LECTURE FIRST.
THE PALaeONTOLOGICAL HISTORY OF PLANTS.
Palaeontology, or the science of ancient organisms, deals, as its subject, with all the plants and animals of all the geologic periods. It bears nearly the same sort of relation to the _physical_ history of the past, that biography does to the civil and political history of the past. For just as a complete biographic system would include every name known to the historian, a complete palaeontologic system would include every fossil known to the geologist. It enumerates and describes all the organic existences of all the extinct creations,--all the existences, too, of the present creation that occur in the fossil or semi-fossil form; and, thus coextensive in s.p.a.ce with the earth"s surface,--nay, greatly more than coextensive with the earth"s surface,--for in the vast hieroglyphic record which our globe composes, page lies beneath page, and inscription covers over inscription,--coextensive, too, in time, with every period in the terrestrial history since being first began upon our planet,--it presents to the student a theme so vast and multifarious, that it might seem but the result, on his part, of a proper modesty, conscious of the limited range of his powers, and of the brief and fleeting term of his life, were he to despair of being ever able effectually to grapple with it. "But," to borrow from one of the most ingenious of our Scottish metaphysicians, "in this, as in other instances in which nature has given us difficulties with which to cope, she has not left us to be wholly overcome." "If," says Dr. Thomas Brown, in his remarks on the cla.s.sifying principle,--"if she has placed us in a labyrinth, she has at the same time furnished us with a clue which may guide us, not, indeed, through all its dark and intricate windings, but through those broad paths which conduct us into day. The single power by which we discover resemblance or relation in general, is a sufficient aid to us in the perplexity or confusion of our first attempts at arrangement. It begins by converting thousands, and more than thousands, into one; and, reducing in the same manner the numbers thus formed, it arrives at last at the few distinctive characters of those great comprehensive tribes on which it ceases to operate, because there is nothing left to oppress the memory or the understanding."
But, is this all? Can the Palaeontologist but say that that cla.s.sifying principle, which in every other department of science yields such a.s.sistance to the memory, is also of use in his, or but urge that it enables him to sort and arrange his facts; and that, by converting one idea into the type and exemplar of many resembling ones, it imparts to him an ability of carrying not inadequate conceptions of the mighty whole in his mind? If this were all, you might well ask, Why obtrude upon us, in connection with your special science, a common semi-metaphysical idea, equally applicable to all the sciences,--in especial, for example, to that botany which is the science of existing plants, and to that zoology which is the science of existing animals?
Nay, I reply, but it is not all. I refer to this cla.s.sifying principle because, while it exists in relation to all other sciences as a principle--to use the words of the metaphysician just quoted--"given to us by nature,"--as a principle of _the mind within_,--it exists in Palaeontological science as a principle of nature itself,--as a principle palpably _external to the mind_. It is a marvellous fact, whose full meaning we can as yet but imperfectly comprehend, that myriads of ages ere there existed a human mind, well nigh the same principles of cla.s.sification now developed by man"s intellect in our better treatises of zoology and botany, were developed on this earth by the successive geologic periods; and that the by-past productions of our planet, animal and vegetable, were chronologically arranged in its history, according to the same laws of thought which impart regularity and order to the works of the later naturalist and phytologists.
I need scarce say how slow and interrupted in both provinces the course of arrangement has been, or how often succeeding writers have had to undo what their predecessors had done, only to have their own cla.s.sifications set aside by _their_ successors in turn. At length, however, when the work appears to be well nigh completed, a new science has arisen, which presents us with a very wonderful means of testing it.
Cowley, in his too eulogistic ode to Hobbes,--smit by the singular ingenuity of the philosophic infidel, and unable to look through his sophisms to the consequences which they involved,--could say, in addressing him, that
"only G.o.d could know Whether the fair idea he did show Agreed entirely with G.o.d"s own or no."
And he then not very wisely added,--
"This, I dare boldly tell, "T is so like truth, "t will serve our turn as well."
