# Popular Science Monthly/Volume 15/May 1879/Clews in Natural History

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* CLEWS IN NATURAL HISTORY.
By Dr. ANDREW WILSON.
IN the exercise of his scientific attainments, there is one aspect in which the naturalist of to-day bears a certain likeness to the detective officer. The latter is perpetually endeavoring to "strike the trail" of the offender through his dexterity in the discovery of clews to the movements of the pursued, and attains his end most surely and speedily when the traces he has selected are of trustworthy kind. The naturalist, on his part, has frequently to follow the history of an animal or plant, or it may be that of a single organ or part in either, through a literal maze of difficulties and possibilities. His search after the relationship of an animal may be fraught with as great difficulty as that which attends the discovery of a "missing heir" or lost relative in actual life; and his success in his mission is found to depend, as does that of the detective's work, simply on the excellence and trustworthiness of the clews he possesses, and on the judicious use to which he puts his "information received." It can not be denied, however, that modern aspects of science and present-day tendencies in research have largely increased the resemblance between the enforced duties of the criminal investigator and the self-imposed task of the biologist. When, formerly, the order of nature was regarded as being of unaltering kind and of stable constitution, naturalists regarded animals and plants simply as they existed. There was of old no looking into the questions of biology in the light of "what might have been," because the day was not yet when change and evolution were regarded as representing the true order of the world. When, however, the idea that the universe both of living and non-living matter had an ordered past dawned upon the minds of scientists, the necessity for tracing that past was forced upon them as a bounden duty. With no written history to guide them, the scientific searchers were forced to read the "sermons in stones" which Nature had delivered ages ago. Without clear and unmistaken records to point the way, they had to seek for clews and traces to nature's meaning in the structure and development of animals and plants; and, as frequently happens in commonplace history, the earnest searcher often found a helping hand where he least thought it might appear, and frequently discovered an important clew in a circumstance or object of the most unlikely kind.
Readers whose tastes are not materially scientific have doubtless heard much of "missing links" of nature, especially in connection with the gaps which exist between the human territory and ape-land. Indeed, the phrase has come to be understood as applying almost entirely and specifically to the absence of connecting forms between man and the apes—forms for which, in one sense, no necessity exists, inasmuch as Mr. ​Darwin's theory does not demand that the gorilla or any of his compeers should be directly connected with man. The gorilla with his nearest relation lives, so to speak, at the top of his own branch in the great tree of life, while man exists at the top of another higher and entirely different bough. The connection between the human and lower types is made theoretically to exist at some lower part of the stem when, from a common ancestor, the human and ape types took divergent roads and ways toward the ranks of nature's aristocracy. But although in some cases the need for "missing links" is seen, even theoretically, to be non-existent, or at least of a widely different nature from that supposed by the popular mind, there are yet cases in which that need is very apparent, and wherein, through the persistent tracing of the clews nature has afforded, the past history of more than one race of animals and plants has been made plain and apparent. Of such clews—which are really mere traces, and nothing more—there are no better examples than the curious fragments of structures found in many animals and plants, and named "rudimentary organs." An animal or plant is thus found to possess a mere trace of an organ or part which, so far as the highest exercise of human judgment may decide, is of not the slightest utility to the being. It is invariable in its presence, and as fixed in its uselessness. It bears no relation to the existing life or wants of the animal, but may in some cases—as, for example, in a certain little rudimentary pocket in man's digestive system, serving as an inconvenient receptacle for plum-stones and like foreign bodies—prove a source of absolute disadvantage or even danger. On what theory can the presence of such organs and parts be accounted for? is a question of extremely natural kind. The replies at the command of intelligent humanity are but two. Either the animal was created with the useless appendage in question—a supposition which includes the idea that Nature, after all, is somewhat of a bungler, and that nothing further or more comprehensible than the fiat "It is so," can be said on the subject—or, secondly, we may elect to explain the puzzle by the assertion that the "rudimentary organ" of the existing animal represents a part once fully developed in that animal's remote ancestors, but now
Dwindled to the shortest span.
