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

*867. In Chelonia also are several such instances. See EDWARDS, Nat. Hist, of Birds, d-c., Pt. iv. 1751, p. 206; MITCHILL, I. c. ; BARBOUR, E. H., Amer. Jour, of Sci., 1888, S. 3, xxxvi. p. 227, PI. v. The last is a particularly interesting case from the circumstance that the behaviour during life was observed to some extent, though only a popular account is given. The two heads seemed to act independently, and it is said that there was no concerted action between the feet of the two sides. BARBOUR'S figures are reproduced in Fig. 207.

In fish-hatching establishments double monstrosity is of frequent occurrence among young Salmon and Trout. A two-headed embryo of a Shark is preserved in Coll. Surg. Mus. (Terat. Cat. 1872, No. 22). Chaetopoda. Duplicity in this Class has been often seen, but that any of the cases are truly congenital cannot be stated. There is evidence that in many Annelids regeneration1 both of head or tail may freely occur, and it is quite possible that the second head or second tail may have grown out from an injured place, though of this there is no actual proof. In cases of posterior bifurcation each tail generally contains all the parts proper to the normal, but in No. 871 one of the tails was without the terminal cirri usual in the species. So far as can be gathered from the evidence it does not appear that the two continuations of the body have always the same number of segments, which might perhaps be expected were both the result of a natural division of the developing body. On the other hand, they do seem generally to have a nearly equal development, and are almost always (in cases of double tails, at least) fairly equal in length, which would not be anticipated if one only were a new growth. Moreover, if the double tail is in some way due to regeneration one would expect to find such duplicity in its minor conditions much more commonly.

Into the details of the structure it is not now proposed to enter, and indeed of most of the cases there is little to be told. The evidence is mentioned here simply in further proof of the power of these individuals, thus greatly departing from the normal of their species, to maintain themselves with no apparent difficulty. It will be noticed that the species concerned are most various, and include not only Errantia, but two cases also in Serpulidfe.

The literature of the subject was collected by CoLLiN2, and a list of the references was independently collected and published with abstracts by ANDREWS3. This list, with a few additions, was republished by FRIEND4. Though many of the accounts are imperfect they are referred 1 The evidence on this point does not come within the scope of this work. References to it may be obtained from ANDREWS, ZEPPELIN, &c. (v. infra). to below, in evidence that the total number of cases is considerable. There are only two certain cases of double head (see Ti/]>«*////ix, Xo. 868, and Allolobophora, No. 873).

*S68. Typosyllis variegata : individual having two small heads, as shewn in Fig. 208. Heads of unequal size, that on the left having 4 segments behind the eyes, while that on the right had two. The appearance suggested that the original head had been broken off and that two new ones had uruwn in its place. LAXGEUIIANS, P., J~«m Acta Ac. C. L. C., XLII. p. 102, I'l. Salmacina incrustans (Serpulidse) : posterior end double. [Two tails shewn in figure as of equal length and in the same straight line, at ri^ht angles to the body. The arrangement of the segmentation at the junction is not clearly shewn.] CLAi'AuV.i'K. MI ni. mx-.jilii/is. t't tl'liiat. mil. (ii'iif-i-f, xx. 1869 — 70, p. 177, PI. xxx. tig. ~> v.

[With these conditions compare Si/llia r<iin»*ii, a form found by the ('Imlli'iiiii-i- in tw« localities, inhabiting a Hexactinellid Sjionge. The body of this creature consisted of vast numbers of branches, abontas thick as thread, passing off at right angles, coiling upon each other and forming inextricable masses. In some specimens no head was found, but a single head was afterwards discovered. It seemed likely that large tracts of the body have no head, but there was no evidence to shew how many heads occur in the colony. Many female buds were found, and a single complete male. Mclxxosn, Chull. Jli'i>., xn. p. I'.l*. I'l. xxxi.]

