Imagens das páginas
PDF
ePub
[merged small][graphic][merged small][graphic][merged small][graphic][merged small]

• Continued from p. 69.

t Sec " Lessons in Elementary Anatomy," Lesson II., p. 22.

[merged small][graphic][merged small][graphic][graphic][merged small]

met with in the Frog's class, and these only very rarely and in an imperfectly developed condition.

As has been mentioned, in two kinds of Frog (Ceratophrys and Ephippifer) the skin of the back is furnished with bony plates. These are found in the deeper layer or dermis, and are therefore " scutes."

The remarkable circumstance,however,is that we have here a lower stage (as it were an incipient condition) of that more developed dermal skeleton which exists in tortoises and turtles. In most of these reptiles both the back and the belly are protected by bony plates which adjoin one another, and together form a solid box in which the body is enclosed. Moreover the bony plates of tortoises and turtles are invested by large horny epidermal scales (" tortoise-shell"), which scales do not agree in either size or number with the bony plates on which they are superimposed.

Again, the middle series of bony .plates of the back are continuous with the subjacent joints of the backbones, and the lateral series of dorsal plates are continuous with the ribs beneath them.

There are certain Chelonians, however—"mud-tortoises" —(of the genus Trionyx), which have the dorsal plates

[graphic][graphic]
[ocr errors]
[ocr errors]

Fic. 33.

Fig. 33.—The Axis Vertebra, c, centrum : s, neural spine ; if, tubercular process: /, capitular process; u, anterior articular surface for atlas; z, postrygapophysis; 0, odontoid process; Ay, median vertical ridge beneath centrum. Fic. 33.—The Atlas Vertebra. st rudiment of neural spine; tt, tubercular process ; /, capitular process; a, articular surface for skull ; liy, plate of bone holding the place of a cranium, and articulating with the odontoid process of the axis vertebra.

much less developed and not connected with the ventral plates save by means of soft structures.

Here then we have in reptiles an interesting approximation to the condition we have seen to exist in those exceptional Anourans, Ceratophrys and Ephippifer. Moreover this resemblance is still further increased by the fact that in Trionyx the bony plates are not covered with any tortoise-shell, but are merely invested by soft skin as in the genera of dorsally-shielded Batrachians.

Have we then here a true sign of genetic affinity? Are these tortoises to be deemed the more specially modified descendants of shielded frogs or of some as yet unknown slightly-shielded animals which were the common ancestors both of frogs and tortoises?

Certainly tortoises cannot be the direct descendants of frogs, they agree with all reptiles in characters which are both too numerous and too important to allow such an opinion to be entertained for a moment.

The other opinion is hardly less untenable; for if all the multitudinous species of frogs (together with a number of reptilian forms more closely allied to the tortoise than any frogs are) descended from slightly shielded animals, how comes it that all frogs and toads, save one or two species in no other way peculiar, have every one of them lost every trace of such shielded structure which nevertheless cannot easily be conceived to have been in any way prejudicial to their existence and survival?

On the other hand, it cannot but strike us with surprise that structures so similar—extending even to the continuity of the dorsal plates with the subjacent joints of the backbone—should have arisen twice in nature spontaneously. Here we seem to have a remarkable example of the independent origin of closely similar structures; and if so, what caution is not necessary before concluding

[merged small][graphic]

Fig. 34.—Lateral, Dorsal, and Ventral view of 6r»t Vertebra of A «r,

really one of our most important organs, and is able to supplement, and to a very slight extent even to replace, the respective actions of the kidneys, the liver, and the lungs. *

In the frog we have this cutaneous activity developed in a much higher degree. Not only does its pcrspirettevr action take place to such an extreme degree that a frog tied where it cannot escape the rays of a summer's sun speedily dies—nay, more, is soon perfectly dried up— but its respiratory action is both constant and important. This has been experimentally demonstrated by the detection of the carbonic acid given out in water by a frog over the head of which a bladder had been so tigbtly tied as to prevent the possibility of the escape of any exhalation from the lungs. The fact of cutaneous respiration has also been proved by the experiment of confining frogs in cages under water for more than two months and a half, and by the cutting out of the lungs, the creature continuing to live without them for forty days. Indeed it is now certain that the skin is so important an agent in the frog's breathing that the lungs do not suffice for the maintenance of life without its aid.