The rudimentary organ or appendage is represented in the animal of to-day as a legitimate heritage derived from its ancestor. It is, in short, a family feature, to which the animal is the "rightful heir," but which has fallen through the operation of natural laws and conditions into disuse and desuetude, and has accordingly suffered with the career of living nature "down the ringing grooves of change." Necessarily, this second and rational explanation of the rudimentary appendages of animals and plants is founded on the supposition that nature and nature's creatures are continually undergoing alterations, and that they have been modified in the past, as they will be in all time to come. The ​explanation thus afforded of the nature and origin of these disused parts is endorsed by the fuller knowledge of their history; while, from a study apparently of insignificant interest, may be shown how certain of our living neighbors, along with ourselves, have, from lower states, and from the dawning epochs of the world, literally taken their place "in the foremost files of time."
As most persons who have attentively looked at any common plant can tell, four parts are included in a perfect flower. These parts or sets
Fig. 1.
of organs, as seen in the wallflower, consist (Fig. 1), firstly, of an outer covering colored green, and named the "calyx" (ca). Then comes the blossom or flower itself, forming the "corolla" (co). Inside the corolla we find certain stalked organs, each bearing a little head or "anther," filled with a yellow dust, the "pollen." These organs are the "stamens" (st). Lastly, in the center of the flower, we note the "pistil" (p), or organ devoted to the production of "ovules." The latter, when duly fertilized by being brought into contact with the "pollen" of the stamens, become "seeds," and are capable of growing up, when planted, into new plants. Now, the botanist will inform us that it is a matter of common experience to find some individual plants of a species with well-developed petals or blossoms, and other individuals of the same species with petals in a rudimentary condition, thus proving that the production of imperfect parts in flowers occurs as an ordinary event under our own eyes, and under the common conditions of plant-life. The natural order of plants to which snapdragon belongs presents a peculiarity, inasmuch as in most of its members one of the five stamens is abortive or rudimentary. It should be borne in mind that the botanist possesses a highly interesting and exact method of ascertaining how many parts or organs should be represented in plants. He places his reliance in this respect on the working of what may be called the "law of symmetry." The operation of this law, which may be said to be founded on wide experience, tends to produce a correspondence in numbers between the parts in the four sets of organs of which we have just noted a flower to be composed. Thus, when we count five parts in the green calyx of a plant, we expect to find five blossoms or petals in its corolla; five stamens (or some multiple of five) and five parts (or ​some multiple of that number) in the pistil. Where there appears to be a lack of this numerical correspondence, the botanist concludes that some violation of the law of symmetry has taken place, and that some parts or organs which should normally have been developed have been altered or suppressed. His reasoning, in fact, proceeds on the plain basis of first establishing, through experience, the normal number and condition of parts in the flower of any given order of plants, and of thereafter accounting by suppression or non-development for the absence of parts he expected to have been represented.
Now, in the snapdragon tribe, we find, as a general rule, five parts in the calyx, five petals in the corolla, but only four stamens. Such a condition of matters is well seen in the flower of frog's mouth (Antirrhinum), where we find four stamens, two being long and two short Fig. 2. (Fig 2, A s$^1$ s$^2$), as the complement of the flower. We account for the absence of a fifth stamen by saving it is abortive. But a natural reflection arises at this point, in the form of the query, Have we any means of ascertaining if our expectation that a fifth stamen should be developed is rational and well founded? May not the plant, in other words, have been "created so?" Fortunately for science, Nature gives us a clew to the discovery of the truth in this as in many other cases. In one genus of these plants (Scrophularia), we find a rudiment of a fifth stamen (Fig. 2, Bs), and in snapdragon itself this fifth stamen becomes occasionally fully developed; while another plant of the order (Mullein) possesses five stamens as its constant provision. Unless, therefore, we are to maintain that Nature is capricious beyond our utmost belief, we are rationally bound to believe that the rudimentary fifth stamen of Scrophularia, and the absent fifth stamen of other plants of its order, present us with an example of modification and suppression respectively. The now rudimentary stamen is the representative of an organ once perfect and fully developed in these flowers, and which it perpetuated by the natural law of inheritance until conditions, to be hereafter noticed, shall have caused it to entirely disappear. The case for the natural modification, and that against the imperfect creation of such flowers, is proved by an ingenious experiment of Kölreuter's, upon plants which have the stamens and pistils situated in different plants, instead of being contained in the same flower, as is ordinarily the case. Some staminate or stamen-possessing flowers had the merest rudiment of the pistil developed, while another set had a well-developed pistil. When these two species were "crossed" in their cultivation, the "hybrids" or mule progeny thus produced evinced a marked increase in the development of the abortive organ. This experiment not only proved that, under ​certain conditions, the rudimentary pistil could be improved and bettered, but also the identity of the two pistils, and the high probability that the abortive organ in the one flower was simply the degraded representative of the well-developed part of the other.
