Evidences Of Organic Evolution; Classification, Palbontology, Distribution, Domestication
( Originally Published 1909 )
IF a child in the kindergarten be given an assortment of cards of various colors and shapes and a number of boxes into which to put them, it becomes evident that the natural tendency is to group together the cards according to their striking resemblances, for most children probably of color. This is simply an elementary exercise in classification and in fundamentals is parallel with the accepted grouping of plants and animals on the basis of resemblances. Many authors have discussed the bearing of classification on the theory of descent, and among the best is the popular statement of the case by Romanes.
"It is a matter of observable fact," he remarks, "that different forms of plants and animals present among themselves more or less pronounced resemblances. From the earliest times, therefore, it has been the aim of philosophical naturalists to classify plants and animals in accordance with these resemblances. Of course the earliest at-tempts at such classification were extremely crude. The oldest of these attempts with which we are acquainted namely, that which is presented in the books of Genesis and Leviticus arranges the whole vegetable kingdom in three simple divisions of Grass, Herbs and Trees, while the animal kingdom is arranged with almost equal simplicity with reference first to habitats in water, earth or air and next as to modes of progression. These, of course, were what may be termed common-sense classifications, having reference merely to external appearances and habits of life.
"But when Aristotle laboriously investigated the comparative anatomy of animals, he could not fail to perceive that their entire structures had to be taken into account in order to classify them scientifically and also that for this purpose the internal parts were of quite as much importance as the external. Indeed, he perceived that they were of greatly more importance in this respect, inasmuch, as they presented so many more points for comparison, and in the result he furnished an astonishingly comprehensive as well as an astonishingly accurate classification of the larger groups of the animal kingdom. On the other hand, classification of the vegetable kingdom continued pretty much as it had been left by the book of Genesis-all plants being divided into three groups, Herbs, Shrubs and Trees. Nor was this primitive state of matters improved upon till the sixteenth century, when Gesner, and still more Cesalpino, laid the foundations of systematic botany.
"But the more that naturalists prosecuted their studies on the anatomy of plants and animals, the more enormously complex did they find the problem of classification become. Therefore they began by forming what are called artificial systems in contradistinction to natural systems. An artificial system of classification is a system based on the more or less arbitrary selection of some one part or set of parts, while a natural classification is one that is based upon a complete knowledge of all the structures of all the organisms which are classified.
"Thus the object of classification has been that of arranging organisms in accordance with their natural affinities, by comparing organism with organism, for the purpose of ascertaining which of the constituent organs are of the most invariable occurrence and therefore of the most typical signification. A porpoise, for instance, has a large number of teeth, and in this feature resembles most fish, while it differs from all mammals. But it also gives suck to its young. Now, looking to these two features alone, should we say that a porpoise ought to be classed as a fish or as a mammal ? Assuredly as a mammal, because the number of teeth is a very variable feature both in fish and mammals, whereas the giving of suck is an invariable feature among mammals and occurs nowhere else in the animal kingdom.
"This, of course, is chosen as a very simple illustration. Were all cases as obvious, there would be but little distinction between natural and artificial systems of classification. But it is because the lines of natural affinity are, as it were, so interwoven throughout the organic world, and because there is, in consequence, so much difficulty in following them that artificial systems have to be made in the first instance as feelers toward eventual discovery of the natural system. In other words, while forming their artificial systems of classification, it has always been the aim of naturalists whether consciously or unconsciously to admit as the bases of their systems those characters which, in the then state of their knowledge, seemed most calculated to play an important part in the eventual construction of the natural system.
"If we were dealing with the history of classification, it would here be interesting to note how the course of it has been marked by gradual change in the principles which naturalists adopted as guides to the selection of characters on which to found their attempts at a natural classification. Some of these changes, indeed, I shall have to mention later on, but at present what has to be specially noted is that through all these changes of theory or principle and through all the ever-advancing construction of their taxonomic science naturalists themselves were unable to give any intelligible reason for the faith that was in them or the faith that over and above the artificial classifications which were made for the mere purpose of cataloguing the living library of organic nature, there was deeply hidden in stature itself a truly natural classification, for the eventual discovery of which artificial systems might prove to be of more or less assistance.