It is no less true that in Batrachians which breathe by means of permanent gills—as, e.g. the Axolotl—such gills are not necessary to life, as the late M. Aug. Dume5ril and Dr. Giinther have established by cutting them awav without inducing any apparent injurious effects. In the whole class of Batrachians skin respiration seems, then, to be of very great importance.

[graphic]

FlG- 35- Fic. 36.

Fig. 35.—Coccyx of Frog, lateral view, a black line indicates the coune of the sciatic nerve. Fig. 36.-Anterior aspect of Coccyx, showin-the double articular concavities placed side by side beneath the neural arch.

The internal skeleton (or the skeleton commonly so called) of the frog presents some points of considerable interest, especially as exhibiting its intermediate position between fishes on the one hand, ?nd higher vertebrates on the other. First, as regards the backbone, it may be remembered that it is made up of distinct bony joints (or vertebra), in which it agrees with all animals above fishes and with bony fishes; its hinder termination, however, is essentially fish-like. It is fish-like, because the terminal piece, as it is called, or "coccyx" (unlike the coccyx in man or in birds) is not formed of rudimentary vertebrxwhich subsequently blend and anchylose together, but is formed by the ossification continuously of the membrane investing (or sheath of) the hindermost part of that primitive continuous rod, or notochord, which, as has been said, precedes, in all vertebrate animals, the development of the backbone, making its appearance beneath the primitive groove.

* See "Elementary Physiology," Lesson V., } 19.

t From Nu'tuc, back, and Xopoit. chord.

The vertebrae are shaped like rings, and enclose within their circuit the spinal marrow upon which, as it were, these rings are strung. From the side of each ring (except at the two ends of the backbone) there juts out a bony prominence called a "transverse process," and to a certain number of these a bony " rib " is in most vertebrate animals attached (though there are none in the frog), often extending round to join the breast-bone in front, and being capable of more or less motion, so as (by their simultaneous movement) to be able to enlarge or to contract the cavity of the chest, which they thus enclose and protect.

That part of each vertebra which is placed next the body cavity is generally the thickest part, and is called the "body," or "centrum." The series of bodies (or centra) occupy the position which was at first tilled by the primitive notochord, the rest of the vertebral rings having been formed in the sides and roof of the canal formed by the upgrowth and union of the two sides of the primitive groove of the embryo.

The frog order is distinguished amongst vetebrates as that which has the abso utely smallest number of joints in the backbone. In the frog there are but nine in the front of the coccyx. In the Pipa toad there are but seven, the eighth vertebra (to the transverse processes of which the haunch bones are attached) having become solidly joined in one bone with the coccyx.

In all higher vertebrates, t.e. in all beasts, birds, and reptiles, the head is supported on an especially ring-like vertebra which—because it so supports—is called the atlas, and this 'in almost all) can turn upon a peculiar vertebra termed (from this circurrstance) the axis, and provided with a toolhlike {odontoid* process, round which, as round a pivot, the "atlas" works. Nothing of the kind exists in any fish.

In the frog (and in all its class) we find but a single vertebra representing these two, but in some allied forms, e.g. in Amp/iiuma, this vertebra develops a median process, reminding us of the odontoid process of the axis.

The frog, as has been said, has no ribs, in spite of the long "transverse processes" which project out on each side of the backbone. Ribs are not necessary to it, for it could apply them to none of the purposes to which ribs are ever applied.

In all beasts ribs aid importantly in respiration, serving by their motions alternately to inflate or empty the lungs by enlarging or contracting the cavity of the chest in the way before mentioned. The frog, however, breathes exclusively, as regards the lungs, by swallowing air by a mechanism which will be described shortly.

