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

Some conception of the magnitude and range of Variation found in single species of Birds may be gained by reference to the beautiful researches of FURBRINGER \ A table is given by Flirbringer, shewing the number and serial position of the spinal nerves which take part in the formation of the brachial plexus in 67 species of 1 Furbringer's memoirs are of such magnitude ami completeness that I have felt it to be somewhat of an impertinence to attempt to make selection from them ; and it must be remembered that from the isolated and typical cases here given, only a distorted view of the evidence can be gained. As regards this subject, therefore, reference to the original work is especially needed.

I'.ndinvestigated by himself. He also gives particulars of the individual variations which were found in certain cases. From thitable tl».- following statement is compiled, shewing the most iin|,'.r!aii! diversities met with and the instances of individual \ a- i-i:iii,,n. In tinmajority of cases the most posterior spinal nerve ,,f the cervical region was the most posterior nerve of the brachial ple\u-. l.ut in a certain number of cases it does not join the plexus ,il : in some other cases the anterior spinal nerve of the dorsal

on al-o takes |> -u-t in forming the plexus. As the table shews, of these plan* has been likewise met with as an individual \anat i"ii Furbringer's table shews 3 as the minimum number of spinal n. TM- found taking p;irt in the formation of the plexus of any bird ininiinuni by other observers iii other birds (v. FlJBBKINGER, p. 24-2. -ome specimenof <'»liiiii1>n. The plexus is generally formed by 4 In cases where -everal individuals were examined, individual variation was generally found, as in A user, Po<l<ir<j«x, /V/'/.s, Gecinus and Garrvlus ; in these '-uses the number of spinal nerves whieh took part in forming the bra.-hial plexus varied between 4 and "), while in Cnlu ,,/lm, the number even varied between 4 and (i.

Variations also ,,,-eiirred in this iv-pe.-t between the t wo sides ,,!' tl,.- b.nlv. 1'or example, in a specimen of Aimer cineim* the plexu^ watermed on the ri^ht side by the nerves XVI, XVII, \ V I 1 1 and XIX while mi the left side it received a strand from the X Xth nerve iii addition to these. A- has been Mated, the lu.st cervical nerve i- -vnerally the last Derve -uppl\in;4 the brachial jilexns but deviations from this plan occur in both directions. These deviations may occur as individual \ar;at i.m> and they may even be unilateral. «.w in^ to the ti'ausit ion between the cervical and dorsal vertebra' being effected al different

Particularar< given respecting the average proportions of the several rootin the different arrangement^, but the arrangement Or -i/'- of the roots r.-latively to each ..ther was not found to bear anv constant relation either to the sv-tematic position of the bird, or to it- -i/.e. or to itcapacity for flight. It was however generally found that there was a certain relation between the relative si/e of the rootan. I the length of the neck in birds with a plexus Coiu- p..~ed o| IMIH- roots. In this case the greatest thickness was generally either in or anterior to the middle roots of the plexus in short- n.-cked bird-, but posterior to the middle .,f the plexus in longnecked bird-, but even this rule was imt at all closely observed and many except ionoccurred. Fi i; r,i;i M; 1:1;, /. <•. p. 24-S.

In \ariation in the ordinal positions (lf the spinal nerves com- pM-ing the plexu-. tinpattern of the plexus as newly constituted commonly bore a resemblance to the original pattern of the plexus, a phenomenon which FtlRBKINGER has called "imitatory Homodynamy " or " Parhomology " of the plexus1 (/. c. p. 245). Correlation between the constitution of the brachial plexus and the position and number of moveable cervical ribs. 65. Anser cinereus, var. domestica. Upon this point Flirbringer has made a series of important observations, especially in the Goose, which enabled him to state that there is, within limits, a certain correlation between the composition of the brachial plexus and the development of the ribs of this region. Speaking generally, those individuals in which the plexus was formed in a more anterior position usually shewed a fairly developed cervical rib on the 18th vertebra (Anser), and even as in Fig. 12, I, a very short but moveable rib on the 17th vertebra ; and in such cases the 19th vertebra generally bore the first true sternal rib. On the other hand, examples with a more posterior development of the brachial plexus shewed not only an entire absence of moveable ribs on the 17th, but even a considerable reduction in the size of the ribs of the 18th and 19th vertebrae, so that these became "transitional" in character, leaving the 20th vertebra as the first vertebra bearing

