Popular Science Monthly/Volume 22/December 1882/Speculative Zoology I
By Professor W. K. BROOKS,
OF JOHNS HOPKINS UNIVERSITY.
AT a time when most naturalists who venture at all beyond the facts of life-science are busied with the attempt to trace the relationship between the various groups of living things, and to express this relationship in a tree-like system of classification, it is startling to hear from one of the highest authorities on life-science the statement that "the time for genealogical trees is past. . . . It seems hardly credible that a school which boasts for its very creed a belief in nothing which is not warranted by common sense should descend to such trifling."
It is true that the context seems to show that the author does not visit all attempts at phylogenetic classification with the sweeping condemnation which the passage quoted seems to imply, yet the fact that a high authority upon the subject has made such a statement at all is a sufficient reason why those who believe that the status of modern morphology is not without a basis of common sense should carefully revise their grounds for this belief, in order to decide for themselves how far, and in what shape, such speculations upon the relationships of organisms are admissible, and favorable to the progress of science.
The belief that the present life of the globe is only a very small part of its total fauna and flora is hardly more firmly fixed in the minds of the present generation of naturalists than the belief that the recent species are the modified descendants of those which are extinct; and there are few who would not acknowledge that their conception of the origin of life would be fairly illustrated by comparing the living things of the past and present to a great, many-branched tree, buried in the ground so that only a few scattered groups of twigs are exposed to our direct observation, although these groups show by their arrangement a vague and indefinite relation to the branches below the ground. The twigs which are exposed are the living things which now people the earth, and those twigs and branches and larger trunks which are below-ground are those organisms which are buried in the past, and which we can study only through their fossil remains.
Most naturalists not only believe that, if we could trace back the history of life, we should find each group bearing evidence of wider and wider relationship as we receded from the present time, but they also believe that we should ultimately find that every form of life is related to every other in such a way as to show that, in the remote past, they all met in a single starting-point—the common ancestor of all living things.
When we come to examine the evidence for this theory of the common origin of all life, we find that it is almost entirely general in its character. There is a nearly complete absence of specific and definite proof. We find an abundance of fossil forms, which we may regard as connecting links between one great group of animals and another; but even in the mo it favorable cases the attempt to follow the history of any particular species back for a considerable length of time soon ends in a total failure, for we lose track of its line of descent entirely, and can go on only by substituting, for the species with which we started, the genus, family, or more comprehensive group to which we have traced it.
Once in a while we find, in the later geological formations, a fossil animal which exhibits such affinity to several closely related recent species, that there is a strong presumption that it is the common ancestor from which they have descended. We have enough evidence to enable us to trace the horse and its allies through several geological periods with considerable accuracy, and to reach a form which is widely different from the horse, and which shows relationship to quite different groups of recent mammals. There are a few other cases where the evidence is equally abundant; but more usually it fails completely, and, although the fossils of the later formations show a very close relationship to their living allies, the resemblance is not exact enough to prove that the fossil forms are the direct ancestors of the recent ones, rather than more distant relations, connected by some unknown fossil form.
In place of the exact evidence which would be necessary to prove that the nearest fossil allies of recent species are in the direct line of descent, we have only the vague general evidence which is furnished by those fossils which unite in themselves the characteristics of widely separated families, or classes, or orders of animals. While the attempt to trace any particular species of bird and any given species of reptile to a common ancestor would be hopeless, we do find fossil organisms in whose structure certain general characteristics of the class Birds are united to certain general characteristics of the class Reptiles, in the way which we might expect if those animals are the descendants of true reptiles and the ancestors of the true birds of the present day. There is no proof that this actually is the case, and it is perfectly possible that they are not in the direct line of descent at all. The evidence is entirely circumstantial and indefinite, and it is impossible to show that any of the reptile-like birds which have been discovered have any descendants at the present day. All we can say is that, if our birds are not their descendants, it is very probable that they are the descendants of some other unknown form very much like them.
At the best it is simply a question of probability, not of direct proof, and paleontological evidence is never definite enough to enable us to reach a specific conclusion which may not possibly be wrong, and, this being the state of the case, we may fairly ask whether such speculations upon probability, in the absence of direct evidence, are entitled to be called science. In order to answer this question, and to show that phylogenetic speculation may be strictly within the legitimate scope of science, we will make use of an imaginary illustration.
Suppose that a large continental area, which is inhabited by a homogeneous human race, is invaded by a band of settlers from another country, about as the first European settlers forced themselves upon the homogeneous inhabitants of the United States.