"Linnaeits, for example, expressly says, 'You ask me for the characters of the natural orders ; I confess that I cannot give them.' Yet he maintains that, altho he cannot define the characters, he knows, by a sort of naturalist's instinct, what in a general way will subsequently be found to be the organs of most importance in the eventual grouping of plants under a natural system. 'I will not give my reasons for the distribution of the natural orders which I have published,' he said; 'you, or some other person, after twenty or after fifty years, will discover them and see that I was right.' Thus we perceive that in forming their provisional or artificial classifications, naturalists have been guided by an instinctive belief in some general principle of natural affinity, the character of which they have not been able to define, and that the structures which they selected as the bases of their classifications when these were consciously artificial were selected because it seemed that they were the structures most likely to prove of use in subsequent attempts at working out the natural system.
"This general principle of natural affinity, of which all naturalists have seen more or less well marked evidence in organic nature, and after which they have all been feeling, has sometimes been regarded as natural, but more often as supernatural. Those who regarded it as super-natural took it to consist in a divine ideal of creation according to types, so that the structural affinities of organisms were to them expressions of an archetypal plan, which might be revealed in its entirety when all organisms on the face of the earth should have been examined. Those, on the other hand, who regarded the general principle of affinity as depending on some natural causes, for the most part concluded that these must have been utilitarian causes; or, in other words, that the fundamental affinities of structure must have depended upon fundamental requirements of function. According to this view, the natural classification would eventually be found to stand upon a basis of physiology.
"Therefore all the systems of classification up to the earlier part of the present century went upon the apparent axiom that characters which are of most importance to the organisms presenting them must be characters most indicative of natural affinities. But the truth of the matter was eventually found to be otherwise. For it was eventually found that there is absolutely no correlation between these two things ; that, therefore, it is a mere chance whether or not organs which are of importance to organisms are likewise of importance as guides to classification, and, in point of fact, that the general tendency in this matter is toward an inverse instead of a direct pro-portion. More often than not the greater the value of a structure for the purpose of indicating natural affinities, the less is its value to the creatures presenting it.
"Enough has now been said to show three things. First. that long before the theory of descent was entertained by naturalists, naturalists perceived the fact of natural affinities and did their best to construct a natural system of classification for the purpose of expressing such affinities. Second, that naturalists had a kind of instinctive belief in some one principle running through the whole organic world, which thus served to bind together organisms in groups subordinate to groups that is, into species, genera, orders, families, classes, sub-kingdoms and kingdoms. Third, that they were not able to give any very intelligible reason for this faith that was in them; sometimes supposing the principle in question to be that of a supernatural plan of organization, sometimes regarding it as dependent on conditions of physiology and sometimes not attempting to account for it at all.
"Of course it is obvious that the theory of descent furnishes the explanation which is required. For it is now evident to evolutionists that, altho these older naturalists did not know what they were doing when they were tracing these lines of natural affinity, and thus helping to construct a natural classification I say it is now evident to evolutionists that these naturalists were simply tracing the lines of generic relationship. The great principle pervading organic nature, which was seen so mysteriously to bind the whole creation together as in a nexus of organic affinity, is now easily understood as nothing more or less than the principle of Heredity."
Darwin first called attention to this line of evidence for evolution in the following words: "Naturalists try to arrange the species, genera and families in each class on what is called the Natural System. But what is meant by this system? Some authors look at it merely as a scheme for arranging together those living objects which are most alike and for separating those which are most unlike or as an artificial method of enunciating, as briefly as possible, general propositions that is, by one sentence to give the characters common, for instance, to all mammals, by another those common to all carnivora, by another those common to the dog-genus, and then, by adding a single sentence, a full description is given of each kind of dog. The ingenuity and utility of this system are indisputable. But many naturalists think that something more is meant by the Natural System; they believe that it reveals the plan of the Creator, but unless it be specified whether order in time or space, or both, or what else is meant by the plan of the Creator, it seems to me that nothing is thus added to our knowledge. Expressions such as that famous one by Linnaeus, which we often meet with in a more or less concealed form, namely, that the characters do not make the genus, but that the genus gives the characters, seem to imply that some deeper bond is included in our classifications than mere resemblance. I believe that this is the case and that community, of descent the one known cause of close similarity in organic beings is the bond, which tho observed by various degrees of modification, is partially revealed to us by our classifications."