In serpents the ribs are the organs of locomotion, as also in the Flying Dragon before referred to ; but in frogs locomotion is effected exclusively by the limbs. In the very aberrant species Pipa and Dactylethra there are on each side of the anterior parts of the body two enormously long transverse processes, each process bearing at its extremity a short flattened, straight osseous or cartilaginous rib. These litile nbs can, however, take no part in such functions as those just referred to.

Ribs, moreover, are found in the other existing orders of the frog's class, i.e. both in the Urodela and Opniomorpka. In none, however, do they join a breast-bone, or sternum, another character in which Batraciiians agree with fishes, though they differ from fishes in that they have a sternum at all. In ascending from fishes through the vertebrate sub-kingdom, a sternum first appears in the class Bratrachia.

In a certain North African Salamander named Pleurodelcs the ribs are not only elongated, but their apices,

• From i'^if. a •^"'il:, and C."*.t, ferm.

if they do not actually perforate the skin, are so prominent as to seem to do so when the finger is drawn from behind forwards along the side of the animal's body.

The several joints of the backbone are connected together by surfaces which are not the same on both the anterior and posterior sides of the centrum, or body, of the same vertebra. Each of the first seven vertebrae is furnished with a round prominence, or head, on the hinder side of its centrum, and each of the precoccygeal vertebrae, except the first and last, has the anterior surface of its centrum excavated as a cup for the reception of the ball of the hinder surface of the vertebra next in front. The first vertebra has in front two concavities, side by side, to articulate with the skull. The eighth vertebra has a concavity at each end of its " body." The ninth vertebra has a body provided with a single convexity in front and a double convexity behind, to articulate with the concavities placed side by side on the front end of the coccyx.

These arrangements are not constant in the frog's order, still less in its class. In Bombinater and Pipa the vertebrae are concave behind each centrum, instead of in front: and the same is the case in Salamandra. In many tailed Batrachians the vertebrae are biconcave, as e.g. in Spelcrpcs, Ainphiuina, Proteus and Siren.

The biconcave shape is an approximation towards the condition which is almost universal in bony fishes, though not quite universal, since the bony pike (Lepidosteus) has a ball at one end of each vertebra and a cup at the other. Moreover, even in some reptiles (e.g. the lizards called dekoes) the vertebrae are biconcave, and the same was the case with the majority of those species of crocodiles the remains of which are found in strata older than the chalk, and even in existing crocodiles the first vertebra of the tail is biconcave.

Vertebrae with a cavity in front of the centrum and a ball behind it are found in the crocodiles now living as well as in the frog, while vertebrae with a ball in front and a concavity behind are found even amongst beasts, as in the joints of the neck of Ruminants, e.g. the sheep. Thus though the vertebrae of the frog's class exhibit no very decided signs of affinity, they show more resemblance to those of fishes than to those of any other non-batrachian class.

The transverse processes of the ninth or last vertebra in front of the coccyx, articulate with the haunch bones, but are not very remarkable in shape. In some frogs and toads the transverse processes of this vertebra become enormously expanded, and the expanded or non-expanded condition of this part is a character made use of in zoological classification. The coccyx is made up mainly, as has been said, of a continuous ossification of the sheath of the notochord, and never consists of distinct vertebra. Nevertheless, the small bony arches which are at first distinct coalesce with it. These arches are called "neural" because they arch over the hinder part of the spinal marrow. The great nerve of the leg (the sciatic nerve) proceeds outwards on each side through a foramen situated at the anterior end of the coccyx from the spinal marrow—the spinal marrow being that structure which gives origin to the great mass of the nerves pervading the entire frame.

St. George Mivart

{To be continued.)

[The author sincerely regrets, that by an inadvertence for which he is exclusively answerable, two cuts introduced into the second of these articles, namely, the figures representing Kina esculenta and Bufozwlgaris, were copied, without sanction, from two illustrations in Professor Bell's " History of British Reptiles,'' published by Mr. Van Voorst, to whom, therefore, this apology is due.]

EARTH-SCULPTURE *
III.

I DO not consider it necessary to defend my facts. They are familiar enough to the geologists of this country, as displayed more or less plainly in every district of our island. I am at present concerned with the counterstatements which the Duke of Argyll would put in their place.