As a final example of the manner in which we receive clews toward the explanation of the modifications of flowers, the case of the wallflower is somewhat interesting. This plant and its neighbors possess the parts of the flower in fours. (Fig. 1, A.) There are four sepals and four petals, while six stamens (Fig. 1, B) are developed; the pistil possessing only two parts. Here the law of symmetry would lead us to expect either four stamens or eight—the latter number being a multiple of four. The clew to this modification is found in the arrangement of the stamens. We find that four of the wallflower's stamens are long (Fig. B, st$^1$), while two (st $^2$) are short. The four stamens form a regular inner series or circle, the two short stamens being placed, in a somewhat solitary fashion, outside the others. This condition of matters clearly points to the suppression of two of an originally complete outer row of four stamens, and we receive a clew as to the probability of this view by finding that in some other flowers of the wallflower's group the stamens may be numerous. It is hardly within the scope of the present article to say anything regarding the causes of the conditions or of the agencies through which the modifications of plants are wrought out. Suffice it to remark that the "law of use and disuse" of organs explains the majority of such cases, by asserting that organs become degraded when they are no longer found to be useful to the economy of their possessors. The degradation of a part is to be looked upon as subservient to the welfare of the animal or plant as a whole, and thus comes to be related to the great law of adaptation in nature which practically ordains that
Whatever is, is right.
The animal world presents us, however, with more obvious and better-marked examples of rudimentary organs than are exhibited by the modifications of flowers—conspicuous as many of these latter instances undoubtedly are. Turning our attention first to lower life, we find among insects some notable and instructive illustrations of abortive organs, and also of the ways and means through which the rudimentary conditions have been attained. In the beetle order, the natural or common condition of the wings—which in insects typically number four—is that whereby the first pair becomes converted into hardened wing-cases, beneath which the hinder and useful wings are concealed when at rest. Now, in some species of beetles, we may meet with certain individuals with normally developed wings; while in other individuals of the species we find the wings to be represented by the merest rudiments, which lie concealed beneath wing-cases, the latter being actually firmly and permanently united together. In such a case the ​modification has been extreme, but there can be no doubt that the ancestors of the beetles with modified wings possessed fully developed appendages; otherwise we must regard the order of nature as being one long string of strange and incoherent paradoxes. Mr. Darwin has given us some instructive hints regarding the modification of beetles' wings and feet in his remarks on the effects of the use and disuse of parts in the animal economy. Kirby, the famous authority on entomology, long ago noted the fact that, in the males of many of the dung-beetles, the front feet were habitually broken off. Mr. Darwin confirms the observation of Kirby, and further says that in one species (Onites apelles) the feet "are so habitually lost that the insect has been described as not having them." In the sacred beetle (Ateuchus) of the Egyptians, the tarsi are not developed at all. Mr. Darwin remarks that necessarily we can not, as yet, lay over-much stress upon the transmission of accidental mutilations from parent to progeny, although, indeed, there is nothing improbable in the supposition; and, moreover, Brown-Séquard noted that, in the young of Guinea-pigs which had been operated upon, the mutilations were reproduced. Epilepsy, artificially produced in these latter animals, is inherited by their progeny. "Hence," says Darwin, "it will perhaps be safest to look at the entire absence of the anterior tarsi (or feet) in Ateuchus, and their rudimentary condition in some other genera, not as cases of inherited mutilations, but as due to the effects of long-continued disuse; for, as many dung-feeding beetles are generally found with their tarsi lost, this must happen in early life; therefore the tarsi can not be of much importance, or be much used by these insects."
