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

The determination of the species is quite uncertain. Welcker in his analysis does not divide the species of Bradypus. In the other cases I have simply taken the name given on the labels. As regards Choloepus the confusion of species is much to be regretted, for according to the received account1 the more northern species, C. hoff'manni, has only 6 cervicals, while C. didactyliis has 7. In the table it will be seen that four specimens in different places have C 6, though generally marked C. didaotylus. Possibly some or all of these are C. hoffmanni, and I have therefore entered them as Choloepus sp. In the case of Bradypus it has not been alleged that the number of cervicals characterizes particular species, so the fact that the species are confused is of less consequence.

l 3 specimens from Brazil said to have 8 cervicals. No detailed account 4 DE BLAINVILLE, Osteogr., Fsc. v., pp. 27, 28 and 64. In the place cited, de Blainville gives C 9, D 16, L 3, S 6, C 9—11 as the normal, but he does not say in how many specimens this formula was seen. I have therefore been unable to tabulate this observation. It will be seen that D 16 is quite exceptional, but as it occurred in the Coll. Surg. specimen no. 3422 it was -described by OWEN as the normal, and this statement has been copied by many authors, perhaps by de Blainville.

"ill-. errical vi [n another of these specimens then1 is a cervical rib On this evidence several comments suggest themselves. First it should be noted that the Bradypodidae strikingly exemplify the principle which Darwin has expressed, that forms which have an exceptional structure often shew an exceptional frequency of Variation. Among Mammals the Sloths are peculiar in having a number of cervicals other than 7, and from the tables given it will be seen that both the range and the frequency of numerical Variation is in them very great, not only as regards the cervicals, but as regards the vertebrae generally.

As concerning the correlation between Variation in the several regions, WELCKER points out that his results go to shew that there is such a relation, and that when the sacrum is far back, the ribs also begin further back, or at least are less developed on the cervicals. As he puts it. with a long trunk there is a long neck. This is a very remarkable conclusion, and it must be admitted that it is, to some extent, borne out by the additional cases given above. The connexion, however, is very irregular. For instance, the Cambridge specimen of Bradypus, though the 29th is the 1st sacral, has had cervical ribs of good size on the 9th vertebra, and even has a small one on the 8th. But taking the whole list together, Welcker's generalization agrees with the great majority of cases. Expressed in the terms defined above, we may therefore say that backward Homceosis of the lumbar segments is generally, though not quite always, correlated with backward Homceosis of the cervicals, and vice versa.

It will be seen further that this Variation concerns every region of the spine, and that even in the total number of prae-sacral vertebrae there is a wide range of variation, viz. from 27 to 29 in Bradypus (52 specimens) and from 30 to 34* in Cholcepus (20 specimens). Perhaps no domestic mammal shews a frequency of variation in the fundamental number of segments comparable with this. In this connexion it may be observed that the absolute number of dorso-lumbars in Cholcepus (25 — 27) is exceptionally large amongst mammals ; but this is not the case in Bradypus.

If the case of Bradypus stood alone, some would of course recognize the occtirrence of cervical ribs on the 9th and 8th vertebrae as an example of atavism, or return to the normal mammalian form with 7 cervicals. The occurrence of normal ribs on the 7th in Choloepus and the occasional presence of cervical ribs on the 6th vertebra in this form, even reaching nearly to the sternum as in Welcker's Leipzig case, obviate the discussion of this hypothesis.

We have, then, in the Bradypodidae an example of mammals in which the vertebrae undergo great Variation as regards both their total number and their regional distribution. As the tables shew this is no trifling thing, concerning merely the number of the caudal vertebrae, the detachment of epiphyses \vhich may then be called ribs. ,,r some other equivocal character, but on the contrary it el i" -id. -s changes in the number of prae-sacral segments

that i- of large portions of the body, each with their proper -u|i|.l\ of nerves, vessels and the like, producing material change in th.- mechanics and economy of the whole body: this moreover in wild animah, struggling for their own lives, depending for their -teneton the perfection and fitness of their bodily organization. Th.- following cases, though few, have an interest as exemplifying \.-rteliral Variation in another Order. Id. Felis doraestica. In all the skeletons of FELIU.E that I have

mined the formula is C 7, D 1 :i, L 7, S 3. A specimen of the domestic Cat ha\in^ C 7, I> II, L7 is described by Struthers. The chain:.- of aiticiilar processes from dorsal to lumbar was completed between the l>th and L 9th vertebra but the posterior zygapophyses of the 17th, though of the dorsal type, have to some extent the characters ..f .1 t ran-it ion joint. A- is -tat«-d below, the change in the domestic Cat iiormallv occur- h.-t \\i-i-n the 1 7th and 18th. I n this case therefore \\ith increa-e in niiinlicrs of ribs the position of the articular change