He states his belief that the Highland mountains have had their contour mainly given to them by " upheavals, subsidences, and lateral pressures, which have folded them and broken them into their present shapes." A belief of any kind must be founded on evidence of some sort, and that evidence must be produced if the owner of the belief desires that it should be accepted by others besides himself. What evidence, then, does his Grace furnish as the basis on which he expects that his "belief" is to supersede what he is pleased to term "the extravagant theories of the younger glacialists"? Having shown "the antecedent improbabilities involved in the extreme theories of erosion," he states that he "proceeds to test them on the field of fact" We follow him anxiously to the field in question, and find that his so-called facts are stated in such words as these: "Loch Fyne . . . occupies, as I believe, the bed (sic) of an immense fault." "The transverse valley of Loch Eck lies across a steep anticlinal, and is due, in my opinion, to the extreme tension to which the crystalline rocks have been subjected.'' "The Pass of Awe is a rupture and chasm." These, and other similar assertions regarding various parts of the Highlands are confidently expressed, but they are accompanied by no evidence by which their accuracy may be tested. In truth, the "facts" which his Grace adduces in support of his "belief" are only other "beliefs" and "opinions" of his own. They may be correct or the reverse, but they cannot legitimately be adduced as evidence in a scientific argument. But they are very far from correct. I utterly deny, for example, the assertion that Loch Fyne lies along the bed of an immense fault, and I ask the Duke of Argyll to try to prove that it does so. Nay more, I challenge him to produce a geological section which would bear a moment's examination on the ground, in which he can show the coincidence of a valley with a line of fault in any part of his own county of Argyll. That cases of this coincidence may be found I do not doubt, but the search for them will be useful in teaching his Grace how exceptional they are.

The Duke of Argyll does indeed offer some explanatory statements regarding some of his assertions of fact. For instance, with regard to Loch Awe, he dwells on the inclinations of the slates and the intrusion of the porphyries among them as evidence that the present contour has been directly the result of subterranean convulsion, and he triumphantly adduces these and similar appearances "ignored" by myself as a demonstration of the truth of his " belief." But any one who knows the H ighland rocks at all may well smile when he is told that a geologist who had ever been over the ground, even in the most cursory way, requires to have these phenomena pointed out to him. In reality I had already granted the existence of these, and far more wonderful evidences of underground movements, for I knew the Highland rocks well, and had mapped their structure over leagues of ground from the mountains of Sutherland to the moors of Forfar, and the headlands of Islay. I was therefore perfectly familiar with the phenomena to which the Duke of Argyll so confidently refers. But I had learned more about them than merely their tale of subterranean turmoil. I had found that they did not bear directly on the origin of hill and valley at all. I had traced everywhere evidence that what

* Op.-nln-; A.Mrcss to the Fdinrm-gh Geological Society, hy Prof. Ceikie, r.R.S. (columned/rum p. yi).

we now see of intruded granite or curved slate has beec laid bare only after the removal of hundreds and thousands of feet of rock under which they once lay. His Grace, it would seem, has still this lesson to learn, and until he has mastered it, and, apart from any theory but simply as a matter of demonstrable fact, has realised what n involves and how vain is the attempt to connect the contorting and hardening of the rocks with the prcser.t surface features of the country, argument with him on thi* question seems hardly possible.

Again, I had quoted the mountain Ben Lawers, with Iu flanking hollow in Loch Tay, as a typical example ciftbe kind of evidence which could be abundantly adductc from all parts of the Highlands to show the relation between geological structure and external form, and tc prove from under what an enormous mass of removec rock the present surface of the Highlands has appeaned. I gave a section to show at a glance the broad facts d the case—a section from which no conclusion is possible but that which I drew. But here, once more. the Duke of Argyll's belief in the all-powerful efrjcacy of granite and igneous rocks, or his thraldom to what he calls "the influence of a preconce:veti theory," brings out in well-marked prominence that obliquity of vision which prevents him from seeing anything but convulsion and fracture. He scents intrusive rocks of some sort along the south bank of Loch Tay. It would be vain to remonstrate that this alleged influence of the igneous rocks is, to say the least, as pure "invention and imagination" as anything which the "younger school " could readily supply, or that the denudation of that region is a momentous fact to be looked in the face and explained, not to be dismissed or denied, no matter what our " theory " or " belief" may be regarding the origin of granite. Without further ceremony, the proofs of enormous denudation at Ben Lawers and Loch Tay, together with their luckless advocate, are all bundled off with the summary judgment, so happily appropriate to its own author, " 1 attach no value whatever to a theory which passes over and ignores this class of facts altogether."