*S73. Allolobophora longa : specimen represented as bearing a second head on the right side of the first segment behind the peristomium. The second head is represented with prostornium, peristomium and one more segment which rests on the peristomium of the normal body. FRIEND, H., Science-Gossip, 1892, July, p. 16 1,./?V/- 876. Acanthodrilus sp. : case of two tails arising from a much thicker anterior portion. Such worms were believed or alleged to be common in a particular district in New Zealand. KIRK, T. W., Trans. N. Zeal. Inst., xix. p. 64, PI.

*878. Chironomus (Gnat): larva with two heads, duplicity beginning from the 5th segment behind the head [important details given, q. v.]. WEYENBERGH, H., Stet. ent, Ztg., 1873, xxxiv. p. 452, Jig. 879. Euscorpius germanicus (Scorpion): tail double from 4th p rajabdominal segment [figure represents each abdomen with one segment too feii), presumably an error]. PAVESI, P., Rend. li. 1st. Lomb., S. n., xiv*. 1881, p. 3'29, fg. 880. [Scorpio africanus :] specimen with two tails. SEBA, Rer/nn Naturalium Thesaurus, 1734, i. p. 112, PI. LXX. Jig. 3. This example was kindly sent me by Mr R. I. Pocock, who tells me that the figure shews the animal to be of the species named.

Conditions, perhaps akin to duplicity, have been seen to occur under three forms. 881. Taenia coenurus : specimen whose head had 6 suckers instead of 4, and 32 hooks instead of 28. Proglottides were 3-sided prisms, in section triangular. Longitudinal vessels 6 instead of 4, two being in each angle. Absolute size of head greater than normal. This abnormal In another form of abnormality the chain of segments has three longitudinal tlangt-s, formed, as it were, by the union of two chains of proglottides having one edge in common. Head not found, but several cases known. Genital openings in one case all upon the common edge. LKTCKAKT, ibid., p. 574. Cp. COBBOLD, Tr<m*. 1'tith. >V., xvn. p. 4:>^: LEVACHER, Comptes rendus, is-U, xin. p. flUl.

Bifurcated chains of proglottides have also been seen, e.g. specimen of Taenia (cysticerci) tenuicollis. which bifurcated several times in terminal portion, though normal in front of this. MOMEZ, Jinll. >W. >/// x. p. 201. See also Taenia saginata ? LEUCKART,/. c., p. 573. FIG. 209. Acdiitliotln/rix xpiiiaxu, No. 888. Case of duplicity. (From P. I. Seen from ventral valve. II. Looking between the valves. 884. Cucumaria acicula : specimen made up of two individuals cohering laterally at posterior ends. SCHMELTZ, Verlt. d. I'er. f. natitrw. Unterhaltung, Hamb., 1877, iv. p. xv.

885. Cucumaria planci : case of second mouth and ring of tentacles borne on a lateral bud-like projection. LUDW1G, H.. Z. f. u\ Z., LIII. Supp. p. 21, PL v. 880. CCELENTKRATA. Forms which are commonly simple, such as Actinia or Snijartin. are rarely found with two discs seemingly due to incomplete division, which in these forms may take place longitudinally ['?] as well as by ordinary budding. GOSSE, P. H., Sea-Anemones, p. NXI., etc. See also GUYOX. Zo<il<>:iixt. p. 70~2>'<. ji:i.

Similar occurrences, not distinguishable from budding, have been seen in Medusae, e.fi., Phialidhim rarinli/li', J>AVIDOFF, Zool. An:., iv. p. 6'20,fi;i.; Ga.-tr"- hlnxtn nijl'mii, LANG, A., Jen. Ztxrltr., xix. p. 735. An interesting case of this kind * \vas seen in Cordylophora lacustris. Several poljstomatous specimens were found mi ii purticiilnr HIUXX df ('<>r(l;il<>}>li<ir<i, but were not found on all colonies gathered with this mass and had not been seen previously in specimens from the same locality. [Further particulars.] PRICE, H., IJ. J. .17. S., 1876, p. 23, ri;ix.