FIG. 12. Diagrams of brachial plexus and cervical ribs in two Geese (Aiiser cinereus, var. domestica) after Fiirbringer (being his specimens D, left, and G, rir>lit). I. Case in which the 17th and 18th vertebrae bear cervical ribs and the 19th bears the first sternal rib. II. Case in which the 17th and 18th vertebras bear cervical ribs, and the 20th bears the first sternal rib. ax axillaris, bri brachialis longus inferior, brs brachialis longus superior, cbri coraco-brachialis internus, ci cutaneus brachii inferior, cs cutaneus brachii superior, i'c iutercostals, Id latissimus dorsi, p pectoralis, r/* rhomboideus, sbsc subscapulares, srpr nerves to levator scapulas and serratus profuudus, srsp nerves to serratus superficialis, stc sterno-coracoideus.

1 The principle denoted by these expressions is nearly the same as that here expressed in the term Hornceosis, which is perhaps more convenient as being a more inclusive expression. iru. -:. inal ribs (Fig. 12, II.). The measurements are given by l-'i i;i:i:isi.i.i: tor 7 specimens, of which those relating to two ex- TM -me cases (here figured) are appended. 60. l-\ <-'.in|i:iri>itii of specimens of the Pigeon, Columba livia, var. domestica, .1 Hiuilar correlation was found to occur, as shewn in KILT. 1') I. ami 1 1. 1 Fiirbringer's specimens A and E).

67. The same correlation was established in the case of the Jay Garrulus glandarius, but an actual variation in the number of moveable cervical ribs is not recorded in this species (see Fig. 14, I. and II., Flirbringer's specimens A and D). FURBRINGER, M., Morpl. JctJirb., 1879, v. p. 375. FIG. 14. Diagrams of the cervical ribs and brachial plexus in two Jays (Garrulus glandarius) after Fiirbringer. I. Case in which the brachial plexus began from the xith nerve, the cervical ribs of 13th and 14th vertebra being longer than in II, a case in which the xuth is the first nerve contributing to the brachial plexus. Letters as in Fig. 12.

The measurements of the two specimens here figured were as follows : Ribs of 15th vert, (with sterno-costal parts), length in rnm. But though this correlation between the nerves and the ribs is on the whole decided and unequivocal, it should be explicitly stated that it only occurs within certain limits and is not universal, and this statement of correlation is far from covering the whole FURBRINGER, /. c. p. 387. *68. Man and other Mammals. By minute dissection of the brachial plexus in fifty-five subjects (32 foetal and 23 adult) HERRINGHAM obtained important evidence as to the parts supplied by the fibres of the several spinal roots forming the plexus, and as to the considerable variation which occurs in respect of this supply. Of the facts thus arrived at, two examples may be quoted

in illustration, concerning the composition of the median and ulnar IM-I jpectively. Tli.- ni'i/iiin is formed by two he-ads from the plexus; into the outer h.ad ili«- Vlth and Vl'lih spinals enter, while the inner is t..im.-d bv branches of the VHIth and IXth, sometimes with the ailditioii of some bundles of the Vllth. The presence of film - froin ill-' \'llth depends on whether the anterior branch of the VI Ith bifurcates, «.r goes wholly to the anterior (outer) cord of the pl.-xu-. In order bo - e whether both Vlllth and IXth contribute

the median, twenty-eight dissections were made, fourteen in infants, fourteen in adults. In one foetus and in one adult no branch tn-m the JXth was found, these being the only except inns the rule that b..th VI lMi and IXth send fibres to the median 'I'll.- median i- ill' n made of the Vlth, Vllth, Vlllth and I \th. but the-,- roots <lo not send to it a constant proportion. The bund!.- fr..m ill.- Vlth varit-s little, that from the Vllth varies con-id.Tablv. that i'r..in tin- Vlllth is sometimes equal to, sometimes smaller, and -.>in> -i mies larger than the bundle from the IXth.