p. 'il, .V«'- . hut the description there given differs in some respects from that -tated alio\e. which i- taken from a letter kindly written l>v l'rnfe--,ir Struthcrin an-u.-r tn mv inquiries. There i- h.-re forward lloiinro-i-, in the development of ribs on the •_' 1 i vertebra, in the alteration iii position of the articular change, and in the fad that the L'Sth i- not united to the sacrum. \ een in -..ni.- other cases, then-fore, with forward Homoeosis the

numher of pne sacral \erteln-;e is increased ; hut as usual owing to the ei|iii\ncal nature of caudal vertehr.-e it is not possible to state that tintotal numher of \eitehrais greater. (•!!. Canis familiaris. Case of ,-.-r\ieal ril> mi left side borne by 7th • •epical. This rili \\.i I in. |oi,.r and articulated with a tubercular elevation on the 1 -t thoracic rih of the same side. The remaining rili- and \ .-rt ehrawere normal. full\ described] CUTHKU, W., A ,-<•!(.

[In connexion \\ it h t he foreifoin^ nli-,T\ at imis it mav he inent inm-d that the arti-'idar chain.'.- does imt take place in the same place in all above described; but in 4 F. domestica, and 2 F. catus in Edinburgh, I F. domestica, I F. catus, 1 F. concolor and 1 Cyncelurus jub<ttn* in Cambridge the change is between the 17th and 18th. Fur information as to the Edinburgh specimens, I am indebted to Professor 50. Galictis vittata. Specimen from ParanA had 10 pairs of ribs,

I 1 true and 5 false ; 5 lumbar, 2 sacral and 2 1 caudal vertebra?. A specimen from Brazil had only 15 pairs of ribs and the same number of lumbar and sacral vertebrae. BURMEISTER, Reise durch d. La Plata-Staaten, Halle, 1861, n. p. 409. [This is therefore another case of forward Homceosis, (as manifested in the presence of an additional pair of ribs) associated with an increase in the number of prajsacral vertebrse.] 51. Halichoerus grypus. Phocidse generally have C 7, D 15, L 5. Specimen of //. yrypus having C 7, D 15, L 6 at Berlin. The anterior of the six lumbars bears a rudimentary rib about 5 cm. in length on the left side. The 28th vertebra is here detached from the sacrum giving S 3, but generally it is united to it, giving S 4. NEHRING, A., Sitzb. naturf. Fr. Berlin,, 1883, pp. 121 and 122. There is here therefore a forward Homceosis in the development of a rib on the 23rd, and also in the detachment of the 28th from the sacrum.

52. -M-1' Boulenger kindly informs me that though the number of ventral shields (which is the same as that of the vertebrae) is as a rule very variable in the several species of Snakes as a whole, there is nevertheless great difference in the degree of variability. A case of maximum variation is that of Polyodontophis subpunctatus, in which the number of ventral shields has been observed to vary from 151 to 240 (BOULENGER , Fauna of Brit. India; Reptilia etc. 18'JO, p. 303).

53. On the other hand the range of variation in Tropidonotus natrix is unusually small. Among 141 specimens examined the number of ventral shields varied from 162 to 190 (SxRAUCH, Mem. Ac. Sci. Pet., 1873, XXL, No. 4, pp. 142 and 144). *54. Gavialis gangeticus. In this animal there are normally present 24 pra?sacral vertebras and 2 sacrals, the first caudal being the 27th. This vertebra has a peculiar form, being biconvex. Specimen described having 25 pnesacrals, 2 sacrals, the 28th being the first caudal. BAUR, G., J. of Morp/i., iv., 1891, p. 334. In this case Baur argues that since the first caudal is clearly, recognizable by its peculiar shape, this vertebra must be " homologous " in the two specimens and he considers that a vertebra must have been " intercalated " at some point anterior to the first caudal by a process similar to that seen in Python (see No. 7). In his judgment this has occurred between the 9th and 10th vertebra?, but no reason for this view is given. On the system here adopted, this would be spoken of as a case of forward Homceosis.

55. Heloderma. The first caudal in the normal form may be distinguished by having a perforation in the small rib connected with it. In thi> it is peculiar. Four specimens shewed the following arrangement II . Lori-ilium N". 1. First caudal is the 36th vertebra (Troschel). a\i- nii'l thinl \. it. lira bore tubercles upon the transverse progapophyses, and in ihar its centrum presented two concavities 11 '• 'I. Vertebral colomna of Frog (JZana temporaria), after BOUKKB.