Before the days of Darwin naturalists had classified plants and animals on a tree-like plan and had rejected the old ladder series of early systematists. The tree system is the one which all naturalists regard as the true one. "According to this system," Romanes points out, "a short trunk may be taken to represent the lowest organisms which cannot properly be termed either plants or animals. This short trunk soon separates into two large trunks, one of which represents the vegetable and the other the animal kingdom. Each of these trunks then gives off large branches signifying classes, and these give off smaller but more numerous branches, signifying families, which ramify again into orders, genera and finally into the leaves, which may be taken to represent species. Now, in such a representative tree of life, the height of any branch from the ground may be taken to indicate the grade of organization which the leaves, or species, present; so that, if we picture to ourselves such a tree, we may understand that while there is a general advance of organization from below upward, there are many deviations in this respect. Sometimes leaves growing on the same branch are growing at a different level especially, of course, if the branch be a large one, corresponding to a class or sub-kingdom. And sometimes leaves growing on different branches are growing at the same level; that is to say, altho they represent species belonging to widely divergent families, orders or even classes, it cannot be said that the one species is more highly organized than the other.
"Now, this tree-like arrangement of species in nature is an arrangement for which Darwin is not responsible. For, as we have seen, the detecting of it has been due to the progressive work of naturalists for centuries past, and even when it was detected, at about the commencement of the present century, naturalists were confessedly unable to explain the reason of it or what was the underlying principle that they were engaged in tracing when they proceeded ever more and more accurately to define these ramifications of natural affinity. But now we can clearly perceive that this underlying principle was none other than Heredity as expressed in family likeness likeness, there fore, growing progressively more unlike with remoteness of ancestral relationship.
"First of all, and from the most general point of view, it is obvious that the tree-like system of classification, which Darwin found already and empirically worked out by the labors of his predecessors, is as suggestive as any-thing could well be of the fact of genetic relationship. For this is the form that every tabulation of family pedigree must assume, and therefore the mere fact that a scientific tabulation of natural affinities was eventually found to take the form of a tree is in itself highly suggestive of the inference that such a tabulation represents a family tree. If all species were separately created, there can be no assignable reason why the ideas of earlier naturalists touching the form which a natural classification would eventually assume should not have represented the truth why, for example, it should not have assumed the form of a ladder (as was anticipated in the seventeenth century), or of a map (as was anticipated in the eighteenth), or, again, of a number of wholly unrelated lines, circles, etc. (as certain speculative writers of the nineteenth century have imagined). But, on the other hand, if all species were separately and independently created, it becomes virtually incredible that we should everywhere observe this progressive arborescence of characters common to larger groups into more and more numerous and more and more delicate ramifications of characters distinctive only of smaller and smaller groups. A man would be deemed in-sane if he were to attribute the origin of every branch and every twig of a real tree to a separate act of special creation, and altho we have not been able to witness the growth of what we may term in a new sense the Tree of Life, the structural relations which are now apparent between its innumerable ramifications bear quite as strong a testimony to the fact of their having been due to an organic growth as is the testimony furnished by the branches of an actual tree."
Summarizing, it is established that "all the general principles and particular facts appertaining to the natural classification of plants and animals are precisely what they ought to be according to the theory of genetic descent, while no one of them is such as might be and, indeed, used to be expected upon the theory of special creation.