Tinorigin of ill.- nliKir n.r\e was traced in thirty-two cases, fourteen being adult-. It \\a< fuiind to arise in four differ, i it \\ays. M..-r .-.1111111. .lily it arose tr<>in tin \'lllth and IXth: this occurred in tuenty-thi. .- cases. With the Vlllth and IXth is sometimes combined a -trand from tin- \'llth, as shewn in five cases (four fn-tal. '.inadult). In thr.-r t'u-tal cases it arose from th'- \" I 1 1 1 h niilv. and in mi'- fo-tal and one adult case from the Vllth and VI I Ith. The Vllth is only added t<> the idnar in some of those -in \\hi.-h it gives a branch to the posterior (inner) cord of the pi. \n-. In several cases the branch t'n .m the X'lIIth was much larger than thai lr..ni the IXth, but the reverse \vas never met with.

K\ ill. -nee similar \« the above is ^i\en respect in-' other nerves tr..m the bra.-hial jilexu-. Ft. .m the re-idtof t he in\e>| igat i«.n ;_;< in rally, it a]i]ieared that the ran^e of N'anatioii tlnnigh considerable was ii"t extravagant, and that uln-n [.arts, usually sii].]ili.d by sdne given nerve root, an- -ii|.|,li. d by some oth.-r n<»\. this otln-r rod i.- then eith. r the one aiiieriitr or the one |>.»teri..r \» the rod lidn \\hich the sit]>]>lv Dormally cornea Sum.- muscl. - seemed to bear definite relations to each other and their nerve >u|>|ily seemed also "to \arv solidly,"

their Derve .-u|.|.lieremaining the same ivlativelv to each other, though d. ii\.d from a different root. "The besl example of this is in the three miis.-les which are attached along the inner side of the bicijiital groove, lh.- siibsca|.ularis, t.-r. - major, and latissimus d.'i'si. The tn-t i- n-iially sU].].li.-d b\ the Vth and Vlth, the Second b\ the \'|th, and the last by the Vllth, and however much th.-y max \ary above and be]..\v their t\pical place they do not change their relations to .a.-h other. A similar relation exists between the t\\.. supinatoi-s and the two radial extensors. rJ'he.s.-

last are sometimes supplied by the Vlth, sometimes by the Vllth, but they are never in any case placed above the supinators. These are always supplied by the Vlth alone. The flexor group in the forearm show a similar fixed relation." Herringham concludes that " the nerve roots are not always composed of the same fibres, but that what is in one case the lower bundle of the Vth may be in another the upper bundle of the Vlth, and what is now the upper bundle of the VHIth will at another time be the lower of the Vllth root." Hence the following principle is enuntiated : "Any given fibre may alter its position relative to the vertebral column, but will maintain its position relative to other fibres."

By physiological methods, SHERRINGTON working chiefly on Macacus, but on other animals also, found that this principle substantially holds good for the outflow of fibres throughout considerable regions of the cord, but that it is not always applicable to great lengths of the cord, for the brachial plexus may be constituted in a region which is near the head end in comparison with the place of origin in other individuals, while in the same individual the sciatic plexus may be constituted in a region which is for it comparatively far back. No exception to the principle was found in the sense that a given efferent fibre which in one individual is anterior to some other particular fibre is ever in any individual of the same species posterior to it. SHERRINGTON, C. S., Proc. Roy. Soc., LI. 1892, p. 76. This principle of Herringham's is analogous to that which in the much simpler case of Variation in vertebra was pointed out on p. 107. It was stated that in such Homceotic variation no gaps are left. If a vertebra assumes a cervical character, it is the 1st dorsal, and so on.

The following noteworthy case is described by HERRINGHAM in an infant. It should be borne in mind that to a normal brachial plexus the I Vth nerve gives a small communication, the Vth, Vlth, Vllth, VIII th and IXth give large cords, while the Xth (or Ilnd dorsal) gives a minute fibre only. In this abnormal specimen, on the left side the part from the Xth was as large as that from the IXth, and this was as large as the VHIth, whereas the natural proportion of Vlllth to IXth is about 2 to 1. The musculo-cutaneous received from the Vllth, instead of from the Vth and Vlth only as more commonly found ; the median received no Vlth (v. supra) ; the teres major was supplied by the Vllth alone, instead of by the Vlth ; the circumflex received from the Vllth, instead of Vth and Vlth alone as seen in 43 cases without any other exception ; the musculo-spiral was formed by the Vllth, VIII th and IXth, instead of by the Vlth, Vllth and Vlllth (and sometimes even Vth) ; the deep branch in the hand received from both Vlllth

