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

1 Animals and Plants under Domestication, ed. 1885, n. chap. xxv. 2 As Darwin mentions, simultaneity in the variations of the hair may be manifested in size and texture as well as in colour. A bay horse was lately exhibited at the Westminster Aquarium standing 16| hands, having the hair of both mane and tail of prodigious length. The longest hairs of the mane measured 14 ft. and those of the tail 13 ft. It did not appear that the hair of the fetlocks or body was unusual in character, but these were kept closely clipped and nothing could be affirmed on this point.

3 By the courtesy of Professor L. VAILLANT I was enabled to examine a number of specimens of the singular breeds of Gold-fish from China in the Paris Museum of Natural History. Some of these are characterized by the great length both of the appendicular fins and of the caudal fin also. Measurement shewed that there was a substantial correspondence between the lengths of these parts, those with long appendicular fins having also very long tails. The correlation between these parts is not however universal in Gold-fishes, and in many of the ordinary "Telescope" Gold-fish the tail may be longer than that of a common Gold-fish of the same size, though the length of the appendicular fins be not exceptional (v. infra).

of Radial Series, the petals of a flower may all together take on the laciniated condition1. Further study will indeed probably lead to the recognition of a principle which may be thus expressed: th&t ports which in aim one body are alike, which have, that is to say, undergone similar V;iri;itinn in the past, may undergo similar variations simultaneously, a principle which, if true at all, is true without regard to the morphological position of the parts in question.

1 For cases see MASTERS, Vegetable Teratology, IHij'j, p. 67. ALL the cases considered in the foregoing chapters have illustrated Variation of parts whose repetition is disposed in Linear Series along the chief axis of the body, being thus arranged directly and immediately with reference to the Major Symmetry of the body. We have now to consider cases of the Meristic Variation of parts which are also repeated in Linear Series but normally possess in some degree the property of symmetry partially completed within the limits of their own series, thus forming a Minor Symmetry.

Of Linear repetitions thus occurring there is a great diversity, and evidence will here be produced regarding two of the chief examples, namely, the digits of vertebrates and the segmentation of antennaB and tarsi of Insects. In each of these groups of organs the parts are frequently formed in such a way as to make an approach to symmetry, about one or more axes within the limits of the appendage to which they belong. This fact will be found to lead to consequences apparent in the manner in which numerical Variation takes place in limbs of the various types.

In these Minor Symmetries Linear Repetition may occur in two forms : there may be repetitions of digits or other parts in lines forming an angle with the axis of an appendage ; and there may be repetitions in the form of joints &c. along the axis of the appendage itself. The cases of Variation in number of joints in the appendages of Insects are chiefly interesting as examples of manifest Discontinuity in Variation, and from the conclusions which they suggest as to the supposed individuality of segments. This latter question arises also in considering the relation of the two phalanges of the pollex and hallux to the three phalanges of the other digits, but the evidence which can be gained from a study

of Variation with reference to this question is so intimately connected with the subject of the variation of digits in general that it cannot be con-idi -r, -d apart. Other cases referring to repetitions in the line of the aaria of appendages will be taken in a subsequent chapter. In studying numerical Variation in the digits of certain animals, e.-p. •.-i.-illv 'the Horse and the Pig, we shall meet with forms of Variation which are peculiar to structures having a bilateral symmetry. In examining the evidence as to Meristic Variation of Bilateral Series further reference to these cases will have to be made, but it appears simplest to describe the facts in the first instance in connexion with the subject of digits.

l-'rom the evidence as to Meristic Variation in digits I propose to make a selection, taking certain groups of cases having a direct and obvious bearing on the general problems of Variation. It will be understood and should be explicitly stated that unless the contrary is declared the principles of form which can be perceived as operating in special cases are not of universal appli- . ,ii ion in the Variation of digits, but are enuntiated as applying onlv to the special cases in which they are perceived. In the human subject, for example, cases of polydactylism will be quoted which when arranged together form a progressive series illustrating the establishment of a novel and curious Symmetry; but though these cases are valuable as illustrations of the way in which the forces of Division and growth can dispose themselves to produce a symmetrical result, yet it must always be borne in mind that very many variations of the digits have been seen in Man, whether consisting in increase in number of digits or in decrease, of which the result is almost shapeless. The case of polydactyle Cats is thus especially interesting from the fact that in this animal the polydactyle condition, though differing in degree of expression in various specimens, yet, in the greater number of eases, occurs in ways which may be interpreted as modifications of one plan, or rather of one plan for the hind foot and of another for the fore foot.