"First of all we must take note that the classification of plants and animals in groups subordinate to groups is not merely arbitrary or undertaken only for a matter of convenience and nomenclature such, for instance, as the classification of stars in constellations. On the contrary, the classification of a naturalist differs from that of an astronomer, in that the objects which he has to classify present structural resemblances and structural differences in numberless degrees, and it is the object of his classification to present a tabular statement of these facts. Now, long before the theory of evolution was entertained, naturalists became fully aware that these facts of structural resemblances running through groups subordinate to groups were really facts of nature and not merely poetic imaginations of the mind. No one could dissect a number of fishes without perceiving that they were all constructed on one anatomical pattern which differed considerably from the equally uniform pattern on which all mammals were constructed, even altho some mammals bore an extraordinary resemblance to fish in external form and habits of life. And similarly with all the smaller divisions of the animal and vegetable kingdoms.
"Everywhere investigation revealed the bonds of close structural resemblances between species of the same genus, resemblance less close between genera of the same family, resemblance still less close between families of the same order, resemblance yet more remote between orders of the same class and resemblance only in fundamental features between classes of the same subkingdom, beyond which limit all anatomical resemblance was found to disappear the different sub-kingdoms being formed on wholly different patterns. Furthermore, in tracing all these grades of structural relationship, naturalists were slowly led to recognise that the form which a natural classification must eventually assume would be that of a tree, wherein the constituent branches would display a progressive advance of organization from below upward.
"Now we have seen that altho this tree like arrangement of natural groups was as suggestive as anything could well be of all the forms of life being bound together by the ties of genetic relationship, such was not the inference which was drawn from it. Dominated by the theory of special creation, naturalists either regarded the resemblance of type subordinate to type as expressive of divine ideals manifested in such creation or else contented them-selves with investigating the facts without venturing to speculate upon their philosophical import. But even those naturalists who abstained from committing themselves to any theory of archetypal plans did not doubt that facts so innumerable and so universal must have been due to some one coordinating principle that, even tho they were not able to suggest what it was, there must have been some hidden bond of connection running through the whole of organic nature. Now, as we have seen, it is manifest to evolutionists that this hidden bond can be nothing else than heredity, and, therefore, that these earlier naturalists, altho they did not know what they were doing, were really tracing the lines of genetic descent as revealed by degrees of structural resemblance that the arborescent grouping of organic forms which their labors led them to begin, and in large measure to execute, was in fact a family tree of life.
"Here, then, is the substance of the argument from classification. The mere fact that all organic nature thus incontestably lends itself to a natural arrangement of group subordinate to group, when due regard is paid to degrees of anatomical resemblance this mere fact of itself tells so weightily in favor of descent with progressive modification in different lines, that even if it stood alone it would be entitled to rank as one of our strongest pieces of evidence. But, as we have seen, it does not stand alone. When we look beyond this large and general fact of all the innumerable forms of life being thus united in a tree-like system by an unquestionable relationship of some kind, to those smaller details in the science of classification which have been found most useful as guides for this kind of research, then we find that all these details, or empirically discovered rules, are exactly what we should have expected them to be, supposing the real meaning of classification to have been that of tracing lines of pedigree."
Equally illumining is the massed evidence from Paleontology. The record of the rocks as shown by fossils has always been one of the most important lines of evidence for the theory of descent. In Darwin's time the critics never grew tired of demanding proof from paleontology. A great deal of such proof has been brought forth, but paleontologists caution against expecting a complete record of the animal and plant life of the past. The record is very imperfect, "but," to quote from Metcalf's 'Organic Evolution' "so far as it goes it is an actual record. Only very unusual circumstances will secure the preservation of any animal or plant as a fossil. An organism, or portion of an organism, to be so preserved usually must be hard; it must be buried beneath soil of the proper kind and when buried must be impregnated with mineral salts or in some other way preserved from disintegration.