We now know, however, that no mere resemblance to truth will for any considerable length of time serve its turn. It is because the resemblances have, like those of Hobbes, been mere resemblances, that so much time and labor have had to be wasted by the pioneers of science in their removal; and, now that a wonderful opportunity has occurred of comparing, in this matter of cla.s.sification, the human with the Divine idea,--the idea embodied by the zoologists and botanists in their respective systems, with the idea embodied by the Creator of all in geologic history,--we cannot perhaps do better, in entering upon our subject, than to glance briefly at the great features in which G.o.d"s order of cla.s.sification, as developed in Palaeontology, agrees with the order in which man has at length learned to range the living productions, plant and animal, by which he is surrounded, and of which he himself forms the most remarkable portion. In an age in which a cla.s.s of writers not without their influence in the world of letters would fain repudiate every argument derived from _design_, and denounce all who hold with Paley and Chalmers as anthropomorphists, that labor to create for themselves a G.o.d of their own type and form, it may be not altogether unprofitable to contemplate the wonderful parallelism which exists between the Divine and human systems of cla.s.sification, and--remembering that the geologists who have discovered the one had no hand in a.s.sisting the naturalists and phytologists who framed the other--soberly to inquire whether we have not a new argument in the fact for an ident.i.ty in const.i.tution and quality of the Divine and human minds,--not a mere fanciful ident.i.ty, the result of a disposition on the part of man to imagine to himself a G.o.d bearing his own likeness, but an ident.i.ty real and actual, and the result of that creative act by which G.o.d formed man in his own image.
The study of plants and animals seems to have been a favorite one with thoughtful men in every age of the world. According to the Psalmist, these great "works of the Lord are sought out of all them that have pleasure therein." The Book of Job, probably the oldest writing in existence, is full of vivid descriptions of the wild denizens of the flood and desert; and it is expressly recorded of the wise old king, that he "spake of trees, from the cedar tree that is in Lebanon, even unto the hyssop that springeth out of the wall; and also of beasts, and of fowl, and of creeping things, and of fishes." Solomon was a zoologist and botanist; and there is palpable cla.s.sification in the manner in which his studies are described. It is a law of the human mind, as has been already said, that, wherever a large stock of facts are acquired, the cla.s.sifying principle steps in to arrange them. "Even the rudest wanderer in the fields," says Dr. Brown, "finds that the profusion of blossoms around him--in the greater number of which he is able himself to discover many striking resemblances--may be reduced to some order of arrangement." But, for many centuries, this arranging faculty labored but to little purpose. As specimens of the strange cla.s.sification that continued to obtain down till comparatively modern times, let us select that of two works which, from the literary celebrity of their authors, still possess a cla.s.sical standing in letters,--Cowley"s "Treatise on Plants," and Goldsmith"s "History of the Earth and Animated Nature." The plants we find arranged by the poet on the simple but very inadequate principle of size and show. Herbs are placed first, as lowest and least conspicuous in the scale; then flowers; and, finally, trees. Among the herbs, at least two of the ferns--the true maidenhair and the spleenwort--are a.s.signed places among plants of such high standing as sage, mint, and rosemary: among the flowers, monocotyledons, such as the iris, the tulip, and the lily, appear among dicotyledons, such as the rose, the violet, the sunflower, and the auricula: and among trees we find the palms placed between the plum and the olive; and the yew, the fir, and the juniper, flanked on one side by the box and the holly, and on the other by the oak. Such, in treating of plants, was the cla.s.sification adopted by one of the most learned of English poets in the year 1657.
Nor was Goldsmith,-who wrote more than a century later, much more fortunate in dealing with the animal kingdom. Buffon had already published his great work; and even he could bethink him of no better mode of dividing his animals than into wild and tame. And in Goldsmith, who adopted, in treating of the mammals, a similar principle, we find the fishes and molluscs placed, in advance of the sauroid, ophidian, and batrachian reptiles,--the whale united in close relationship to the sharks and rays,--animals of the tortoise kind cla.s.sed among animals of the lobster kind, and both among sh.e.l.l fish, such as the snail, the nautilus, and the oyster. And yet Goldsmith was engaged on his work little more than eighty years ago. In fine, the true principles of cla.s.sification in the animal kingdom are of well nigh as recent development as geologic science itself, and not greatly more ancient in even the _vegetable_ kingdom. It would, of course, be wholly out of place to attempt giving a minute history here of the progress of arrangement in either department; but it can scarce be held that the natural system of plants was other than very incomplete previous to 1789, when Jussieu first enunciated his scheme of cla.s.sification; nor did it receive its later improvements until so late as 1846, when, after the publication, in succession, of the schemes of De Candolle and Endlicher, Lindley communicated his finished system to the world. And there certainly existed no even tolerably perfect system of zoology until 1816, when the "Animal Kingdom" of Cuvier appeared. Later naturalists,--such as Aga.s.siz, in his own special department, the history of fishes, and Professor Owen in the invertebrate divisions,--have improved on the cla.s.sification of even the great Frenchman; but for purposes of comparison between the scheme developed in geologic history and that at length elaborated by the human mind, the system of Cuvier will be found, for at least our present purpose, sufficiently complete. And in tracing through time the course of the vegetable kingdom, let us adopt, as our standard to measure it by, the system of Lindley.