I arrange the evidence primarily according to the animal concerned, Cat, Man and Apes, Equida>, Artiodactyles, &c. To these are added a few facts as to digital variations in Birds, but from the scantiness of the evidence and the difficulty of determining the morphology of the parts I have not found it possible to give a profitable account or these phenomena in other vertebrates below Mammalia. In most of the groups increase in number of digits may be seen to occur in several distinct ways ; and, just as in the case of teeth, mamma'. &c., it is possible to recognize cases of division of single members of series, and cases of addition to the series

either at one of its ends (often associated with remodelling of other members of the series) or in the middle of the series. Reduction in number of digits, or ectrodactylism as it is often called, is usually so irregular in the manner of its occurrence that little could be done as yet beyond a recitation of large numbers of cases amongst which no system can be perceived. For the present therefore the interest of these observations for the student of Variation is comparatively small and they are for the most part omitted.

To the irregularity of ectrodactylism in general certain cases of syndactylism are a marked exception and of these an account will be given. After stating the morphological evidence as to numerical Variation in digits in the several groups, reference will be made to some collateral points of interest concerning such variations. There is a good deal of evidence respecting the recurrence of digital variations in those lines of descent wherein they have appeared. Facts of this kind have been frequently seen in the case of Man, and other examples are known in the Cat, the Pig, the Ox, Deer, Sheep, &c. References to these cases will be given.

It will be seen that the facts contained in this section of evidence are of consequence rather as indicating the limits set on Variation, and from their bearing on the question of the nature of Symmetry and of Homology, than from any more direct application to the problem of Species, but even this cannot be said with much confidence. There are in certain groups limbs such as the pes of Macro- or that of Peramelidas whose appearance forcibly recalls what is seen in some teratological cases and the possibility that they may have had such a sudden origin may well be kept in

The apprehension of the chief features in the evidence as to digital variation in the Cat will be made more easy if a general account of the subject be given as a preliminary. In order to understand the peculiar phenomena seen in the limbs of polydactyle cats certain points of normal structure are to be remembered. Of these the most important relate to the claws and their disposition with regard to the second phalanx ; for it is by this character that the relation of digits to the symmetry of the limb may be determined.

1 In the case named this is all the more likely from the circumstance that according to THOMAS, Cat. Marsup. Brit. Mus., p. 220, there is reason for supposing that the extraordinary condition of the digits II and III was attained independently in these two groups. The phenomena seen in the case of the hind foot are in some r. -pects simpler than those of the variations in the fore foot, and t'uithU reason they may conveniently be described first.

If the phalanges of the index of the hind foot, for example, be examined, it will be seen that the proximal phalanx is nearly bilaterally symmetrical about a longitudinal axis, but that the -econd phalanx is deeply hollowed out upon the external or fibular >ide. Into this excavation the ungual phalanx is withdrawn when the claw is in the retracted position. The retraction is chiefly effected by a large elastic ligament running from the outside of the distal head of the second phalanx and inserted into the upper angle of the last phalanx (see OWEN, Anat. and Phys. of Vert., in. p. 70, tig. W>). Tlie same plan is found in the digits II to V both of the fore foot and of the hind foot. By this asymmetrical retraction of the claw a digit of the right side may be differentiated at a glance from one of the left side, for the claw is retracted to the right side of a right digit and to the left side of a left digit. The importance of this fact will be seen on turning to the evidence, for it is found that with variation in the number of digits there is a correlated variation in their symmetry.