"When once converted into a fossil it must escape destruction at the hands of those agencies that are constantly destroying the rocks ; heat, pressure, the disintegration that comes from exposure to the atmosphere, abrasion by ice and especially erosion by water. The character of whole continents has been repeatedly changed by these agencies. No wonder then, since fossilization is rare and the destruction of fossils when once formed so easy, that our record of past faunas and floras is so scant. It is a cause for con-gratulation that we have so much of a record as we do possess. Thousands of species of fossil plants and animals are known, and as yet but a small portion of the earth has been searched." Attention will be given to but a few illustrations of the kind of record found in the fossil-bearing rocks, and those records naturally will be chosen that are fairly complete.
Turning to a few illustrations of the origin of particular species or organs, the same principle of gradual increase in complexity is found in coming from the older to the younger geological formations. The record of the evolution of branching antlers in the deer, as before mentioned, is fairly complete. The first deer in the early Miocene had no antlers at all. In the middle Miocene are found deer with two-pronged antlers of small size. In the upper Miocene and lower Pliocene are found three-pronged antlers somewhat larger. In the later Pliocene four-pronged and five-pronged antlers and still larger are met, while in the Pleistocene clays are seen arborescent antlers like those of the modern deer. (Fig. 21.)
Out of the mass of evidence, one further illustration must needs be sufficient. The record of the origin of the horse, worked out by American paleontologists from American fossils, is probably the best example of paleontological evidence of evolution. The horse is especially peculiar in the character of its feet and teeth, and attention will be directed to these points as shown in the accompanying illustrations. In the lower Eocene rocks an animal, Phenacodus, about the size of a fox, is found having five well developed toes on each foot, and with short and but moderately corrugated teeth. This is one of the simplest known relatives of the hoofed mammals, and from forms something like Phenacodus must have been developed the elephant, rhinoceros, hog, sheep, camel, and all the other hooted mammals, including the horse and its long line of ancestors. Observe the steps in the transformation of the five toed limb of a form like Phenacodus into the one toed limb of the horse.
These are, of course, but illustrations of the kind of testimony which the study of the rocks contributes to-ward the proof of the theory of descent. There is, how-ever, an enormous body of uniform evidence to prove two general facts of the highest importance in regard to the theory. The first of these general facts is that an increase in the diversity of types both of plants and animals has been constant and progressive from the earliest to the latest times, as it is rational to anticipate that it must have been on the theory of descent in ever-ramifying lines of pedigree. And the second general fact is that through all these branching lines of ever-multiplying types, from the first appearance of each of them to their latest known conditions, there is overwhelming evidence of one, great law of organic nature—the law of gradual advance from the general to the special, from the low to the high, from the simple to the complex.
The argument for the theory of descent deduced from a comparative study of the phenomena of paleontology the distribution of species in time can be supplemented by a like argument derived from a comparative study of the phenomena of geographical distribution, a distribution of the species of today in space. In considering the distribution of living things on the earth, the first thing to be noted is that there is a decided difference in the animals and plants of different regions. At first sight, differences of climate and other physical conditions would seem to account for this, but there are many phenomena of distribution which cannot thus be explained. Countries exceedingly similar in climate and physical conditions may have quite a different fauna and flora, while those that are unlike may be characterized by similar species of plants and animals.
If regions in Australia, South Africa and western South America, between latitudes twenty-five and thirty-five degrees, are compared, it will be found that they are extremely similar in climate and physical conditions; yet it would not be possible to point out three faunas and floras more utterly dissimilar. Or, on the other hand, if the productions of South America south of latitude thirty five be compared with those north of twenty five degrees it will he noted that altho climatic conditions are decidedly different, these productions are incomparably more closely related than they are to the productions of Australia or Africa under almost the same climatic conditions.
The most interesting evidences of all, however, are de-rived from a study of distribution in combination with geological records. A good sample of this sort of evidence is to be found in the distribution of marsupials (kangaroos, bandicoots, etc.) over the earth. This singular and lowly organized type of mammals constitutes almost the sole representative of the class in Australia and New Guinea, while it is entirely unknown in Asia, Africa or Europe. It reappearsin America, where several species of opossums are found. This anomaly of distribution at first was a puzzle to evolutionists, for it seemed unreasonable to postulate an earlier direct connection of these countries, which are too distant from each other to allow for migration in any other way. When, however, the geological history of the marsupial is taken into consideration the difficulty vanishes, for fossil records give abundant evidence that at one time before the more complex types of mammals came into existence the marsupials were spread over the whole eastern and western hemispheres and that as the higher mammals developed they exterminated the more primitive marsupials, except that in Australia and New Guinea the earlier forms persisted and in America the opossums remained.