To attempt at this stage any summary of conclusions would be misleading. The first object of this work is not to set forth in the present a doctrine, or to advertise a solution of the problem of Species, but rather to bring together materials that may help others hereafter to proceed with the solution of that problem. A general enumeration of particular conclusions is therefore to be avoided. Indeed, from the scantiness of the evidence, its present value is chiefly in suggestion, and the facts must therefore be themselves still studied in detail. The reader must interpret as he will.

But, as often happens, that which may not shew the right road is enough to shew that the way taken has been wrong, and so is it with this evidence. Upon the accepted view it is held that the Discontinuity of Species has been brought about by a Natural Selection of particular terms in a continuous series of variations. Of the difficulties besetting this doctrine enough was said in the introductory pages. These difficulties have oppressed all who have thought upon these matters for themselves, and they have caused some anxiety even to the faithful. And if in face of the difficulties reasonable men have still held on, it has not been that the obstacles were unseen, but rather that they have hoped a way through them would be found.

Now the evidence, of which a sample has been here presented, gives hope that though there be no way through the difficulties, there is still perhaps a way round them. For since all the difficulties grew out of the assumption that the course of Variation is continuous, with evidence that Variation may be discontinuous, for the present at least the course is clear again. Such evidence as to certain selected forms of variations has, I submit, been given in these chapters, and so far a presumption is created that the Discontinuity of which Species is an expression has its origin not in the environment, nor in any phenomenon of Adaptation, but in the intrinsic nature of organisms themselves, manifested in the original Discontinuity of Variation.

doubt whether tinvariationhere detailed arc such as go to the building of Specific Differences (a doubt which, it must be granted, does fairly attach to some part of the evidence), yet the existence of sudden and discontinuous Variation, the existence-, that is to say, of new forms having from their first beginning more or less of the kind of jn'/-/'> -r//o// that we associate with normality, is a fact that di-poses, once and for all, of the attempt to int. -rpr- •! all perfection and detiniteiiess of form as the work of Selection. The -tudy of Variation leads us into the presence of whole classes of phenomena that are plainly incapable of ,-uch interpretation.

The existence of Discontinuity in Variation is therefore a final proof that the accepted hypot he-is is inadeijuate. If the evidence wen! no further than this the result would be of use, though ituse would be rather to destroy than to build up. But besides this negative result there is a positive result too, and the same Discontinuity which in the old structure had no place, may be made the framework round which a new structure may be built. For if distinct and "perfect" varieties may come into existence discontinuously, may not the Discontinuity of Species have had a similar origin ? If we accept the postulate of Common Descent this expectation is hard to resist. In accepting that postulate it was admitted that the detiniteness and Discontinuity of Species depends upon the greater permanence or stability of certain terms in the series of Descent. The evidence of Variation suggests that this greater stability depends primarily not on a relation between organism and environment, not, that is to say, on Adaptation, but on the Discontinuity of Variation. It suggests in brief tin it the Discontinuity of Species result* //•<>/// the Dim:i>nti/n/itt/ <>f \r<in /fion.

This suggestion is in a word the one clear and positive indication borne on the face of the fact-. Though as vet it is but an indication, there is scarcely a problem in the comparison of -tinctures where it may not be applied with profit, The magnitude and Discontinuity of Variation depends on many elements. So far as Meristic Variation is concerned, this Discontinuity is primarily associated with and results from the fact that the bodies of living things are mostly made up of repeated parts — of organs or groups of organs, that is to say, which exhibit the property of "unity," or, as it is generally called, "individuality. Upon this phenomenon depends the tact that Meristic Variation in number of parts is often integral, and thus discontinuous.