The beetles of Madeira present us with a remarkable state of matters, which very typically illustrates how rudimentary wings may have been produced in insects. Two hundred beetles, out of over five hundred species known to inhabit Madeira, are "so far deficient in wings that they can not fly." Of twenty-nine genera confined to the island, twenty-three genera include species wholly unable to wing their way through the air. Now, beetles are frequently observed to perish when blown out to sea; and the beetles of Madeira lie concealed until the storm ceases. The proportion of wingless beetles is said by Mr. Wollaston to be "larger in the exposed Desertas than in Madeira itself"; while most notable is the fact that several extensive groups of beetles which are numerous elsewhere, which fly well, and which "absolutely require the use of their wings," are almost entirely absent from Madeira. How may the absence of wings in the Madeiran beetles be accounted for? Let Mr. Darwin reply: "Several considerations make me believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection, combined probably with disuse. For during many successive generations each individual beetle which flew least, either from its wings having been ever so little less perfectly developed, or from indolent habit, will have had the best ​chance of surviving from not being blown out to sea; and on the other hand those beetles which most readily took to flight would oftenest have been blown to sea, and thus destroyed." An instinct of laziness, so to speak, alone or aided by a shortness of wing, developed stay-at-home habits; and such habits would necessarily tend toward the survival and increase of wingless forms. Other Madeiran insects—such as butterflies, moths, and flower-feeding beetles—have well-developed wings, or possess wings relatively larger than they exhibit elsewhere. This observation, remarks Mr. Darwin, is quite in consistency with the theory of the law of natural selection which favors the survival of the fittest. "For when a new insect first arrived on the island, the tendency of natural selection to enlarge or to reduce the wings would depend on whether a greater number of individuals were saved by successfully battling with the winds, or by giving up the attempt, and rarely or never flying."
Among animals of higher rank in the scale than insects, the presence of rudimentary organs is frequently to be demonstrated. What explanation, other than that of degradation and decay owing to disuse, can be offered of the case of the crabs from the Kentucky Cave? Fig. 3. Crabs possess compound eyes borne at the extremities of highly movable stalks, these stalks in the sentinel crab (Fig. 3) being extremely elongated. In some of the Mammoth Cave crabs the stalk remains, but the eye has completely disappeared. As the eyes in such a case could in no sense disappear from any reason connected with injury to the animal, we are absolutely without any reason for their absence other than that of disuse. Professor Silliman captured a cave rat which, despite its blindness, has large, lustrous eyes. After an exposure for about a month to carefully regulated light, the animal began to exercise a feeble sense of sight. Here the modification or darkness has simply affected the function of the eye; in due time the effects of disuse would certainly alter and render abortive the entire organ of sight.
The possession of flying powers is so notable a characteristic of the class of birds that any exception to this rule, and the want of aerial habits, may be rightly regarded as presenting us with a highly anomalous state of matters. Yet instances of rudimentary wings in birds are far from uncommon; and several groups are, in fact, more notable on account of the absence of powers of flight than for any other structural features. The ostrich, for instance, represents a bird the wings of which are mere apologies for organs of flight, and which are used, as every one knows, simply as aerial paddles. The curious Apteryx or ​kiwi-kiwi (Fig. 4) of New Zealand, a near relative of the ostriches and running-birds in general, represents a still more degraded condition of Fig. 4. the organs of flight, for the wing is reduced in size to an extraordinary degree, and exists in a highly abortive condition; while only one complete finger is represented in the hand—other birds, as a rule, possessing three modified fingers. The logger-headed duck of South America has wings so reduced that it can but "flap along the surface of the water," a condition of matters closely imitated among ourselves by the Aylesbury duck—although, indeed, the young ducks are able to fly. The wing of the penguin (Fig. 5) is a mere scaly appendage utterly useless for flight, but useful as a veritable fin, enabling it to swim under water with great facility; and of the auk's wing the same remark holds good. In the
Fig. 5.
birds, then, there is ample evidence of deterioration of organs in the rudimentary nature of the wings of many species. How these conditions have been brought about is not difficult to explain in most instances. In New Zealand, where we find a singular absence of quadrupeds, wingless birds—many being extinct—of which the apteryx is a good example, take the place of the four-footed population. In view of an immunity from the attack of other animals, the ground-feeding habits of these birds would become more and more strongly settled as their special way of life; and, in the pursuit of such habits, the wings, ​seldom used for flight, would degenerate as time passed. The later advent of man, in turn, has exterminated certain races of the wingless birds—such as the Dodo (Fig. 6) and Solitaire (Fig. 7) in Mauritius and Rodriguez—while the wingless and giant Dinornis of New Zealand and its contemporaries have probably been hunted to the death of their species by their human co-tenants of these strange lands.
