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

sense, includes the cases of irregularity. The difficulty is to understand the causes of regularity and of symmetry ; but if we could be sure of these it would not be hard to concave disturbances resulting in irregularity. In the pigs are found, first, cases of six on both sides in pairs, and also of seven on both sides in pairs ; besides these there were cases of G — 7 and of 7 — 8. Of these there were some in which two on one side stood in positions which geometrically balanced that of one of the other side, the others being arranged in pairs. In such cases the appearances suggest that there has been a division of one mamma to form two, and that the two have then separated or travelled apart. The division of organs into two is of course a common occurrence, and may naturally be supposed to be a phenomenon of the same nature as the division of single cells. The case of mammae is perhaps instructive inasmuch as it bears witness to the fact that such division must take place at a remotely early period in development. For while in cases to be given hereafter of division, for example, between teeth, it may be supposed that the travelling apart of the two resulting teeth is mechanical, in the sense that the two growing teeth may simply push apart from each other just as two cartilagecells, &c., may separate by the concentric deposition of material, the separation cannot be supposed to occur in the mamma? by these late changes, but the process of mechanical separation, though the same in kind as that in the case of teeth, must be conceived as beginning early in the history of segmentation.

At this point a circumstance, very often to be seen in other cases, should be mentioned. When an organ, single on one side, corresponds geometrically with two organs on the other side, each of the latter is frequently of the same size and developed to a like extent as the single one of the other side. This of course would be expected on the hypothesis that the division of organs is a phenomenon similar to the division of cells, that is to say, not merely a division, but a reproduction.

But the supposition of division of single members of the series is not sufficient to account for all the facts of Variation seen. We have to consider not only the case in which one organ of one side balances two of the other. We have to deal also with the cases of six on each side and seven on each side all corresponding in pairs. In these there is no indication that there has been a division of a single member on each side. The spacing is regular in each case and there is no obvious crowding at any part of the series. Even if therefore in the former case there is a suggestion that the germs of single mammae have divided into two at a period of development after the series of mammae was constituted as a series, there is no such suggestion in the present case. We must, I think, in the latter suppose that the existences of all the mammae, whether

six ..r seven, are determined together. How or at what stage such <1. -t. •rtninatiou is made, there is no direct evidence to shew. The \ari.ius arrangements seen suggest then that the relative positions ormpied l>y the mamma? depend partly on the number ti,:,t are present, :unf thsit the position of each mamma is to some extent dependent on the position of other mamma?, especially of its rhbours In this connexion the cases F and L are interesting oni - Fii •" H. In L for example, the 1st on the left is at a higher |,-v.-l than th.- 1st on the right. It is succeeded by a rudimentary 2nd ha\mur none "ii the same level on the other side. The left :{rd is behind the right "2nd, but posterior to this point the nipples

approximately paired. These appearances suggest that the displacement ofthelsl and .Srd on the left are in some way connected with the | '!• sence of the rudimentary left 2nd. Similarly in F the |, .ft :inl and ."ith are -paced out for the rudimentary 4th. From its position and Mnall size it might fairly be supposed that this is a •• stipernnmerarv "I'gan, for at all events it is visibly different from th,. others: l"it in the case of seven on each side in pairs, no one mamma rat ht-r than another can be pointed out as obviously -npernnmeiarv \\hrn compared with a similar series of six. It seems therefore that ..f the factors determining the relative position^ nf tinmaniina- :d<>ng the mammary lines, the number of the mamma- i- one, and that the positions of the mamma? are in some way and t<» a limited extent correlated with each other. That there :ir.' oth'-r factors at work, also, is sufficiently shown by the existence of cases of apparently utter irregularity.

In sec 'kin1.; t" go 1 H -y ond this and inquire as to the way in which this correlation i> brought about there is, in the present Maliof kliovjed-e of tinmechanics of Division, HOt lllUcll to be :i,-d. Keference may be madibo recent observations published in ali-tract 1,\ ( ). Si -iifi.TXK1. According to him there is in young embryoof -, -\.-ral mammals (Pig 1'5 cm. long; Rabbit 13 — 14 dav>, iVc. ) a rid^'1 running along the dorso-latenil aspect on each -id'f and at points upon this the mamma' and nipples are eventually fornu'd. (The formation of the true nipples is preceded by tinraising of the epidermis into small elevations, "primitive beats," \\lii'-li afterwards disappear.) The two mammary lines are li\- -nli-i-i|iient rhaniM-s and growth of the body brought into the \entro-lati-ral position. The i|Urst ion of t lie ^»^\\ ion of the mamma' thei-etoi. resolves itself into this: what determines the positions at which mammary centres, to borrow the word used in the case of hone, are to lio formed on the inaininarv lilies? In a subsequent place I shall contend tha» the tacts given are only intelligible on the \ie\v that the forces dctei-miuing the points of growth of mammaancompounded into one -y>tem of forces. But to the i|iie-iioii \\hat are these forces tin-re is no answer.