The second factor that most contributes to the Discontinuity of Variation is Symmetry, manifesting its control in the first place directly, leading often to a result that we recognize as definite and perfect because it is symmetrical. But besides this direct control that we associate with Symmetry, other effects greatly contributing to the magnitude of Variation can be traced to a factor not clearly to be distinguished from Symmetry itself. For, as has been explained, Symmetry, whether Bilateral or Radial, is only a particular case of that phenomenon of Repetition of Parts so universally characteristic of living bodies; and that resemblance between two counterparts, which we call Bilateral Symmetry, is akin to the resemblance between parts repeated in Series, though, as is shewn by their geometrical relations, the processes of division by which the parts were originally set off, must be in some respects distinct. Bilateral Symmetry of Variation is thus only a special case of the similar and simultaneous Variation of repeated parts.

The greatness of the observed change from the normal is often largely due to this possibility of simultaneity in Variation, the change thus manifesting itself not in one part only, but in many or all of the members of a series of repeated parts. Instances of such similar and simultaneous Variation of serial parts in animals have now been given. Examples still more marked may be seen abundantly among plants. A variation, for example, in the form or degree of fission of the leaf, slight perhaps by itself, when taken up and repeated in every leaf in its degree, constitutes a definite and conspicuous distinction. Everyone has observed this common fact. Few illustrations of it are more evident than that of the common Hawthorn. In a quickset hedge soon after the leaves begin to unfold almost each separate plant can be recognized even at a distance, and its branches can be traced by their special characters, by the shapes and tints of the leaves, by the angles that they make with the stem, by the manner of unfolding of the buds, and so forth. These variations, sometimes slight in themselves, by their similarity and simultaneity build up a conspicuous result.

The phenomenon of serial resemblance is in fact an expression of the capacity of repeated parts to vary similarly and simultaneously. In proportion as in their variations such parts retain this capacity the relationship is preserved, and in proportion as it is lost, and the parts begin to vary independently, exhibiting differentiation, the relationship is set aside. It will be noticed that to render the converse true we must extend the conception of Serial Homology in special cases to organs not commonly regarded as serially homologous with each other, but which having assumed some common character thereafter may vary together (cp. p. 309).

In the power of independent Variation, members of series once more exhibit the property of "unity" that we have already noticed as appearing in the manner in which the number of the members is changed. The fact that members of series should be capable of varying as " individuals " is paradoxical. Such members, teeth, digits, segments of Arthropods, and the like, are each made up of various tissues endowed with miscellaneous functions and dissimilar in their morphological nature. Nevertheless each group is capable

<>f indi-pi-nd'-nt division and of separate Variation. Single digits for instance may thus be independently hypertrophied as a whole, single segments or single appendages or pairs of appendages may be differentiated in some special way, and ^> forth. At this point reference may again be made to that extraordinary Discontinuity of Variation appearing in what I have called Ho- mceosis, so strikingly seen in the few Arthropod cases given (p. 14<i i, and so common in flowering plants. In these changes a limb, a floral segment, or some other member, though itself a group of miscellaneous tissues, may suddenly appear in the likeness of some other member of the series, assuming at one step the condition to which the member copied attained presumably by a long course of Evolution.

Many times in the course of this work we have had occasion to r the modifications in the conception of Homology demanded by the facts of Variation. It is needless to speak further of this matter here, and the reader is referred to pp. 12o, 191, 2G!>, M'.i4 and 417, where the .subject is discussed in relation to Linear Sen.-- of several kinds, and to the facts given in Chapter XVI and at p. 433 bearing on the same questions in their application to Radial Series. The outcome of these considerations shews, as I think, that the attribution of strict individuality to each member of a series of repeated parts leads to absurdity, and that in Variation such individuality may be set aside even in a series of differentiated members. It appears that the number of the series may be increased in several ways not absolutely distinct, that a single member of the series may be represented by two members, that a terminal member may be added to the series, and also that the number of the members may change, no member precisely corresponding in the new total to any one member of the old series : in short, that with numerical change resulting from Meristic Variation there may be a redistribution of differentiation.