Commencing at the bottom of the scale, we find the Thallogens, or flowerless plants which lack proper stems and leaves,--a cla.s.s which includes all the algae. Next succeed the Acrogens, or flowerless plants that possess both stems and leaves,--such as the ferns and their allies.
Next, omitting an inconspicuous cla.s.s, represented by but a few parasitical plants incapable of preservation as fossils, come the Endogens,--monocotyledonous flowering plants, that include the palms, the liliaceae, and several other families, all characterized by the parallel venation of their leaves. Next, omitting another inconspicuous tribe, there follows a very important cla.s.s,--the Gymnogens,--polycotyledonous trees, represented by the conifers; and cycadaceae. And, last of all, come the Dicotyledonous Exogens,--a cla.s.s to which all our fruit, and what are known as our "forest trees,"
belong, with a vastly preponderating majority of the herbs and flowers that impart fertility and beauty to our gardens and meadows. This last cla.s.s, though but one, now occupies much greater s.p.a.ce in the vegetable kingdom than all the others united.
Such is the arrangement of Lindley, or rather an arrangement the slow growth of ages, to which this distinguished botanist has given the last finishing touches. And let us now mark how closely it resembles the geologic arrangement as developed in the successive stages of the earth"s history.
[Ill.u.s.tration: Fig. 1.[4]
-+------------------------- | Thallogens.
Silurian. | | Acrogens.
-+-----+------------------- | | | Gymnogens.
Old Red. | | | | | | -+-----+-----+------------- | | | | Monocotyledons.
Carboniferous. | | | | | | | | -+-----+-----+-----+------- Permian. | | | | -+-----+-----+-----+------- Tria.s.sic. | | | | -+-----+-----+-----+------- | | | | : Dicotyledons.
Oolitic. | | | | : | | | | | -+-----+-----+-----+-----+- | | | | | Cretaceous. | | | | | | | | | | -+-----+-----+-----+-----+- Dicotyledonous Trees.
| | | | | Tertiary. | | | | | | | | | | -+-----+-----+-----+-----+- Geologic [Thal. Ac. Gy. Mon. Dic.] arrangement.
Lindley"s [Thal. Ac. Mon. Gy. Dic.] arrangement.
THE GENEALOGY OF PLANTS.]
The most ancient period of whose organisms any trace remains in the rocks seems to have been, prevailingly at least, a period of Thallogens. We must, of course, take into account the fact, that it has yielded no land plants, and that the sea is everywhere now, as of old, the great habitat of the algae,--one of the four great orders into which the Thallogens are divided. There appear no traces of a terrestrial vegetation until we reach the uppermost beds of the Upper Silurian System. But, account for the fact as we may, it is at least worthy of notice, that, alike in the systems of our botanists and in the chronological arrangements of our geologists, the first or introductory cla.s.s which occurs in the ascending order is this humble Thallogenic cla.s.s. There is some trace in the Lower Silurians of Scotland of a vegetable structure which may have belonged to one of the humbler Endogens, of which, at least, a single genus, the _Zosteraceae,_ still exists in salt water; but the trace is faint and doubtful, and, even were it established, it would form merely a solitary exception to the general evidence that the first known period of vegetable existence was a period of Thallogens. The terrestrial remains of the Upper Silurians of England, the oldest yet known, consist chiefly of spore-like bodies, which belonged, says Dr. Hooker, to Lycopodiaceae,--an order of the second or acrogenic cla.s.s. And, in the second great geologic period,--that of the Old Red Sandstone,--we find this second cla.s.s not inadequately represented. In its lowest fossiliferous beds we detect a Lycopodite which not a little resembles one of the commonest of our club mosses,--_Lycopodium clavatum_,--with a minute fern and a large striated plant resembling a calamite, and evidently allied to an existing genus of Acrogens, the equisetaceae. In the Middle Old Red Sandstone there also occurs a small fern, with some trace of a larger; and one of its best preserved vegetable organisms is a lepidodendron,--an extinct ally of the Lycopodiums; while in the upper beds of the system, especially as developed in the south of Ireland, the n.o.ble fern known as _Cyclopteris Hibernicus_ is very abundant. This fern has been detected also in the Upper Old Red of our own country, mingled with fragments of contemporary calamites. With, however, these earliest plants of the land yet known, there occurs a true wood, which belonged, as shown by its structure, to a gymnospermous or polycotyledonous tree, and which we find a.s.sociated with remains of Coccosteus and Diplacanthus.