Suppose that these settlers, increasing in numbers, gradually spread over the whole country, interbreeding with the autochthones, until, in later generations, the population comes to consist of two equally distributed races, represented by individuals of pure descent, with strongly marked race-characteristics; and, in addition to these, a great number of hybrids, presenting the characteristics of the two pure races in all degrees and manners of union.
As time goes on, imagine this latter class to increase at the expense of the others, until few persons of pure blood are left; and meanwhile suppose that a number of persons of a third race are introduced, about as the negroes were introduced into this country, and, after this immigration has lasted for a time, suppose it to stop, and let this third race spread and increase, and, after a time, gradually mix with the other two. Let the same process take place again, until the population comes to be made up of four quite dissimilar races with well-marked race characteristics, crossed in such a way that no individuals of the original race or of the first immigration are of perfectly pure blood, while there are a few nearly pure types of the third race, and still more of the fourth. Suppose, now, that an anthropologist undertakes to study the inhabitants of the country in order to learn what he can of their origin and history, and let him begin by attempting to classify them. Any attempt to divide them up into groups will fail, on account of the complexity of their relationships; and, although there are traces of four types, it is not possible to arrange them in four classes, since most of them have resemblances to more than one type. After long study of their relationships, and an enumeration of all the forms which are distinguishable, we may suppose him to hit upon some such expedient for tabulating their resemblances as that of arranging them in four intersecting sets of concentric circles.
One type, the latest arrival, would be represented by a series of larger and larger circles around a center—the center standing for the few individuals of pure blood; the next ring, overlapping the other sets a little, would represent those persons, more numerous than the pure specimens, in whom the characteristics of the race are slightly obscured by characteristics of the other types. The next still larger ring, intersecting still more rings in the other sets, represents the still greater number of individuals of less pure descent, and so on; each larger circle, intersecting the other sets at more points, will represent the manner in which the number of individuals increases as the purity of the type disappears.
The race which has been a little longer in the country will, if it has been equally prolific, and equally inclined to mix with the others, be represented by a system with no center, but with a few very small rings so near the center that they have few points of intersection with the other sets—that is, there will be a few persons with nearly pure blood, but none of perfect purity. The two older races will be represented by systems which are made up entirely of large intersecting circles. After his studies have earned him thus far, we may suppose the anthropologist to speculate how or why it is that the complicated resemblances of this mixed people should follow a system which admits of such a peculiar system of tabulation. He might perhaps invent an hypothesis to explain it—the hypothesis of immigration. As this hypothesis would account for all his facts, there would be a probability in its favor sufficient to justify him in following it out as far as possible, to see what it would lead to, and we may suppose him to continue his studies historically. He would now find that the number of pure specimens of the race which entered the country last was greater a few generations back than at present, while the number of persons who exhibit only slight traces of the characteristics of this race become less numerous as he traces the history backward. Going still further back he would find that the pure-blooded specimens of this race not only become more numerous in proportion to those of mixed blood, but also more restricted in their distribution over the country.
Still farther back he would find records of the entrance of a few perfectly pure representatives of this race into the country, and, continuing his studies, he would meet with no evidence of the presence of any of them before this date.
Continuing his studies he would find that the second race gradually became more pure and more restricted, and, although he might not meet with any record of their first appearance except vague and contradictory traditions, he would find that there was no mention of them in any of the records before a certain date. The older monuments and records would contain references to only two races, and, although there might be no traditions to show that either of these had occupied the country longer than the other, the history of the language and the architectural and other remains would bear witness to the gradual introduction of one of them, and would show that in still more ancient times there were no traces of the existence of more than one race.
The interpretation of the relationships of the people to which he had been led by the study of the living inhabitants would now be far more than a working hypothesis, for everything which had been learned about their past history could be shown to be exactly what we should, by the hypothesis, expect, and he might therefore claim that this was a strictly scientific explanation of the theory of their origin. Although he would have definite proof of only one immigration, he would be fully warranted in concluding that the present inhabitants are the result of the crossing of four races; that at one time only one of these races inhabited the country, and that the other three have been introduced from outside at different times in a definite order.
The mass of evidence which he could bring forward in favor of this opinion would be sufficient to render it more probable than any other explanation of their origin, and would fully justify its acceptance as a scientific truth, but a little examination will show that the evidence is entirely circumstantial, and does not by any means amount to absolute proof, but only to a very great degree of probability. Although it may be true that it is very much more probable than any other explanation, this may be due to imperfect knowledge, and some other explanation may possibly be the true one.