FROM the consideration of numerical Variation in mammae we may proceed to an examination of like phenomena in the case of the teeth of vertebrates. The modes of Variation in these organs are, as might be expected, in many ways similar, but several circumstances combine to make the Variations of teeth more complicated than those of mammae. Teeth arise developmentally by special differentiation at points along the jaws, much as the mammae arise by differentiation at points along the mammary lines ; and as in the case of mammae, so in the case of teeth, we are concerned first with changes in the number of points at which such differentiation takes place, and next with the general changes or accommodations which occur in the series in association with numerical changes. As in mammae, so also in teeth, numerical Variation may occur sometimes by the division of a single member of the series into two, and sometimes by a reconstitution of at least a considerable part of the series.

Between the case of mammae and that of teeth, there is however an important point of distinction. The series of mammas is practically an uudifferentiated series. There is between mammas standing in one mammary line no obvious qualitative differentiation. Though not all identical in structure, the differences between them are of size and of quantity, not of form or quality. If such qualitative difference is present it must be trifling. In considering Variation in mammae we have thus to deal only with changes in number, and with the geometrical and perhaps mechanical question of the relative positions of the mammae. The teeth of most Vertebrates, however, are differentiated to form a series of organs of differing forms and functions, and the study of Variation in teeth may thus be complicated by the occurrence of qualitative changes in addition to simply numerical ones. In teeth, in fact, there are not only Meristic variations, but Substantive variations

also; and \}\ns, AS in the case of vertebra?, for instance, in any given example «)f a numerical change qualitative changes must be A- a preliminary to the consideration of evidence relating to the Vaiiati.in «.f te.'th it may be useful to call attention to cert.-iin peculiarities of teeth considered as a Meristic Series. In the lntr."lucti"ii. S.-cti.in V, it was pointed out that in order to get auv conception of the Evolution of parts repeated in an animal, tli.'- ta.t of this II. -petition must be recognized, and it must be alwavs remembered that we are seeking for the mode in which not one part but a series of similar parts has been produced. The simple^ case to \\hi.-h this principle applies is that of organs paiivd about th.- middliline, and in the steps by which such parts have taken .>n a gi\en f.-nn it is clear that similar variations must have ..ccurred .,11 thetwo sides. In the absence of evidence it mi^ht I..- suppos.-d fit her that such variations had occurred little l.\ little on thf two sides independently, or on the other hand, that Variation had come in symmetrically and simultaneously on the t\\o sides. Upon the answer given to this question the success of all attempts t.. form a just estimate of the magnitude of the int.-gral steps of Variation depends. In many examples already gi\en it has now been -h.-wn that though in the case of paired organs Variation ma\ be a-ymmet rical, yet it is not rarely symmetrical, ami m part t In- quest ion has thus been answered.

In the evidence that remains many more cases of such symmetrical variations will be described, and it may be taken as e-tablished that \\lien the organs stand in bilateral symmetry, that i- t<- say, as images <,n eitlier side of a middle line, their Variation ///'/// be similar and symmetrical. The teeth present this problem .if the Variation of parts standing as images, in an unusual and peculiar way. For in the case of teeth we ha\e to consider not only the steps by which the right and left sides i,t' each jaw have maintained their similarity and symmetry, but in addition the further question as to the relation of the teeth in the upper jaw to tho-e in the lower jaw. There are many animals in which there is very great difference between the upper and lower rows of teeth, and it must of course be remembered that perhaps in no animal are the teeth in the upper jaw an exact cnp\ ..f those in the lower, but nevertheless there is often a substantial similarity between them, and in such cases we have to consider the In, ml er kinship bet \\een the upper and lo\\er teeth whereby they have become similar or remained so. F"i- 11 may b.- state.! at .nice that there is s..me evidence that the teeth iii the upper and lower jaws may vary similarly and shnultaneoiisl\. though such cases are decidedly rare, especially in nuiiH-rieal \aiiati.m. and are much less c.iiiimon than symmetrical Variation . .n the two sides of the same jaw.