But though this is, in my judgment, a fact of great consequence, its relation to the Study of Variation is merely incidental. It is not so much that to enlarge the conception of Homology so as to include the phenomena of Meristic Variation is a direct help, as that to maintain the old view is a hindrance and keeps up an obstacle in the way of any attempt to apprehend the real nature of the phenomena of Division, and hence of Meristic Variation. So long as it is supposed that each member of a series of repeated parts is literally individual, it is impossible to form any conception of Division that shall include the facts of Meristic Variation, for in Variation it is found that the members are divisible.

It is an unfortunate thing that the study of Homology lias been raised from its proper place. The study of Homologies was at first undertaken as a means of analyzing the structural evidences of relationship, and hence of Evolution. This is its proper work and use ; but the pursuit of this search as an aim in itself has led to confusion, and has tended to conceal the fact that there are phenomena to which the strict conception of individual Homology is not applicable.

This exaggerated estimate of the fixity of the relationship of Homology has delayed recognition of the Discontinuity of Meristic Variation, and has fostered the view that numerical Variation must be a gradual process. This view the evidence shews to be wrong, as it was also improbable. Brief allusion may be made to three separate points of minor unimportance. It is perhaps true that, on the whole, series containing large numbers of undifferentiated parts more often shew Meristic Variation than series made up of a few parts much differentiated, but throughout the evidence a good many of the latter class are nevertheless to be seen.

Reference may be made to a point that might with advantage be examined at length. The fact that Meristic Variation may take place .suddenly leads to a deduction of some importance bearing on the expectation that the history of development is a representation of the course of Descent. In so far as Descent may occur cliscontinuously it will, I think, hardly be expected that an indication of the previous term will appear in the ontogeny. For example, if the four-rayed Tetracrinus may suddenly vary to both a five-rayed and also to a three-rayed form (see p. 437) it is scarcely likely that either of these should go through a definitely four-rayed stage ; and if the origin of the four-rayed form itself from the five-rayed form came similarly as a sudden change, it would not be expected that a five-rayed stage would be found in its ontogeny. Similarly, if a flower with five regular segments arise as a sport from a flower with four, it would not, I suppose, be expected that the fifth segment would arise in the bud later than the other four. I suggest these examples from Radial Series, as in them the question is simpler, but similar reasoning may be applied to many cases of Lineal- Series also.

It will be noted that the attempt to apply to numerical variations the conception of Variation as an oscillation about one mean is not easy, difliculty arising especially in regard to the choice of a unit for the estimation of divergence. En few cases can facts be collected in quantity sufficient even to sketch the outline of such an investigation; but, to judge from the scanty indications available, it seems that in cases of numerical change variations to numbers greater than the normal number, and to numbers less than it are not generally of equal frequency. Probably no one would expect that they should be so.

As was stated in the Introduction, we are concerned here with the manner of origin of variations, not with .the manner of their perpetuation. The latter forms properly a distinct subject. We may note however, in passing, how little do the few known facts bearing on this part of the problem accord with those ready-made principles with which we are all t'ainiliar. Upon the special fallacy of the belief that great Variation is much rarer in wild than in domesticated animals we have often had occasion to dwell. As was pointed MIII in the discussion of tinevidence on Teeth (p. 2GG) this belief arises from the tact that domesticated animals are for the most part variable, and that we have every opportunity of observing and preserving their variations. To compare rightly their varialiility with that of wild animals choice should be made of animals that are aNo variable though wild. Taken in this way the comparison is fair, and as 1 have already >aid. if we examine the variation in the vertebra:' of the Sloths, in the teeth of the Anthropoid Apes, in the colour of the Dog-whelks (Purpura lapillus), *v«-., we find a fiv . (iieney and a range of Variation matched only by the most variable of domesticated animals.