In order to show this, let us suppose that the people of the country have another account of their origin, and believe that the four types were formed ages ago by the breaking up of an homogeneous race into four castes, which have ever since borne substantially the same relations to each other. Suppose that this belief bears such a relation to other traditions, and to certain manners and customs, that it has assumed, in their minds, a very great importance, and is regarded as a point of grave significance, resting upon irreproachable evidence. They might answer the anthropologist somewhat in this manner: You have certainly made out a very ingenious story, and have brought together a mass of evidence which appears to render your view very probable, but you have not shown that no other explanation is possible, and, as we do hold quite a different opinion upon evidence which is satisfactory to us, you must be mistaken. We are open to conviction, and are not unreasonable, but we must have proofs which leave no room for doubt, for as long as there is a chance that you are wrong we must hold to our old view. If you say that all natural knowledge is simply probable, and open to the possibility of error, give us proofs which are as direct as those which are furnished by the physical sciences, instead of the general and circumstantial evidence which you adduce. The student of physics does not ask us to believe that all bodies attract each other according to the law of inverse squares until he has shown us that he is able to prove that every body we bring him does behave in this way; and the chemist shows us that he can separate every specimen of pure water which we furnish into oxygen and hydrogen before he expects us to believe that all water is compounded of these two substances. This is the sort of proof we want; something exact and specific in place of your generalizations. When you can trace back the ancestry of any man we bring you with what you call negro characteristics, tell us who his father and grandfather were, and so on, until you reach one of your negro immigrants—when you can do this with all our inhabitants, and show us that every man with these characteristics is the descendant of one of these immigrants, and that every man with European characteristics has some of the blood of one of your European immigrants in his body, you may claim that you have given us scientific proof of your hypothesis.
If that is too much to ask, trace one of our people back in this way, for it must be plain to you that, if you are not able to do this, your hypothesis is only a probability.
You trace us back for a generation or two with some exactness, but then you make a great leap to some one whom you find mentioned in history, and you trace his ancestors and descendants for a generation or two, and then comes another break. There is no certainty that he has any living descendants, nor is there any certainty that he is at all related to any of your immigrants. We acknowledge your proofs of a negro immigration, and we know that a few other negroes have come to our country from time to time, but their race soon dies out, and you must remember that we have satisfactory evidence that our race had its present character long before the time when you say the foreign elements were introduced.
Even if we grant the accuracy of all the facts which you claim to have discovered, they only show that the history which you have traced out is such a history as your hypothesis would lead you to expect, but this does not prove the truth of the hypothesis. You have only got at a few facts here and there, and future discoveries may show that you are wrong. We are glad to know about the foreign settlers, but you have by no means proved that they were ancestors of ours.
I think that this illustration gives us a fair statement of the value of the evidence for evolution which is furnished by paleontology. There is an absolute and total lack of direct proof, and there must be by the nature of the case, so there is no room to hope for the conversion of any one who is determined to reject the theory as long as doubt is possible; but the end of science is not to proselyte but to discover, and the circumstantial evidence converges in such a way as to give in this case every assurance of substantial accuracy.
There is not a single wild species of animal which can be traced, by direct unbroken pedigree, to an ancestor belonging to a different genus, and no zoölogist has any hope of ever obtaining anything more than circumstantial evidence of such a pedigree; but we can hardly overestimate the vast and increasing stores of evidence in favor of evolution which are yielded by the structural, geographical, and chronological relations between the fauna of the present day and that of the past. It is true that all this evidence is circumstantial, and, although it renders the theory of evolution vastly more probable than any other explanation of the origin of species, it still leaves a possibility that some other explanation may be the true one. Although the investigator who is fully acquainted with all the evidence may feel justified in ignoring this possibility, it still exists.