The dogmatic assertions which the Duke of ArgyD makes regarding the influence of granite and other rocks upon the surface, and as to the existence of fractures and depressions along the line of valley and glen, are really most flagrant examples of the petitioprincipii. In effect, his Grace tells us, "The 'inventions and imaginations' of these younger men are based upon'assumed facts' which ' are, in my opinion, to a large extent purely hypothetical.' I am 'suspicious of the influence which a preconceived theory has had on their estimate of evidence. I therefore 'attach no value whatever' to their statements, and do not consider it necessary to lose time in weighing what they actually mean by this denudation of theirs, and all which, as they contend, must flow from it. My belief is that valleys are due to fractures and depressions. The Highlands abound in valleys, and therefore it must be evident to everyone capable of forming an opinion on the subject, that they abound also in proofs of fracture and depression."

In the foregoing remarks I have been dealing only with the Duke of Argyll's paper of February 1868, which in his recent vigorously-worded address he cites as still unanswered, and which, therefore, we may suppose still to express his views. And yet no one can peruse that address without perceiving that it betokens a considerable change of opinion. Especially gratifying must it be to that "younger school" of geologists against which the Duke has so vehemently lifted up his protest, to observe that the lapse of time which he would not allow to have had much denuding effect upon the rocks, has yet been able to strip off from himsell some of that crust of preconceived "theory" against which no argument or adverse fact could once make any impression. It is true that his Grace formerly thought it necessary to assure us that Time could do nothing by itself, "nothing except by the aid of its great ally Force—Force working in Time." Well, we shall not quarrel about the use of words, but cheerfully admit that the change which has become perceptible in the opinions of the Duke of Argyll is wholly the result of " Force working in Time," and not a very long time, for it cannot be stretched out beyond five years. Surely if the lapse of so brief a space, with all the amount of Force which we can crowd into it, can have modified geological opinions which certainly seemed as solidly and unalterably fixed as his own Ben More itself, it can hardly be too much to hold that by the end of another lustrum still further modification may justify the confident belief that his Grace may still come to join the "younger school" heart and hand. We can assure him a jubilant welcome.

But it may be asked what is the nature of this present alteration of view? In brief, it may be put thus: the Duke of Argyll finds that, after all, denudation is one of those disagreeable facts which will insist on being prominent—" chiels that winna ding." He has discovered that it really has had some share in the shaping of the present outlines of the land. He now admits in words " that the forms of hill and valley which preceded the coming-on of glacial conditions [during the Ice age] had been themselves determined in a large degree by previous denudations." And even though this general admission is neutralised by statements which follow it, it is most welcome as an indication doubtless of the effect of those "more extended opportunities of observation " which his Grace tells us he has since enjoyed, and on the continuance of which our hopes of his secession to the ranks of the " younger school" are mainly based.

The Duke of Argyll appeals once more to the details 6f geological structure. Most gladly do we accede to the appeal. He points to the contorted condition of the older rocks as evidence of the extent to which they have been affected by subterranean movements. But no geologists are more familiar with these facts than his maligned "younger school." He conceives that it was alter such movements that the forces of denudation began to work. Most assuredly ; this has been explained over and over again. He affirms that " so long as such hills and mountains last at all, and wherever they are exposed to view, they bear upon them the unmistakeable impress of their origin and of the mighty subterranean forces to which their structure is due." This sentence is rather ambiguous. If it means th.it contorted rocks retain evidence of contortion, such an obvious truism was hardly worth a sentence to itself. If it means that a mountain made of contorted rocks has had its form determined at the time of contortion, the statement is mere assertion and a begging of the very question to be proved.