[Ill.u.s.tration: Fig. 2.
CYCLOPTERIS HIBERNICUS.
(Nat. size.)]
[Ill.u.s.tration: Fig. 3.
CONIFER OF THE LOWER OLD RED SANDSTONE.
Cromarty.
(Mag. forty diameters.)]
And here let me remark, that the facts of Palaeontological science compel us to blend, in some degree, with the cla.s.sification of our modern botanists, that of the botanists of an earlier time. In a pa.s.sage already quoted, Solomon is said to have discoursed of plants, "from the cedar tree that is in Lebanon, to the hyssop that springeth out of the wall,"--from the great tree to the minute herb; and Cowley rose, in his metrical treatise, as has been shown, from descriptions of herbs and flowers to descriptions of fruit and forest trees. And as in every age in which there existed a terrestrial vegetation there seem to have been "trees" as certainly as "herbs," the palaeontological botantist finds that he has, in consequence, to range his cla.s.ses, not in one series, but in two,--the Gymnogens, or cone-bearing trees, in a line nearly parallel with the Acrogens, or flowerless, spore-bearing herbs. But the arrangement is in no degree the less striking from the circ.u.mstance that it is ranged, not in one, but in two lines. It is, however, an untoward arrangement for the purposes of the Lamarckian, whose peculiar hypothesis would imperatively demand, not a double, but a single column, in which the ferns and club mosses would stand far in advance, in point of time, of the Coniferae. In the Coal Measures, so remarkable for the great luxuriance of their flora, both the Gymnogens and Acrogens are largely developed, with a very puzzling intermediate cla.s.s, that, while they attained to the size of trees, like the former, retained in a remarkable degree, as in the Lepidodendra and the Calamites, the peculiar features of the latter. And with these there appear, though more sparingly, the Endogens,--monocotyledonous plants, represented by a few palm-like trees (Palmacites), a few date-like fruits (Trigonocarpum), and a few gra.s.s-like herbs (Poacites). In the great Secondary division, the true dicotyledonous plants first appear; but, so far as is yet known, no dicotyledonous wood. In the earlier formations of the division a degree of doubt attaches to even the few leaves of this cla.s.s. .h.i.therto detected; but in the Lower Cretaceous strata they become at once unequivocal in their character, and comparatively abundant, both as individuals and species; and in the Tertiary deposits they greatly outnumber all the humbler cla.s.ses, and appear not only as herbs, but also as great trees. Not, however, until shortly before the introduction of man do some of their highest orders, such as the Rosaceae, come upon the scene, as plants of that great garden--including the fields of the agriculturist--which it has been part of man"s set task upon earth to keep and to dress. And such seems to be the order of cla.s.sification in the vegetable kingdom, as developed in creation, and determined by the geologic periods.
[Ill.u.s.tration:
Fig. 4.[5]
-+-----+-----+------------------------------- | | | Rad. Art. Mol.
Silurian. | | | | | | | Fishes.
-+-----+-----+-----+------------------------- | | | | Old Red. | | | | | | | | | Reptiles.
-+-----+-----+-----+-----+------------------- | | | | | Carboniferous. | | | | | | | | | | -+-----+-----+-----+-----+------------------- Permian. | | | | | -+-----+-----+-----+-----+------------------- Tria.s.sic. | | | | | -+-----+-----+-----+-----+------------------- | | | | | | Birds.
Oolitic. | | | | | | : Mammals.
| | | | | | : -+-----+-----+-----+-----+-----+-----+------- | | | | | | : Cretaceous. | | | | | | : | | | | | | : -+-----+-----+-----+-----+-----+-----+------- | | | | | | | Pla. Mam.