If the evidence which we have is so circumstantial that it does not amount to absolute proof, it is clear that, even if we fully believe in evolution, we can not hope to trace, with anything like minute accuracy, the past history of any particular form of life; but perhaps an illustration may help to make this clearer:
Let the dots A, B, and C (Fig. 1), represent a number of recent species, each of which has distinctive characteristics of its own, together with other characteristics which are common to all; and let D, E, and F be another set of species related to each other in the same way; and suppose that certain of the common characteristics of D, E, and F are also common to A, B, and C, while others distinguish the one set as a whole from the other as a whole. According to the theory of evolution, we believe that A, B, and C are the descendants of an ancestor from whom they inherit all that they have in common; and that D, E, and F are related to each other in the same way; that the common ancestor of all the forms in the first group had, together with distinctive characteristics of its own, certain other characteristics which it shared with the ancestor of the forms in the second group, and that this similarity was due to inheritance from a still more remote ancestor common to both. This system of relationship might be expressed by a phylogenetic tree, like that which is shown by dotted lines. . . . in the diagram, with six ultimate ramules, two large branches, and a common stem, G. Now, suppose that we discover, in a recent geological formation, a fossil form, M, which resembles A, B, C, D, E, and F in all the features which they have in common. It is possible that this fossil is the form G, from which A, B, C, D, E, and F are descended, but it is not probable that this is the case, for the analogy of recent species compels us to believe that the fossil M was one of several closely related species, G, H, I, K, and L, any one of which may have been the ancestor of the recent forms, and, as M is only one out of several species, the chances are that it is not the root G from which A, B, C, D, E, and F are descended. Even if it could be proved to be the only species of its genus, and therefore the direct ancestor of the recent species, the precise manner in which these are related to it would still remain in uncertainty, for it may have given rise, at about the same time, to two divergent variations, represented by the dotted lines, and these may have led to the two forms a and b, each of which gave rise to variations which resulted in the recent species A, B,
C, and D, E, F, or it may have persisted for a long time, and become slowly modified, through the stages c, d, e, f, into the form g, different from the form G, and still more different from the known fossil M, before it gave rise to the ancestors of the two recent genera. In this case the phylogenetic tree would be represented by the continuous light lines of the diagram. It is possible, again, that A, B, and C are not equally related, but that two of them, B and C, for instance, have had a common ancestor which was not in the line which led to A, and in this case their relationships would be expressed by the broken lines —.—.—.— of the diagram; or, finally, it is perfectly possible, and many naturalists think it is very probable, that evolution has been guided by some more fundamental law than that of the natural selection of divergent variations, and, if this is so, the characteristics which distinguish A, B, and C may be due to something else than descent from a common ancestor, a, different from the common ancestor b of D, E, and F, and all may be the descendants of G, in the way shown by the heavy lines, or A may be the descendant of I; B of K; C of G, etc. If we were to attempt to indicate all the possible ways in which the six living species, A, B, C, D, E, and F, may be related to the fossil M, the diagram would become a confused mass of lines, and we have pointed out enough to show that, in a very simple case, where there are only two living genera and only six species, the attempt to follow them back only two stages to a common ancestor leads to so many possible systems of relationship that there is a very great chance against the truthfulness of any particular one, and we may fairly ask whether the attempt to express the relationships of animals in a tree-like classification can have any scientific value if the chances against its correctness are so very great. At first sight it may seem as if no good could be expected from this sort of speculation, and we may feel inclined to condemn the construction of phylogenetic trees as unscientific; but a little examination will show that all the lines in the diagram agree in one important particular, and trace the recent animals, A, B, C, D, E, and F, back to a remote common ancestor with a general resemblance to M. This, after all, is the essential thing, the gist of the whole matter, for the precise line of descent has no more scientific interest than the exact pedigree of each person would have to the anthropologist of our illustration. Such an exact pedigree would have a certain value as a bit of specific information, but the general evidence is of such a character that it is more logical to accept the conclusion than it is to reject it, and it is as truly scientific as the conclusion of our anthropologist.
We find that living things are related to each other in a peculiar way, which can be explained upon the assumption that they are the modified descendants of more ancient generalized forms, with wider relationships, and this assumption can be readily expressed in the form of a phylogenetic tree. We find, too, that so far as the higher groups of vertebrates, the mammals, reptiles, and birds, are concerned—groups which are of comparatively recent appearance, like the last races of immigrants in our imaginary case—the fossil forms which we meet with are such as our assumption would lead us to expect. The presumption is, therefore, very great that the genetic relations of living things may be expressed with general accuracy by a phylogenetic tree, although the chances of minute accuracy of detail in favor of any particular tree which is drawn up from paleontological evidence are very slight. This lack of minute accuracy can not be urged as an objection to all attempts at following out, in a general way, the lines of evolution of our present groups of animals, according to the best evidence which is attainable, and we must remember that only a very small part of this evidence is furnished by paleontology. If no fossils were known, the facts of comparative anatomy, of embryology, and of geographical distribution would be enough to satisfy us that the living things known to us are the divergent descendants of more generalized ancestors, and that their relationships can be most exactly expressed by a system of converging lines, which meet and form larger branches to represent the extinct ancestors from which our divergent species are descended. The evidence is circumstantial, and only leads to general conclusions, and a complete series of fossil forms is the only absolute proof which we could have; but, in the absence of this proof, the conclusions drawn from the study of living animals are rendered extremely probable by the fact that the fossil members of the more modern groups of animals, such as the mammals and birds, are just such forms as the evidence from other sources leads us to expect, and the attempt to read and interpret such records as we have, and to trace the history of life with as much accuracy as possible, is therefore perfectly legitimate, and may fairly claim the attention of the morphologist.