The ascent to the quadrupeds brings in review before us still more striking illustrations of the apparently incomplete rendering of the structures of animal life. No better instance of the "rudimentary organs" of the naturalist can be found than in the group of the whales, and more especially in the species from which we obtain the commercial whalebone and oil—the Greenland or right whale. This whale possesses no teeth in its adult state, but before birth teeth are found in the gum. These teeth, however, are gradually absorbed, and utterly disappear from the jaws, the adult whale possessing, as is well known, a great double fringe of whalebone-plates depending from the palate. The same remark holds good of the unborn young of ruminants, or animals which "chew the cud"; these animals in their adult state possessing no front teeth in the upper jaw, but in their immature condition developing
| Fig. 6. | Fig. 7. |
these organs—which, by the way, never cut the gum—only to lose them by a natural process of absorption. Now, here there can be no question of use; and certainly no adequate explanation of their occurrence exists, save that which regards these fœtal teeth as the remnants of structures once well developed in the ancestors of the whale-bone whales and ruminants. To this supposition the evidence—avowedly incomplete—obtained from geology gives no contradiction, even if it does not by any means supply the "missing links" in an adequate fashion. We do know that among the oldest of the great leviathans of the past was the Zeuglodon, which had teeth developed much in excess of anything we find represented in the dental arrangements of the ​whales of to-day—a creature this, of which, as regards its teeth at least, modern whales are but shadowy reproductions. While under the shelter of great authority we may declare this ancestor of the whale to have been intermediate in nature between the seals and whales, or between the whales and their neighbors the manatees or sea-cows and dugongs. In either case, the intermediate character of the animal argues in favor of its having been the likely parent of a race dentally degraded in these latter days.
There is little need to specialize further instances of the occurrence of rudimentary organs in the higher animals, save to remark that not the least interesting feature of such cases is contained in the fact that the milk-glands of male animals among quadrupeds—organs which exist in a rudimentary condition—have been known to become functionally active and to secrete milk; this peculiarity having been known to occur even in the human subject. Among the higher quadrupeds, however, there yet remains for extended notice one special instance of the occurrence of "rudimentary organs," wherein, not merely is the nature of the parts thoroughly determined, but the stages of their degradation can be clearly traced through the remarkable and fortunate discovery of the "missing links." Moreover, the case in point, that of the horse, so clearly illustrates what is meant by progressive development or evolution of a species of animals, that it is highly instructive, even if regarded from the latter point of view. Fig. 8 When we look at the skeleton of a horse's fore-limb, we are able, without much or any previous acquaintance with the facts of comparative anatomy, to see that it is modeled upon a type similar to that of the arm of man. Were we further to compare the wing of the bird, the paddle of the whale, the fore-limb of the bat, and the fore-leg of a lizard, with the equine limb, we should find the same fundamental type of structure to be represented in all. Thus we find in the arm of man (Fig. 8)—to select the most familiar example from the series just mentioned—a single bone, the humerus ($^3$), forming the upper arm; two bones (radius ($^4$) and ulna ($^5$)) constituting the fore-arm; eight small bones forming the wrist (carpus): five bones—one for each finger—forming the palm or metacarpus and five fingers, each composed of three small bones, named phalanges, with the exception of the thumb, in which, by a mere inspection of that digit, we may satisfy ourselves only two joints exist. In the wing of the bird (Fig. 9) we find similarly an upper-arm bone or humerus, two bones (radius and ulna) in the forearm; a wrist (b), a thumb (g), and two fingers (c f e d). Now, turning to the fore-limb of a horse (Fig. 10)—the hind-limb being essentially similar, in its general conformation, and corresponding as closely with man's lower limb—we find its conformation to correspond in a remarkable fashion to that ​of man's arm. First, there is the humerus (h), a bone of the horse's upper arm, concealed, however, beneath the skin and muscles, and being, therefore, inconspicuous in the living animal. The horse's forearm, like that of man, contains two bones—radius (r) and ulna (u)—it is true; but the ulna has degenerated in a marked degree, and exists as a mere strip of bone which is tolerably distinct at its upper end, but unites with
| Fig. 9. | Fig. 10. | Fig. 11. |
and merges into the other bone, the well-developed radius. The wrist (w) of the horse naturally succeeds its forearm, but from the fact of the upper arm being concealed beneath the skin and muscles, the wrist is commonly mistaken for the horse's knee. Thus, when a horse chips its "knee," it in reality suffers a contusion of its wrist. Man possesses eight bones in his wrist, the horse has only seven, but the equine wrist is readily recognizable as corresponding with the similar region in man. The greatest difference between the human limb and that of the horse is found in the regions which succeed the wrist, and which constitute the palm and hand. Man has five palm-bones: the horse has apparently but one long bone, the "cannon-bone" (m$^1$), in place of the five. Now, to which of man's palm-bones does this "cannon-bone" correspond? The anatomist replies, "To that supporting the third or middle finger"; and attached to this single great palm-bone the horse has ​three joints or "phalanges" (1, 2, 3) composing his third finger. These joints are well known in ordinary life as the "pastern," "coronary," and "coffin bones"; and the last bears the greatly developed nail we call the "hoof."