Tertiary. | | | | | | | | | | | | | | | | | | | | | -+-----+-----+-----+-----+-----+-----+-----+- Recent. | | | | | | | | Man.
-+-----+-----+-----+-----+-----+-----+-----+- Geologic [Rad. Art. Mol. Fish. Rep. Bird. Mam. Man.] Arrangement.
Cuvier"s [Rad. Art. Mol. Fish. Rep. Bird. Mam. Man.] Arrangement.
THE GENEALOGY OF ANIMALS.]
The parallelism which exists between the course of creation, as exhibited in the animal kingdom, and the cla.s.sification of the greatest zoologist of modern times, is perhaps still more remarkable. Cuvier divides all animals into vertebrate and invertebrate; the invertebrates consisting, according to his arrangement, of three great divisions,--mollusca, articulata, and radiata; and the vertebrates, of four great cla.s.ses,--the mammals, the birds, the reptiles, and the fishes. From the lowest zone at which organic remains occur, up till the higher beds of the Lower Silurian System, all the animal remains yet found belong to the invertebrate divisions. The numerous tables of stone which compose the leaves of this first and earliest of the geologic volumes correspond in their contents with that concluding volume of Cuvier"s great work in which he deals with the mollusca, articulata, and radiata; with, however, this difference, that the three great divisions, instead of occurring in a continnous series, are ranged, like the terrestrial herbs and trees, in parallel columns. The chain of animal being on its first appearance is, if I may so express myself, a threefold chain;--a fact nicely correspondent with the further fact, that we cannot in the present creation range _serially_, as either higher or lower in the scale, at least two of these divisions,--the mollusca and articulata. In one of the higher beds of the Upper Silurian System,--a bed which borders on the base of the Old Red Sandstone,--the vertebrates make their earliest appearance in their fourth or ichthyic cla.s.s; and we find ourselves in that volume of the geologic record which corresponds to Cuvier"s volume on the fishes. In the many-folded pages of the Old Red Sandstone, till we reach the highest and last, there occur the remains of no other vertebrates than those of this fourth cla.s.s; but in its uppermost deposits there appear traces of the third or reptilian cla.s.s; and in pa.s.sing upwards still, through the Carboniferous, Permian, and Tria.s.sic Systems, we find reptiles continuing the master existences of the time. The geologic volume in which these great formations are included corresponds to the Cuvierian one devoted to the Reptilia. Early in the Oolitic System, birds, Cuvier"s second cla.s.s of the vertebrata, make their first appearance, though their remains, like those of birds in the present time, are rare and infrequent; and, for at least the earlier periods of their existence, we know that they were,--that they haunted for food the waters of the period, and waded in their shallows,--only from marks similar to those by which Crusoe became first aware of the visits paid to his island by his savage neighbors,--their footprints, left impressed on the sands over which they stalked of old. This early Oolitic volume corresponds in its contents to the section devoted by Cuvier, in his great work, to his second cla.s.s, the birds. And in the Stonisfield slate,--a deposit interposed between the "Inferior" and "Great Oolites," we detect the earliest indications of his first or mammaliferous cla.s.s, apparently represented, however, by but one order,--the Marsupiata, or pouched animals, to whose special place in the scale I shall afterwards have occasion to refer. Not until we reach the times of the Tertiary division do the mammals in their higher orders appear. The great Tertiary volume corresponds to those volumes of Cuvier which treat of the placental animals that suckle their young. And finally,--last born of creation,--man appears upon the scene, in his several races and varieties; the sublime arch of animal being at length receives its keystone; and the finished work stands up complete, from foundation to pinnacle, at once an admirably adjusted occupant of s.p.a.ce, and a wonderful monument of Divine arrangement and cla.s.sification, as it exists in time. Save at two special points, to which I shall afterwards advert, the particular arrangement unfolded by geologic history is exactly that which the greatest and most philosophic of the naturalists had, just previous to its discovery, originated and adopted as most conformable to nature: the arrangements of geologic history as exhibited in time, if, commencing at the earliest ages, we pursue it downwards, is exactly that of the "Animal Kingdom" of Cuvier read backwards.