That this wide distribution of marsupials was possible in early geologic times is further supported by the fact that other geological evidence shows that Australia and New Guinea were once connected, or nearly connected, with the Malay Peninsula, making migration from the mainland possible. Moreover, the evidence shows that at that early time a very mild climate prevailed far up into the Arctic regions, hence it is not difficult to believe that migration from Europe and Asia was not unusual. There are many other kinds of evidence to be derived from the study of distribution, some of the illustrations being even more striking than those quoted, but these will be sufficient to show the relation of this kind of evidence to the doctrine of evolution.
The last line of evidence to be considered is that of 'domestication.' Every one is familiar with the great modifications which have been brought about in domestic animals, such as horses, cattle, dogs, pigeons, canary birds, etc.; in food plants, as cereals, cabbages, lettuce, radishes, berries, fruits, etc. ; and in ornamental plants, as roses, carnations, dahlias, pansies and a host of other forms. Moreover, the ease with which varieties of a given form can be produced is attested to by the work of experimental breeders.
Moreover, many of these varieties differ markedly from the forms from which they were derived. "Even during the brief history of man," says Le Conte, "have been formed races of different domesticated animals and useful and ornamental plants, differing so greatly from each other that if found in the wild state they would unhesitatingly be called different species, or even in some cases different genera."
It is on this last point that objection is often taken to this line of proof. Are even the extreme artificial varieties of any form distinct species or must they be considered but varieties of the original form? This question shows the opportunity for opposing views hinging on the definition to be given to the term species.
It might well be asked, What are the differences between the artificially-made extreme varieties (often called races), equivalent, so far as difference of form is concerned, to species, and real natural species? On this question much technical discussion could be given. It will be necessary, however, to allude here only to the most broadly recognised difference, namely, that artificially-made varieties intercross freely in breeding, producing offspring which are indefinitely fertile, while natural species do not intercross.
A great deal of study has been given this phenomenon. Darwin sums up his attitude toward it as follows : "It can no longer be maintained that varieties when crossed are invariably quite fertile. From the great difficulty of ascertaining the infertility of varieties in a state of nature, for a supposed variety, if proved to be infertile in any degree, would almost universally be ranked as a species; from man attending only to external characters in his domestic varieties, and from such varieties not having been exposed for very long periods to uniform conditions of life; from these several considerations we may conclude that fertility does not constitute a fundamental distinction between varieties and species when crossed. The general sterility of crossed species may safely be looked at, not as a special acquirement or endowment, but as incidental on changes of an unknown nature in their sexual elements."
It is true that horticulturists and breeders are intent only on making varieties along the lines of use or beauty from man's standpoint that is, in size, structure, color, habits, etc., so called morphological varieties and not on making physiological species. There is, however, little doubt that mutually infertile races could be bred if the selection of individuals for breeding should be chosen with this idea in mind. Breeders have not cared to breed with this object in view. On the contrary, cross-sterile varieties would be a positive disadvantage to them in limiting the range of their experiments.
The important point so far as evidence of the truth of organic descent is concerned is to be found in the known changes in type which have been seen to occur in domesticated animals and plants and in the diversity of forms which are known to have been derived from a few simpler types. As Professor Bailey has said, "If the prejudices of scientists respecting the so-called artificial production of species could be overcome, he could just as well draw his proofs of evolution from what has already been done with cultivated plants and domesticated animals as from similar results which might arise in the future from independent efforts."
The trend of the evidence which has been used to prove the truth of the theory of descent having been outlined, the various causes or factors which have brought about organic evolution demand attention. These bear within themselves many problems of no little subtilty and are borne out by certain correlations of facts which are of absorbing interest.