With respect to the tarsus little need be said. The proximal part of the tarsus contains three bones, the calcaneum, astragalus and navicular. The distal row consists of four bones, the cuboid and three cuneiform bones. In the majority of polydactyle cats that I have seen in which the tarsus is affected, the cuboid is normal and the ecto-cuneiform is also normal and recognizable ; internal to the latter there are three small cuneiforms articulating with the navicular instead of two, making four cuneiforms in all. In some specimens there is no actual separation between the two innermost of these cuneiforms, but the lines of division between them are clearly marked.

In the normal hind foot of the Cat there are four fully formed toes, commonly regarded as II, III, IV and Y, each having thn-e phalanges. In the place where the hallux would be there is a small cylindrical bone articulating at the side of the internal cuneiform. As usually seen, all the four digits are formed on a >imilar plan, each having its claw retracted to the external or fibular side of the second phalanx, the four digits of a right foot being all right digits and those of left feet being all left digits. The rudimentary hallux has of course no claw.

Starting from this normal as the least number of digits, it will be found that a large proportion of cases are such that they may be arranged in an ascending or progressive series. In this series the following Conditions have been observed. In the schematic representations of the limbs the words 'Eight' or 'Left' signify that a digit is shaped as a right or as a left. The Roman numeral indicates that the digit to which it is assigned has the tarsal or carpal relations of the digit so numbered in the normal. For brevity each is described as a riyltt foot.

I. The normal, consisting of four three-phalanged digits, each retracting its claw to the external, viz. right side, and a rudimentary hallux with no claw. In this foot therefore the digits enumerated from the external side are II. Five digits, each with three phalanges. Of these the minimus and annularis borne by a normal cuboid are normal and are formed as right digits. The medius is borne by a normal ectocuneiform and is also a true right digit. Internal to this is a fullsized digit having the relations of an index and borne by a bone placed as a middle cuneiform. But the claw of this digit cannot be retracted to the external side of the limb, for the second phalanx is not excavated on this side. There is on the contrary a slight excavation on the internal side of the second phalanx, but this is very incomplete and the claw cannot be fully retracted, being in fact almost upon the middle line of the digit when bent back. This digit is thus intermediate between a right and a left. Nevertheless it is truly the index of this right foot, for it has the tarsal relations of an index.

Internal to this digit is another, which by all rules of homology should be the hallux, but it has three phalanges and is fashioned as a left digit, retracting its claw to the left (internal) side of the digit. This digit (Fig. 85, II, d1) is borne jointly by two cuneiforms, c1 and c'2, as shewn in the figure. There is thus one cuneiform more than there is in the normal. In this foot therefore the digits enumerated from the external side are as follows :—

Between this state and the normal I have as yet met no intermediate. It might perhaps have been expected that a foot having four three-phalanged digits and a hallux with two phalanges would be a common form of variation. Such a condition has not however been seen, so far as I know. III. The foot shewn in Fig. 85, 1 exemplifies the next condition. In it the three external digits, which are structurally the minimus, annularis and medius of a normal foot are normal in form, position and manner of articulation with the tarsus. Internal to the medius are three digits, of which the innermost has two phalanges (Fig. 85, I, d1) and a claw which cannot be retracted, like the pollex of the normal fore foot. The other digits, d3 and d\ are fashioned as left digits, retracting their claws to the internal or left side of the limb. It will be seen that of them d3 has the

relations to the tarsus which an index should have. The tarsus is as in tinlast Condition. In tinspecimen seen, c1 and c2 were not actually separate from ea«-h other, but there was a distinct line of division between them. Henthen the digits enumerated from the external side are as follows:— IV. The stage next beyond the last is shewn in Fig. 87, II. [The drawing is from a left foot.] Here there are six digits, each with three phalanges. The three externals are normal and true rights as before. The other three are all formed as lefts. Tarsus as before.