In the same address the noble president declares that "denudation has done its work along the lines determined by upheaval, by fracture, and by unequal subsidence." This h;is never been denied by anyone. A main object of my book was to show how, by means of denudation along such lines, much of the present contour of Scotland has been produced. Again we are told—" All sedimentary beds must have had an edge somewhere; and if they are lifted into a vertical position and the edges come to be exposed, the removal of a small amount of material may result in a horizontal surface, or in surfaces cutting across the lines of structure at every variety of angle."' If the Duke intends this explanation to apply not to a mere hand specimen, but to any district of convoluted and vertical rocks, such as the hills of Wales or the Southern Uplands or Highlands of Scotland, he cannot have noticed the string of physical absurdities which it involves. The rocks are often vertical, or nearly so, for miles at a stretch. Could we put them into something like their original horizontal or gently inclined posi

tion their present edges would end off in a cliff many miles high. Can his Grace expect anyone to believe that the beds, which certainly "must have had an edge somewhere," ever ended off in that fashion? But this would be only a part of the feat. In actual fact the rocks have been violently contorted, so that a series several hundreds or even thousands of feet in thickness is folded again and again upon itself. The present surface has been cut across these foldings, and in great part has its inequalities independent of them. If we could flatten these curved rocks out again from their present condition they would show a series of deep sharp troughs separated by steep pyramidal ridges of flat strata. And from the Duke of Argyll's teaching we should learn that this wonderful arrangement was the normal plan in old times of laying down sediment which, instead of always going to the bottom and filling up the hollows as it does nowadays, contrived then to ascend, layer after layer, like the tiers of the Great Pyramid, as if it were under the impulse not of mere gravity or of the play of ocean currents, but of the methodical action of organisms like the coral polyps. We should further learn that these neatly-shaped sand and mud ridges and troughs were so accurately laid down that when subterranean forces came into action and crumpled the whole up, every ridge popped conveniently into a trough below, as if a trap-door had been opened for its reception, and with such nice adjustment as to bring its top to the same general level as the bottom of the former troughs!

The truth is, and, in common fairness I am bound frankly to state it, that such assertions as these with which I am dealing, could never be made if geological structure were really understood and kept in view. This is a matter of science, and is only to be mastered by the same patient toil '.vhich is required in other scientific inquiries. Moreover, it is by no means so easily mastered as it seems to be. The first absolute requisite for overcoming our ignorance, is to reduce our facts to the test of ocular proof and measurement. Let us construct a section across the tract of which wc would master the structure, and to avoid risks of error from exaggeration of proportion, let us begin by making the section as nearly as possible on a true scale, that is, giving the same value

I to length as to height. With the outline of the ground accurately traced we may then, section in hand, insert upon it at the proper places, and with the true angle of

\ dip, such rocks as we be able to see exposed. Having fixed these data in this patient way, we may expect with some confidence to understand and fill in the geological structure of the ground for ourselves, and to make it intelligible and credible to others. Until we have gone through such a training ourselves, or have learnt adequately to appreciate what it is from the labours of others, we have no right to utter an opinion on the relations between geological structure and external form, for we are destitute of one of the necessary qualifications for dealing with the problem.

The greater part of the recent address of the president of the Geological Society deals with the traces of iceaction in this country, and the manner in which they are to be accounted for. In his remarks upon this subject, the Duke agpin places himself in opposition to the views of the ">ounger school," and expresses opinions from which every member of that school would, I am sure, emphatically dissent. It is no part of my present purpose to enter upon these. I cannot, however, pass by one statement in the address. His Grace asserts that these restless "younger geologists" have recently made a most complete change of front. He therefore directs his attack against this new position. He says that they no longer maintain the existing systems of hill and valley to have been cut out of the solid by an enormous glacier, but

'admit the general contour of the country to have been

j very much the same before the Ice age as after it, all the

« AnteriorContinuar »