IXth (instead of Vlllrh alone, as seen in five cases outof six). But 1 !i"U_di in all these respects the nerve-supply of the plexus was in ordinal position posterior to the normal, nevertheless the 1\ th • a cMinmunieation to the Vth (as it does normally) and the Buprascapular and subscapular were given off normally. Here, then, the -upply to the plexus began at the normal place, though it extended further back than it normally does. On the right side the branch from th- Xth was slightly bigger than usual, but otherwise

th dy abnormality noted was that the IXth sent a branch to the P »•:;.".. In \iew of FURBRINGEE'S evidence (see Nos. 65 and (i~), it iiii^lit be expected that the first rib would be reduced in correlation with the irregular forward Homceosis of the nerves. In replv however to a question on the subject, Dr Herringham has kindly informed me that no abnormality in the ribs was seen, but thai this point was not specially considered. ( 'oinpare also L\\i.s case, No. '24, in which similarly a large branch from the Xth joined the plexus on the right side and the firs) rib was rudimentary, both structures thus shewing a correlated forward I b >mO3< isis.

*70. r,\ ph\ -i"l'. -leal methods SHERRINGTON found that the supply to i iie lumbo-sacral jilcxus \aried considerably with regard to its origin from the spinal nerves. This was seen in Macacus, in the 1 and in the Frog. In none of these animals was any one arran-'-nient found sufficient ly often to justify its selection as a"normal type. In each case it was found convenient to divide the dit'f. rent forms of arrangement into two classes, the one in which the -upply to the pi. AIIS uas in ordinal position more anterior ("pie-axial." Sherrini;toni. tl ther being more posterior ("postaxial." Sherrin-toii ». Particulars respecting the distribution of the ial nerves and the movements resulting from their stimulation in the two classes, are ^\\<-\\ in detail (i|. v.). In Macacus, 31 in- «li\idua|s bel'.n^ed to the nioiv a 1 1 1 e i' i , ,]• ( • la ss, and 21 to the more

posterior. In the ( 'at the number of individuals in the two classes was -- and :!!l respectively. It is stated generally that •'I'll.- iliMrilnitioii of tinperipheral nerve t nniks is not obviously iliH'rivnt, \\liether, I »y iis rooi f«,niiatiitii the plexus In-long to the pre-axial class, or to the p.-i axial. The peripheral nerve-trunks are, D o-.l- th.-ir OlUScleS, relatively stable in eomp.-iri.son with the spinal !••• \\"hen the inin-rvation of the liiiili-inuseles is of the pre-axial

. 0 also i- that ..t' the anus, vagina and bladder; and conversely/' 71. Primates. Si ..... • m examining the tacts of Variation we are g for evidence as t,, the m..des in which specific differences originate, allusion may therefore lie made to some facts of normal structure in differing forms in illustration of the nature of such differences, and for comparison with the differences which are seen to occur by Variation. The arrangement of the lumbo-sacral plexus in the Primates well exemplifies some of these points. In Man, Chimpanzee and Gorilla the 1st sacral vertebra is the 25th ; in the Orang it is the 26th ; in the Baboons, e.g. Macacus inuus (=Inuus pit/teens Is. Geoff., the Barbary Ape) it is the 27th. Now, as Rosenberg says, seeing that in Man the sacral plexus receives one whole prse-sacral root, the XXVth, and part of the XXIVth, it might be supposed that this plexus in the Orang would receive two whole pras-sacral roots and part of a third, or that in Macacus it would receive three prae-sacral roots and part of a fourth. But, as a matter of fact, in each of these forms, Chimpanzee, Orang and Macacus, according to Rosenberg, only one whole prae-sacral root and part of the next above it enter the sacral plexus, just as in Man, though the ordinal positions of the nerve-roots are different.

The Chimpanzee, however, which Rosenberg examined, was the specimen described (No. 34), having the 25th as a transitional lumbo-sacral vertebra, and rudimentary ribs on the 21st. In this specimen the prag-sacral nerves received by the sacral plexus were the XXVIth and part of the XXVth, thus bearing the same ordinal relations to the sacrum that the nerves of the lumbo-sacral cord do in the other forms and in Man, though each is ordinally one lower in the whole series than it is in Man. The same was true of the spinal roots composing the obturator and crural. ROSENBERG, E., Morph. Jahrb., i. 1876, pp. 148, 149 and Tables, note 19.