Right. Right. Right. Left. Left Left V IV III II digit digit As far as I have seen the last or fourth Condition is the commoner. There are doubtless many variants on these plans. No. 477 is an especially noteworthy modification of the third Condition and the cases of the hind feet in No. 478 must also be specially studied us not conforming truly to either Condition. I. The normal right fore foot has four digits II — V each with throe phalanges all differentiated as rights, and a pollex with two phalanges, the last being non-retractile but bearing a claw. It may be represented thus :—

Departures from this normal are more irregular than they are in the case of the hind foot. Those given in this summary being only a selection. For the others the evidence must be examined. II. One specimen, No. 474, has the four external digits normal. The pollex however has three phalanges and is formed as a digit of the other side, thus :— IV. In the majority of polydactyle cats the manus has the digits II — V normal in shape and symmetry. Internal to the • li^it II are two digits more or less united in their proximal parts; -oinrtinirs the niftaearpal only, sometimes the metacarpal and first phalanx are common to both. Of these two digits the external,

that is, the one iiext to the digit II, is in some degree shapeless and imperfect, but the external branch is as a digit of the other side in form. Internal to this double digit is a seventh digit, sometimes with two phalanges, sometimes with three, but in either case the claw is as a rule non-retractile, and the digit is in this respect not differentiated as either right or left. Such a manus may be thus represented (cp. Fig. 86 a left manus) : —

As regards the carpus its changes are like those of the tarsus. When there are six metacarpals there are three carpals in the distal row internal to the magnum. That next the magnum may be supposed to be trapezoid, and the other two may be spoken of as first and second trapezium. In correspondence the length of the scapho-lunar is increased. No comment can increase the interest of these curious facts. In the pes, as has been stated, with change in the number of digits there is change in the grouping and symmetry of the series of digits, and in particular the digit having the relations of the index or digit II is formed as the optical image of its neighbour III instead of forming a successive series with it. There is thus a new axis of symmetry developed in the limb, passing between the parts which form the digits II and III of the normal.

*472. Cat having the digital series of each extremity abnormal, being that preserved in the Coll. Surg. Mus., Teratological Catalogue, 1872, Nos. 305 and 306. Right pes (Fig. 85, II). Digits III, IV and V normal right digits. Internal to these are two digits each having three phalanges and claws. That lettered d1 is formed as a left digit but d? is almost indifferent, the second phalanx being slightly hollowed on the inside. Internal to the external cuneiform there are three small bones, of which the inner two together bear the digit d1. [This is the Condition II of the pes.]

Left pes has the same structure as the right so far as can be seen from the preparation (in which the muscles remain). The digits III, IV and V are normal left digits, but internally to them there are two digits each with three phalanges, of which the external is an indifferent digit, while the internal is formed as a right. [Condition II of the pes.] Left manus. The digits II, III, IV and V are normal. But the carpal of the distal series (trapezoid) which bears the digit II is imperfectly separated from a similar bone placed internal to it. This second part of the trapezoid bears a metacarpal which articulates with a full-sized digit of three phalanges formed as a right digit. From the external side of the first phalanx of this

digit there is given off a rudimentary digit, which has however a complete claw, but its bones do not differentiate it as right or left. FIG. 85. I. Right pes of Cat No. 473, skewing condition III of the pes. II. Right pes of Cat No. 472 shewing Condition II of the pes. UK. astragalus, c1, c-, c3, three ossifications representing the ento- and mesociiMi'ifnnn- "I 'the normal, rb, cuboid, clc, calcaneum. rf1 — d6, the digits numbered from the inside, ect, cu, ecto-cuneiform. nav, navicular.

The " pollex", dl, has two phalanges and is rather slender. The t r:ij it '/in in which bears it is not separated from the scaphoid. Rigid manus. This is exactly like the left manus so far as can be seen from the dissect ion, except for the fact that the rudimentary 'li.^it borne by the large digit external to the "pollex" is much more reduced than in the case of the left manus. The digit which supports it is fashioned as a left digit. [Condition IV of the manus.]