In speaking of the relation of the series of the upper jaw to that of the lower jaw as one of images, it must be remembered that the expression is only very loosely applicable. In particular it should be noticed that though in so far as the lower teeth are a copy of the upper ones the resemblance is one of images, yet the teeth which resemble each other do not usually stand opposite to each other in the bite, but members of the upper series alternate with those of the lower. The incisors, as a rule, however, and the back teeth of a certain number of forms do bite opposite each other, and in them the relation of images is fairly close.

The importance of the recognition of the relation of images as subsisting between the teeth of the upper and lower jaws will be seen when this case is compared with that of the two sides of the body. For ordinary bilateral symmetry is, as has already been suggested, an expression of the original equality and similarity of the two halves into which the ovum was divided by the first cleavage-plane, or by one of the cleavages shortly succeeding upon this. The fact that the two halves of the body are images of each other is thus both an evidence and a consequence of the fact that the forces dividing the ovum into two similar halves are equal and opposite to each other. The bilateral symmetry of Variation is thus only a special case of this principle.

In view of the fact that the teeth in the upper and lower jaws may vary simultaneously and similarly, just as the two halves of the body may do, it seems likely that the division of the tissues to form the mouth-slit must be a process in this respect comparable with a cleavage along the future middle line of the body. It is difficult, however, to realize the actual occurrence of such a process of division in the case of the slit forming the original stomodoeum, and this difficulty is increased by the recent observations of SEDGWICK1 to the effect that in the Elasmobranchs examined by him the mouth-slit first appears as a longitudinal row of pores. If this is so the relation of images must exist in the case of the mouth, not only in respect of the two sides of the slit, but also in respect of the anterior and posterior extensions of the slit. But whatever may be the processes by which the tissues bounding the mouth of a vertebrate come apart from each other,the result is clearly in many cases to produce an anterior series of organs in the upper jaw, related to a posterior series of organs in the lower jaw, much in the same way that the right side of a jaw is related to the left of the same jaw. This relation may appear as has been stated, not only in the normal resemblances between the upper and lower teeth, but also in the fact that similar and simultaneous Variation is possible to them.

In another respect the Repetition of teeth may differ from that of other Linear Series already considered. In many animals, the Pike, the Alligator, or the Toothed Whales, for example, the teeth • I in ,i P -pillar and usually continuous series, differing from • ad, other chiefly in size, ranging from small teeth in from, through large teeth, and often down to small teeth again ai the 1 a.-k '• .f t h.- jaw. Such a 'homodont' series as a rule passes through ,,nlv one ni'axinium. Most mammals, however, are 'heterodont,' iha't LB to say, the teeth can be distinguished into at least two groups, ih' incisors and canines on the one hand, and the premolarfi and molars on the other; and in a large number of animalhaving tinarrangement the anterior members of the series ot premolars and molars are small, increasing regularly in size from before l.a«-k\\ards, reaching a maximum usually in some tooth anterior to th.- last. Though instances will be given of Variation, and especially "f reduplication, occurring in most of the teeth, even in those which stand well in the middle of the series of back- •h. auch as tin upper carnassials of the Cat, or the fourth premolai-; of ill, Seal, ye\ on the whole Variation in heterodont forms i-, more common at tinanterior and posterior ends of the series of back -teeth. In view of this fact it is of some importance to recognize that th<- -mall members at the beginning of the premolaiseries are as n -gards their relatively small size, in the condition of terminal memb.-rs of series, and exhibit the variability of ti -rniinal memb.-ix almost as much as the last molars.

With these n-niarks by way of preface, evidence as to the num. -i-ical \'ariatiou of teeth in certain groups will be given in full. Thiaccount will lor the most part be confined to a brief description of the conditions piv-mted by the specimens. In the next chapter the principles which may be perceived to underlie these facts and the general conclusions to which they appear to lead will be -••parately discussed. The evidence here given relates to certain selected groups1 of Mammal-, ami chiefly to the lYimate-, (excepting Lemuroidea), Carnivora i( 'anida-, Felida-, Yiverrida-, Mustelidae and Pinnipediae), and Marsupialia I 1'halangerida', 1 )a>yiirida-t I)idelphyidai, part of Macropodida-. >Vc.).