Thus the horse walks upon a single finger or digit—the third; and it behooves us to ask what has become of the remaining five—the highest number of fingers and toes found in mammals or quadrupeds? We find that, with the exception of other two the second and fourth fingers—the horse's digits have completely disappeared. The second and fourth fingers have left mere traces, it is true, but it is exactly these rudimentary fingers which serve as the chief clews to the whole history of the equine race. On each side of the single palm-bone of the horse's great finger, we see two thin strips of bone (one of which is represented at m$^2$ Fig. 10), which veterinary surgeons familiarly term "splint-bones." (See also Fig. 12 A, d). But these "splints" bear no finger-bones, and the condition of the horse's "hand" or fore-foot is therefore seen to be of most noteworthy and curious conformation. It may, indeed, sometimes happen that the small pieces of gristle or cartilage may be found at the base of the splint-bones, and comparative anatomists incline to regard these gristly pieces as the representatives of the first and fifth fingers. But the ordinary condition of the horse's hand may be summed up by saying that the animal walks on one well-developed finger—the third—and possesses the rudiments, in the form of the "splint-bones," of other two fingers, the second and fourth. These latter, it need hardly be added, are completely concealed beneath the skin and other tissues of the limb. In the hind-limb of the horse (Fig. 11), a similar modification is observed. The thigh-bone (f e) and knee-cap (p) are readily observed. There is but one toe—the third ($^l$, $^2$, $^3$)—supported by a single cannon-bone (mt$^2$); and there are likewise two splint-bones (one depicted at m$^2$ ), representing the rudiments of the second and fourth toes. The horse's heel, like his wrist, appears out of place, and is popularly named his "hock." The shin-bone (t) is the chief bone of the leg, and has united to it the other bone (fi) succeeding the thigh, named the fibula, and which is seen in man's leg, and in that of quadrupeds at large.
To the eyes even of an unscientific observer, who sees the skeleton of a horse placed in a museum, in contrast with the bony frames of other and nearly related animals, the equine type is admittedly a very peculiar and much modified one. In place of five toes, we find but one; and in the matter of its teeth, as well as in other features of its frame, the horse may be said to present us with an animal form which appears as a literal example of Salanio's remark that
Nature hath framed strange fellows in her time.
A person of a thoroughly skeptical turn of mind might possibly demand to know the exact reasons for the assumption that the ​splint-bones of the horse are in reality the rudiments of the fingers we have represented them to be, and might further demand proof positive of their nature. Fortunately, geology and the science of fossils together come to our aid, with as brilliant a demonstration of the steps and stages of the degradation of the horse's fingers as the most sanguine evolutionist could hope to see. From Mother Earth, whose kindly shelter has sufficed to preserve for us the remains of so many of. the forms of the past, we obtain the means for constructing a genealogical tree of the equine race, by methods of certain kind, and through the exhibition of fossils, each bearing an impress of its history, which, to use Cuvier's expression, "is a surer mark than all those of Zadig."