Jahrb., ivj, ,i,. ,,. 818, appeared too Int.- t.. permit me to incorporate the valuabli I icta it contains. for articulation with a tenth vertebra. The right zygapophysis was well formed and articulated with the tenth, but the left was rudimentary. The tenth vertebra itself had an imperfect centrum and the neural arch though complete was markedly asymmetrical. Posteriorly its centrum presented two convexities for articulation with the urostyle. [For details see original figures.] BOURNE, A. G., Quart. J. Micr. Sci., xxiv. 1884, p. 87.

This is a case of some importance as exhibiting Meristic Variation in a simple form. Of course, as Bourne says, we may say that in this specimen the end of the urostyle has been segmented off and that it is composed of " potential " vertebra?, and as he also remarks, it is interesting in this connexion to notice that some Anura, e.g. Discoylossus, present one or two pairs of transverse processes placed one behind the other at the proximal end of the urostyle. But this description is still some way from expressing all that has happened in this case ; for beyond the separation of a tenth segment from the general mass of the urostyle there is Substantive Variation in the ninth vertebra in correlation with this Meristic Variation. For the ninth has developed a zygapophysis and has two concavities behind, like the vertebra? which in the normal frog are anterior to the ninth. There is therefore a forward Homoeosis, associated with an increase in number of segments, just as there is in such a case as that of Man (No. 26) or in that of Galictis vittata (No. 50).

It is also interesting in this case to see that the actually last free vertebra here, though it is the 10th, has two convex articular surfaces behind like the 9th, which is the last in the normal frog, thus shewing a similar forward Homoeosis. Now applying the ordinary conception of Homology to this case, we may, as Bourne says, prove that the 9th in it is homologous with the 9th in a normal frog for its transverse processes are enlarged in the characteristic manner to carry the pelvic girdle. But similarly we may prove also that the tenth in this case is homologous with the ninth of the normal, for its centrum has the peculiar convexities characterizing the last free vertebra, Baur's proof that the first caudal was homologous in the two specimens of Gavialis (see No. 54) rested on the same class of evidence, and for the moment is satisfying, but as here seen this method though so long established leads to a dead-lock. Upon this case it may be well to lay some stress, for the issues raised are here so easily seen. Besides this the imperfect condition of the extra vertebra enables us to see the phenomenon of increase in a transitional state, a condition rarely found. In the instances recorded in Gavialis (No. 54), owing to the perfection and completeness of the variation, the characters of the 1st caudal are definitely present in the 28th though normally proper to the 27th, and therefore it may be argued that the 28th here is the 27th of the type. The frog here described shews that in this conclusion other possibilities are not met. On the analogy

of several cases already given, it is not impossible that it' the variation -<•• -n in this frog had gone further, the 10th vertebra iiii^lit alone support the ilium (cp. Nos. 57 and GO) and thus pre-.-nt the characters of the normal 9th in their completeness. Jf thichange had taken place, we should have a case like that of 1,'i'i'ilin, and there would be nothing to shew that the new loth •• l.ra was not the 9th of the normal. The truth then seems to I- that owing to the correlation between Meristic Variation producing change in number, and simultaneous Substantive Variation producing a change of form or rather a redistribution of characters, tinattempt to trace individual homologies must necessarily fail ; t«-r while such determination must be based either on ordinal P»Mtion or on structural differentiation, neither of these criterions are really sound. As I have tried to shew, the belief that they are SO depends rather on preconception than on the facts of Variation.

d. -eril.ed t,\ Ho\\i:s. In this case the 9th had a posterior isveree process of the 9th was not larger than that of the 8th and did not >upport tinilium, which on the left side was entin-lv 1-orii'- 1-y tiplarge transverse proct-ss of the 10th. On the right -id<- the tran.-\er-e processes of both 9th and 10th were developed to -uiiport th'- ilium, neither being in it-elf so large as that of the loth on tipleft side. Tin- !Mh \\a- concave in front instead of convex a- ii-ual. and thus tin- Mh which i> normally biconcave is ci.nv-x In-hind. Tinposterior t'aci-s of l»otli !Uh and 10th bore two convexities Midi as .-innormal to the !»th. The nrostvle was normal, having \\.-ll-dev.-loped aperture^ for exit uf the last pair

In thicase th.- d.-partur«- from the normal, exemplified by No. .".»;, lia> gone -till furtln-r, and the new 10th vertebra bears the ilium wholly ,>\\ th<- 1- tt *[<{<• and in part on the right. The condition U thuagain int.-rmcdiat.- between the normal and a complct.- traii-torination ot tin- !»th into a trunk vertebra and the introduction .,(' a 10th to b.-ar the ilium (as in Xo. GO). As regard > the hoinologi.-s of the vertebrae, the same issues are a-ain raided which w.-n- indicated in regard to No. 56.