The tacts to be given relate chiefly to increase in number of teeth. In the case of terminal members of series, such as the moM anterior pivmolar or the last molar, some reliable facts as to i-asi - of alr-^nce \\.-re found, but )oithe most part the evidence as to the absence of teeth is ambiguous and each case requires -••parate treatment. 1 K\id. ];'•,• as to tinilriitul variations of .Man is not hnv introduced. Coniiilili- o>lliTti«>ns ft' siK'li diets have li.'cii niailf liy MAUITOT (Amnii. <lu ni/st.

liiimiin variations air indinl. .1 piniiomeua dil'tVi.'iit in kind from those seen in zoological position of the groups concerned. In several cases variations of similar nature were seen in different groups ; cases of this kind will be brought into association in the next chapter. As regards nomenclature I have in the main followed the common English system, numbering both the premolars and molars from in front backwards. In one respect I have departed from the practice now much followed. It has seemed on the whole better that the premolar which in any given jaw stands first, should be called p1, even though in certain cases there may be reasons for doubting whether it is the true homologue of the jj1 of other cases1. Theoretical views of this kind can only at best be used as a substitute for the obvious nomenclature in a few restricted cases, such as that of the Cat, in which by the application of the methods of reasoning ordinarily adopted in Comparative Anatomy the first upper premolar would be looked on as the equivalent of F2 in the Dog. There are, however, few who would feel confident in extending this reasoning to many other cases, that of Man, for instance, and I believe it is on the whole simpler to number the teeth according to their visible and actual relations. As I have already attempted to shew in another place2, in the light of the facts of A^ariation, it is to be doubted whether in their variations teeth do follow those strict rules of individual homology by which naturalists have sought to relate the arrangements in different types with each other.

The material examined has consisted chiefly of specimens in the British Museum and the Museums of the College of Surgeons, Leyden, Oxford and Cambridge, the Paris Museum of Natural History, and some smaller collections. I have to thank the authorities of these several museums for the great kindness I have received from them; and in particular I must express my indebtedness to Mr Oldfield Thomas, of the British Museum, for the constant help and advice which he has given me, both as regards the subject of teeth generally and especially in examining the specimens in the British Museum3.

SIMIID^E. The Anthropoid Apes (Orang, Chimpanzee, and Gorilla). *165. The teeth of the three large Anthropoids are perhaps more variable, both in number and position, than those of any other 1 In cases where confusion might arise any change from common nomenclature is notified in the text. :1 In the following descriptions B.M. stands for British Museum; C.S.M. for Museum of the Eoyal College of Surgeons; C.M., O.M., U.C.M., Leyd. M., P.M., for the Cambridge, Oxford, University College London, Leyden and Paris Museums respectively.

group of mammals of which I have been able to examine a considerable numb. T. lii different collections 14'2 normal adult skulls were BCCE and 12 cases of extra teeth. Of these one was a case of extra inei-or (dorilla, No. 186), one of anomalous teeth (Gorilla, \.< I ^7 i. ami the remainder molars. Thus far therefore there are 1 1. -aily s per i -i nt. cases of extra teeth. This figure is remarkable in roinpari-on with the rarity of such cases in nyldbates (51 skulls ii. all normal), and the like rarity in other Old World monkey- > li'.'i normalami "1 cases of extra teeth).

wnumerary molars. l»iii. Adult mallhaving additional posterior molar (m4) behind and in series \\ith the normal teeth, on both sides in upper jaws and on left -id.- iii lower jaw. In each case the m* is rather smaller than in . hut all are well formed, having each four cusps and the normal • •"iiijil. nifiit of l'aii;4-, viz., one in front and one behind in the |.I\\,T jaw. ami two mi outer and one on inner side in upper jaw. < in ri_dit .»ide of lower jaw there is no trace of additional molar, thoii-h there is almost as much room for it as on the left side.

His. Sj.rcinicn \\ith largr alveolus on each side for m*. L. M., 24. lii!'. Sjierinicn ( Korn. -o) having ^ in right lower jaw, behind and in lee with the nnrnial teeth. The tooth is of rather small size, but is regular in jiu-itiun and t'-.r'ii. 15. M., 3, m. S|,eeinien "remarkable t'..i- absence of tinupper right third m. ilar and f.,r absence "f na-al bones, \\hich are gn-atly reduced Variations in position »/tr,-flt. Th..ugh not directly pertaining

to the sul.jeet her,- e,,|lHdered. the following examples'of considerable departure from the normal arrangement may be perhaps usefully introduced in illustration of the peculiar variability of the dentition of the group. *174. A skull from Borneo in the Oxford University Museum (numbered 2043 o) has the following extraordinary arrangement. All the teeth are normal and in place except the second premolar of each side in the upper jaw. On both sides there is a large diastema between pl and «^. The diastema on the left side is of about the same size as the normal second premolar, but that on the right side is considerably too small for a normal tooth. The singularity of this specimen lies in the fact that the missing tooth of the right side is present in the skull, but instead of being in its proper place it stands up from the roof of the mouth within the arcade immediately in front of the right canine and almost exactly on the level of the second incisor, being in the premaxilla, at some distance in front of the maxillary suture.