Our theoretical journey backward into the ages begins with the Recent or last-formed deposits—those which lie nearest the outer surface of our earth. The Recent or Quaternary period forms a division of the Tertiary period, that is, the latest of the three great epochs into which, for purposes of classifying fossil forms by their relative ages, the geologist divides the rock-formations. The Tertiary rocks, commencing the list, with the last-formed or uppermost strata, begin with the Quaternary or Recent deposits; next in order succeed the older Pliocene rocks; then come the Miocene formations, and lastly succeed the Eocene rocks. These last are the oldest of the Tertiary period, and lie in natural order upon the Cretaceous or Chalk Rocks, which themselves belong to an entirely different and anterior (Mesozoic) period in the history of our globe. The first fossil—that is, the last deceased—horses we meet with are found in the Quaternary and Pliocene, or the last-formed deposits of the Tertiary system. Between these earlier Pliocene horses and our own Equidæ there are no material differences; and the limbs of these forms may therefore be diagrammatized as depicted in Fig. 12, A A $^1$; the cannon-bone in all of these figures being marked a; the splint-bones dd; the "pastern" and "coronary" bone b, c, and the "coffin-bone" f.
But near the beginning; of the Pliocene formations of the Old World, and in the oldest of the Miocene rocks which lie below them, we find a member of the horse family which differs in certain important respects from the horses of the Recent period, and from those of to-day. The fossil horses alluded to are found not merely in Europe, but in the Sewalik Hills in India, and they must therefore have possessed a very wide range of distribution. When first discovered, M. de Christol called this species of horse Hipparion, a name which has been still retained for it, amid that constant alteration in zoölogical nomenclature which is the labor of the foolish and the sadness of the wise among us. What are the chief peculiarities of Hipparion? Briefly stated, in the larger development of the "splint-bones" (Fig. 12, C C$^1$), which, according to Owen, must have "dangled by the side of the large and functional hoof (or third toe) like the pair of spurious hoofs behind those forming the cloven foot in the ox." This conformation, continues Owen, "would ​cause the foot of the Hipparion to sink less deep into swampy soil, and be more easily withdrawn than the more simplified horse's foot." Furthermore, the ulna or bone of the forearm, deficient in the horse of to-day, is tolerably well developed in Hipparion.
Backward in time, and in the older Miocene formations of Europe, another fossil horse was disentombed, and was duly described under the name of Anchitherium. This latter horse possesses a completely developed ulna in the forearm, and fibula in the leg; but its chief point of interest lies in the fact that each foot possessed three fully developed toes (Fig. 12, DD$^1$ d, d, c) which apparently must have touched the
Fig. 12.
ground in walking. Already, our splint-bones are seen to better their condition as we pass backward through the ages, and to appear as the natural supports of well-developed second and fourth toes. Here the geological history of the horse in the Old World may be said practically to end. Modern history assures us that the first horses which peopled the New World, and whose descendants roam over American prairies as the famed mustangs, were imported by the Spaniards at the period of the Mexican conquest. Geology has a more curious tale to relate of the New World horses and their history, and gives them an antiquity compared with which the events of man's primitive history in either world are but as yesterday. Recent researches among the rock formations of Western America, in particular, have shown us that it is to the New World we must look for a perfect pedigree of the horse. For, beginning with the horse of to-day, with its splint-bones, we are carried gradually backward in time to the Pliocene horse of the New World named Pliohippus (BB$^1$)—a form not differing materially from the living horse, but serving in a very graduated fashion to introduce us to the older Protohippus, the New World representative of our own fossil Hipparion (CC$^1$), and in some respects a more typical three-toed ​horse than the latter. Our own Anchitherium (D D$^1$) corresponds to the next specimen of the New World—Miohippus by name; and Miohippus evinces a still more important modification in that it possesses a rudiment of the fifth or little finger in addition to the second, third, and fourth digits with which the fore-feet are provided.