Let us then, in grappling with the vast multiplicity of our subject, attempt reducing and simplifying it by means of the cla.s.sifying principle; not simply, however,--again to recur to the remark of the metaphysician,--as an internal principle given us by nature, but as an external principle _exemplified_ by nature. Let us take the organisms of the old geologic periods in the order in which they occur in time; secure, as has been shown, that if our chronology be correct, our cla.s.sification will, as a consequence, be good. It will be for the natural theologians of the coming age to show the bearing of this wonderful fact on the progress of man towards the just and the solid, and on the being and character of man"s Creator,--to establish, on the one hand, against the undue depreciators of intellect and its results, that in certain departments of mind, such as that which deals with the arrangement and development of the scheme of organic being, human thought is not profitlessly revolving in an idle circle, but progressing G.o.dwards, and gradually unlocking the order of creation. And, on the other hand, it will be equally his proper business to demand of the Pantheist how,--seeing that only _persons_ (such as the Cuviers and Lindleys) could have wrought out for themselves the real arrangement of this scheme,--how, I say, or on what principle, it is to be held that it was a scheme originated and established at the beginning, not by a _personal_, but by an impersonal G.o.d. But our present business is with the _fact_ of the parallel arrangements, Divine and human,--not with the inferences legitimately deducible from it.
[Ill.u.s.tration: Fig. 5.
OLDHAMIA ANTIQUA;--the oldest known Zoophyte.
Wrae Head, Ireland.]
[Ill.u.s.tration: Fig. 6.
PALaeOCHORDA MINOR.
(One half nat. size.)]
Beginning with the plants, let us, however, remark, that they do not precede in the order of their appearance the humbler animals. No more ancient organism than the _Oldhamia_ of the Lowest Irish Silurians, a plant-like zoophyte somewhat resembling our modern sertularia, has yet been detected by the geologist; though only a few months ago the researches of Mr. Salter in the ancient rocks of the Longmynd, Shropshire, previously deemed unfossiliferous, have given, to it what seem to be contemporary vegetable organisms, in a few ill-preserved fucoids. So far as is yet known, plants and animals appear together. The long upward march of the animal kingdom takes its departure at its starting point from a thick forest of algae. In Bohemia, in Norway, in Sweden, in the British Islands, in North America, wherever, in fine, what appears to be the lowest, or at least one of the lowest, zones of life has yet been detected, the rocks are found to be darkened by the remains of algae, so abundantly developed in some cases, that they compose, as in the ancient Lower Silurians of Dumfriesshire, impure beds of anthracite several feet in thickness. Apparently, from the original looseness of their texture, the individual plants are but indifferently preserved; nor can we expect that organisms so ancient should exhibit any _very_ close resemblance to the plants which darken the half-tide rocks and skerries of our coasts at the present time. We do detect, however, in some of these primordial fossils, at least a noticeable likeness to families familiar to the modern algaeologist. The cord-like plant, _Chorda filum_, known to our children as "dead men"s ropes,"
from its proving fatal at times to the too adventurous swimmer who gets entangled in its thick wreaths, had a Lower Silurian representative, known to the Palaeontologist as the _Palaeochorda_, or ancient chorda, which existed apparently in two species,--a larger and smaller. The still better known _Chondrus crispus_, the Irish moss or carrageen of our cookery-books, has likewise its apparent though more distant representative in _Chondritis_, a Lower Silurian algae, of which there seems to exist at least three species. The fucoids, or kelp weeds, appear to have had also their representatives in such plants as _Fucoides gracilis_ of the Lower Silurians of the Malverns; in short, the Thallogens of the first ages of vegetable life seem to have resembled, in the group, and in at least their more prominent features, the algae of the existing time. And with the first indications of land we pa.s.s direct from the Thallogens to the Acrogens,--from the sea weeds to the fern allies. The Lycopodiaceae;, or club mosses, bear in the axils of their leaves minute circular cases, which form the receptacles of their spore-like seeds. And when, high in the Upper Silurian System, and just when preparing to quit it for the Lower Old Red Sandstone, we detect our earliest terrestrial organisms, we find that they are composed exclusively of those little spore receptacles. The number of land plants gradually increases as we ascend into the overlying system. Still, however, the Flora of even the Old Red is but meagre and poor; and you will perhaps permit me to lighten this part of my subject, which threatens too palpably to partake of the poverty of that with which it deals, by a simple ill.u.s.tration.
[Ill.u.s.tration: Fig. 7.