That this tooth is actually the second premolar which has by some means been shifted into this position there can be no doubt whatever. It has the exact form of the normal second premolar, and is of full size. It stands nearly vertically but is a little inclined towards the outside. The canine is by the growth of this tooth slightly separated from the second incisor, and the first premolar is consequently pushed also somewhat further back. Hence it happens that the diastema for the second premolar on the right side is not of full size. This should be understood, as it might otherwise be imagined that the contraction was due to a complementary increase in the size of the other teeth, of which there is no evidence.

On the left side of the palate there was a very slight elevation at a point homologous and symmetrical with that at which the second premolar of the right side was placed. As it seemed possible that the missing tooth of the left side might be concealed beneath this elevation, a small piece of bone was here cut away, with the result that a tooth of about the same size and formation as Fj was found imbedded in the bone. In this case therefore the second premolar of the right side and of the left side have travelled away from their proper positions and taken up new and symmetrical positions in the palate, anterior to the canines. The facts of this case go to shew that the germ of a tooth contains within itself all the elements necessary to its development into its own true form, provided of course that nutrition is unrestricted. This might no doubt be reasonably expected ; but since the forms of organs and of teeth in particular are by some attributed to the mechanical effects of growth under mutual pressure, it may be well to call special attention to this case, which goes far to disprove such a view.

175. Specimen having the teeth of the two sides in the lower jaw in extraordinarily asymmetrical disposition. The bone of the jaw does i,.,r m t.» have been broken, but there appears to have been disease IT i- displaced l..i.-k wards and lies to the outer side of the tirst premoln. wlii.-Ii it ha- j.u.-du'd towards the middle line." C. S. M.. 41 (see 177. < -i whi.-h upper right canine occupies a position within and on a level with the tirst premolar, which is pushed outwards. C. S. M.,

»178 Specimen having on right side in upper jaw a very small square tool h behind /// ', in the arcade (Fig. 35) ; and in the left upper II. A n«inn;il CuiiH]' mx.ee of approximately the same size. jaw .-in .-mpty aUvolus in the similar plact-, shewing clearly that -imilar tu..th lias lic.'ii j.iv-,-nt : lower jaw iiurmal. C. S. M., 1. 17 ' Sp.-cinn-ii in which ti-fth all ;_;<>nc', but alveoli exist behind th'isi- i.f tinnormal ti-rth on both sides in upper jaw, and there is little doubt that there was here a fourth molar on each side.

S|ieeiiiien in \\liieh teeth all goiii-, luit alveoli shew clearly that theiv was a toiirtli upper molar on right side; evidence on left side inconclusive : lower jaw gone. <\ S. M.. 12. Specimen ,,\' '/'. calvus having an extra ,7"' in lower jaw on right side. Tins to..th is about one ijuarter of' the size of m?, ie-,-mb|iiIL>r that in case No. 17s. Thi^ -pecimen is in the private collection . .(' I'IM)'. MILM: KDWAKDS, who was so kind as to shew it to mi- Gorilla savagei i < i-rilla). Normal a«!ult -kullseen, "'•"'.

*182. Specimen having m* behind and in series with the others on both sides in lower jaw and on right side in upper jaw. On left side both teeth are square and somewhat worn, but the right m* is a curious conical tooth. Gallery of P. M., A, 505, described by GERVAIS, P., Journ. de ZooL, in. p. 164. PL 183. Two cases of four molars in each upper jaw. MAGITOT, Anom. du *yst. <:lent., p. 100, PL v. tig. 8. [Of these one is in collection of Dr Auzoux; the other is No. 121 in P. M., but as I did not see it when examining the collection it is not reckoned in the statistics given above.]

184. Specimen having supernumerary molar which had not quite pierced bone [no statement as to position]. WYMAN, JEFFRIES, Proc. Boston T. //. £., v. p. 160. 185. Specimen having extra molar in crypt on each side in upper jaw behind w». L. M., 3. *186. Fully adult male from Congo having an extra incisor in lower jaw. There are thus five incisors in lower jaw (Fig. 36), of which FIG. 36. Lower incisors and canines of Gorilla No. 186. x, y and z are three central incisors. The upper figure shews the tooth y as seen from the side. (Specimen in Coll. Sunj. Mas., 21, A.)