present in addition a very singular structural variation. This consists essentially in the presence of a notch of greater or less depth occurring belcro tin anterior end of the dorsal tin above theeye (Fig. 152). By I his cleft the anterior end of the dorsal fin is separated from the l>aek of the head and is borne on a process or horn projectin -• anteriorly so as to continue the contour of the body above the