This case is interesting as an example of forward Homceosis in the vertebrae associated with forward Homoeosis in the sacral plexus. When compared with the following case of a Chimpanzee1 having normal lumbo-sacral vertebra?, several discrepancies will be seen beyond those which can be accounted for by the single change of one in the ordinal position of the roots. No doubt for the larger nerves Rosenberg's account is correct, but as he states that the specimen was so badly preserved that the nerves could not be satisfactorily traced, it is possible that some of the branches may have been missed. However this may be, the specimen dissected by Champneys had important features of difference, notably that the sacral plexus received from the XXIInd spinal, while the highest recorded as entering it in Rosenberg s case was the XXVth, a greater difference than can be accounted for on the simple hypothesis of a change of one place throughout. Though, speaking generally, Rosenberg is right in saying that the evidence of the normal condition in Macacus and Orang as compared with each other and with Man 1 CHAMPNEYS, F., Journ. A nut. Phijs., Ser. 2, v. 1872, p. 176.

;|iat tinvariation of the vertebral regions goes hand in hand \\itli that t>f the plexus, and though a comparison be- t\\.i ii ll'i-i-nherg's al)iionnal Chimpanzee with that dissected by ( 'liampii'-v- lar-i-ly bears out this suggestion, yet it is also clear that tin- e..nv|ation is not a precise one, as indeed has already a | >| I. -a i. -i I in several instances. In ;_ri\iir_r the compositions of the several nerves of the lumbo- }» \-^ '•!' the nerves in the whole series for simplicity of comparison.

li will In- remembered that a Chimpanzee has one pair of ribs in. MI- than Man. tin- XX 1st nerve is the 1st lumbar in Man, but i- i In- loth dursal iii Chimpanzee, the XXVIth nerve being the !-• sacral in l'"th forms. The table given shews, as Champneys similar to that in Man, but that the nerves are very differently 'I'll'' "li-m of tinnerves is therefore in several cases lower in Man than in the Chimpanzee, although in the absence of ribs "ii 'I"- -<>th vertebra Man shews a character which, as compared with tinpresence of ribs in this position in the Chimpanzee represents a backward 1 1> >mo'o.-.is.

Man. With the l'..re--oin'4. compare the case mention. -d above (No. -VJi in \\lii.-h two entire Inmliar nerves joined the sacral pl'Aii> in a human subject having no ril.s on the 19th vertebra, ~-. I''""' int'oniiatinn as bo the \ariatiuns of the luml>o-s;uTal plexus in I" one of these tin- Hi. ..f the i:iih dorsal ii'iMh vertebra) \\°is nut il.-Nrlnp,.,!, this vertebra I.einformed as a lumbar and thus itself sh'-w in- a I. . 'i. -k ward ||, ...... ^is ii, currelation to that of the nerves

(vox JHERIXG, I. c. p. 182, pi. iv. tig. 2). Descriptions and diagrams of similar cases are given throughout the work, but as some of them represent specimens described by others (e.g. STRUTHERS and ROSEN- BERG) originally without diagrams, it is difficult to know how far the accounts given are schematic. For this reason reference to the original work must be made. *73. Bradypodidae. II rachial plexus. As examples of normal differences the Sloths are especially interesting, but unfortunately an extended investigation of the nerves in several individuals has not been made. The results found by SOLGER relate to one specimen of B. tridactylus and one of C. didactylus. The latter was a perfect specimen, but the former had been partially dissected and the details of the nerves were largely imperfect. The Cholcapus was a specimen with seven cervicals, and the Bradypus had nine, the last bearing rudi-

FIG. 15. Diagrams shewing the composition of the brachial plexus in I. a Cholcepus, II. a Bradypus. v1 — t;11, the vertebra. IV, VII, X, XII, fourth, seventh, tenth and twelfth cervical nerves. A, dorsal cord. B, ventral cord, a, phrenic, b, dorsalis scapulae, c, suprascapular. d, subscapular. rnentary ribs. As the figure shews (Fig. 15), there was a close but not a perfect resemblance between the composition of the plexus in the two cases, that of Bradypus being in nearly each case two roots lower than that in Cholo&pus. In the latter the IVth nerve gave a branch to the Vth, but whether in Bradypus the Vlth gave a branch to the Vllth was not determined with certainty owing to the condition of the specimen. [For details see original paper] SOLGER, B., Morph. Jahrb., 1875, i. p. 199, PL vi.