The American horses now continue the history of the race in time past without aid or representative from the eastern hemisphere, in so far, at least, as the latest research has shown. To Miohippus succeeds the Mesohippus (E E$^1$) from the American Miocene, which has three well-developed toes, and in addition shows the rudiment of the little finger (E e) of the fore-feet (seen also in Miohippus, D e) in an enlarged condition. Passing to the Eocene formations, the oldest series of the Tertiary rocks, we meet with the next step in the form of the Orohippus (F F$^1$), in which the little finger (e) appears as a veritable member of the hand, the hind-feet still possessing three well-developed toes only: while, consistently with the development of the toes, the ulna of the forearm and fibula of the leg appear as bones of legitimate size, and present a striking contrast to their rudiments in the horse of to-day. The last discovered horse is from the oldest of the Eocene beds; it has been appropriately named Eohippus, and presents us with four complete toes (second, third, fourth, and fifth) on the fore-feet, and a rudiment of the first toe as well; with a trace of the fifth toe of the hind-feet—this last member being, as we have seen, unrepresented in any of the other forms. When the Chalk. Rocks shall have yielded up their fossil horses, it is consistent with logic and reason to expect that the primitive stock of the horses will be discovered with its complete provision of five toes, and its corresponding modifications of form.
To what conclusions, of reasonable kind, do these stable facts regarding the pedigree of the horse naturally lead? The answer is toward a belief in the slow and progressive modification and evolution of the one-toed modern horse from a five-toed ancestor. This process of modification must, of course, have affected its entire frame, but it is sufficient for our present purpose to point out that in the structure of the foot alone we discern the evidence for evolution, as clearly as in the entire organization of the animal. An increase of speed, and obvious advantage over its enemies, would be gained by the horse, as its toes grew "small by degrees and beautifully less"; and the single-toed race has thus practically come to the front in the world of to-day, as the plain and favorable result of the work of degradation among its digits.
Two bony shreds or rudiments thus lay the foundation of a grave conclusion regarding the horse and its manner of development, and exemplify the adage that great and unlooked-for results sometimes spring from beginnings of apparently the most trifling kind. The "splint-bones" form, in fact, a clew which, when rightly pursued, leads not merely to a knowledge of the evolution of the horse, but to an understanding of the entire scheme of nature. The idea, then, of ​"special creation" of the horses does not look well, it must be confessed, in the face of the gradual and obvious modification exhibited by the series of fossil horses, which leads without a break from Eohippus to the modern horse. At most, it may be said, there is but a choice of probabilities offered us. And in the adoption of a scheme of development, and in face of the facts laid before us, it is hard to see any grounds whereon the special-creation theory can be maintained, or the theory of progressive development and evolution denied. For if evolution is the law of the horse's history, it must logically follow that it represents the scheme of nature throughout: since the uniformity of nature, in which we are bound to believe, and to which we are bound to appeal, would utterly negative the idea that evolution should hold good for the horse, and be inapplicable to any other living thing. Because the missing links are not so completely supplied to us in other cases as in the horse, we are not on that account entitled to assume that the theory of development is invalid. We may not see an oak-tree grow inch by inch, but we are as positive as our mental nature will admit, that the oak was once an acorn, and that there has been a progressive growth and increase which might not be apparent to us were we to watch the tree for weeks together. Applying this reasoning to the case before us, it would be as illogical to deny that the order of nature was that of development, as to insist that the oak was created as it stands. The extent of human knowledge, and the duration of human existence, are together inadequate to enable us to discern the progress of this world's order after the fashion whereby, from a lofty elevation, we may trace every winding of a stream. But the probabilities of the case are as overwhelmingly for progressive development, as the direct evidence at hand—exemplified by the horse's pedigree—tells against special and independent creation having been the way of the First Cause in the making of the world and its living things.
The entire scheme of scientific discovery thus depends very largely upon the use made of the hints which nature is continually presenting to the searcher, and on the correct interpretation of the facts he is fortunate enough to elicit in his search. The study of the rudiments of animal and plant structures may well exemplify, from the importance of its results, the value of gathering up the veriest fragments of knowledge. For, as Mr. A. R. Wallace has remarked regarding rudimentary organs, "There must be a cause for them; they must be the necessary results of some great law." And again are this author's words most appropriate when he says: "Many more of these modifications should we behold, and more complete series of them, had we a view of all the forms which have ceased to live. The great gaps that exist between fishes, reptiles, birds, and mammals (that between reptiles and birds is now wellnigh obliterated) would then, no doubt, be softened down by intermediate groups, and the whole organic world would be seen to be an unbroken and harmonious system."—Gentleman’s Magazine.