Rana teraporaria : Case in \\hi.-h transverse processes were present in tinatlas \.-rtcln-a and the transverse processes of several of the vertebra were al. normal (Fig. 11. [.). The atlas possessed welldeveloped t ran ^\ . •!•>•• pr. •.•••^•-i-v. In i lie axis the transverse processes are direete.l forwards instead Of Kackwank and that of the I. ft -Me presents an indication of bifurtion al M - e\t remit v. which are joined together for two-thirds of their length. The fourth

vertebra presents a transverse process on the right side which is bifurcated at its extremity. The remaining vertebra1, though slightly asymmetrical, present no special peculiarity, except that the neural arch of the ninth vertebra is feebly developed. BOURNE, A. G., Quart. Joura. Micr. Sci., 1884, xxiv., p. 86, Plate. There is here backward Homoeosis of the atlas, the only case of the kind I have met with1. The reduplication of the transverse processes of the third vertebra should be studied in connexion with the cases of double vertebrae in Python (No. 7) and the cases of bitid rib (in Man, No. 12), for they present a variation perhaps intermediate between these two phenomena.

Bombinator igneus. In this form there is a considerable range of variation in the development of the transverse processes for the attachment of the pelvic girdle. 59. GOTTE figures a specimen in which the flat expanded transverse processes have a similar extent on the two sides, but while that on the right side is made up of the processes of the 9th and 10th vertebne (in about the proportions of two to one), that on the left side is entirely formed by the transverse process of the 10th vertebra. GOTTE, Entw. d. Unke, Atlas, PI. xix., tig. 346.

*60. Sardinian specimen figured in which the processes for the attachment of the pelvic girdle seem to be composed entirely by those of the 10th vertebra while those of the 9th are not developed. GENK, J., Mem. Reale Ac. di Torino, S. 2, i., PL v., fig. 4. 61. Specimen figured in which both transverse processes of 9th and of the lUth are almost equally developed to carry the pelvic girdle. CAMEKANO, L., Atti R.Ac. tici. Torino, 1880, \v.,fg. 3.

62. Specimen in which the left transverse process of the 9th bears the pelvic girdle on the left side, and the right transverse process of the 10th bears it on the right side, while the corresponding processes of the opposite sides were not developed. Similar case recorded in Alytes obstetricans by LATASTE, Rev. int. des Sci., in., p. 49, 1879 [not seen, W.B.] ; ibid. Jig. 4. (j3. Specimen in which the transverse processes of the 9th alone were developed to carry pelvic girdle, but the proximal end of the urostyle was laterally expanded more than usual, ibid. p. 7, Jig. 3.

Recapitulation of important features of Variation as seen in the vertebral column. ADOLPHI, L c., p. 352, PI. xn. fig. 3 gives an account of a specimen of Bufo variabilis in which the atlas bore a transverse process on the left side only. In this specimen the first two vertebras were united and their total length was reduced. 4. The frequency of substantial if imperfect bilateral symmetry in the variations, but the occasional occurrence of asymmetry also.

'». The special variability of some types, e.g. Simla xutyrus: the Bradypodidae ; Borribinator igvwus. ('). The evidence iliat this variability may occur without the influence of civilization <>r domestication, I The occasional, though not universal, association of forward Homujosis with increase in number and of backward Homueosis with reduction in number. '1. Tli-- frequent correlation between Variation in several regions, such correlated Variation being sometimes unilateral.

THE spinal nerves compose a Meristic Series in many respects similar to that of the vertebrye. As between the vertebras, so between the spinal nerves, there is differentiation according to the ordinal succession of the members, certain distributions and functions being proper to nerves in certain ordinal positions. The study of the way in which Variation occurs in this series is one of great interest, but unfortunately it is extremely complicated. For while as regards vertebras the distribution of structural differentiation can be recognized on inspection, in the spinal nerves to obtain a true knowledge of the arrangement in any one case physiological investigation or at least elaborate and special methods of dissection are needed. Though it is therefore impossible to introduce any account which should at all adequately represent the great diversity of possible arrangements, it is nevertheless necessary to refer briefly to the chief results attained by these methods and to the principles which have been detected in the Variation of the nerves. It must of course be foreign to our purposes to examine the many diversities of pattern produced by the divisions and anastomoses of nerve-cords in the formation of plexuses, &c., and we must confine our consideration to cases of Variation in the distribution of differentiation among the spinal nerves, that is to say, in the segmentation of the nervous system in so far as it may be judged from the arrangement of spinal nerves.