Popular Science Monthly/Volume 22/January 1883/Speculative Zoology II
By Professor W. K. BROOKS,
OF JOHNS HOPKINS UNIVERSITY.
WE will now examine our various sources of information, in order to see how far the evidence which they furnish can be used to establish phylogenies.
Comparative anatomy might not at first sight be expected to yield much of this kind of evidence, for the only animals which we can study thoroughly are those recent ones which have diverged very widely from their remote ancestors; and, while it is true that the study of the structure of living animals does not furnish direct evidence, the doctrine of homology supplies a means of sifting out, by general comparisons, what has been recently acquired from what is more deep seated, and of thus arranging animals in a series of groups of greater and greater extent and less and less contact. This classification of animals upon morphological grounds is essentially phylogenetic, for the difference between a system of converging lines and a system of more and more inclusive definitions is simply a difference in the manner of expression; nor can it be said that the one method assumes the disputed point, genetic relationship, any more than the other, for the naturalist who believes that classification is not simply a matter of convenience, but that there is one natural system, and that, according to this system, living things fall into a few great groups, each of which is characterized by certain general features, and that each of these groups is divided into smaller groups distinguished in a similar way, and these again into smaller groups, and so on, tacitly assumes that the natural system of classification or relationship is what we should expect it to be if the theory of descent with modification is true.
If there is a natural "systematic classification," it must be exactly the same as a phylogenetic tree, and the idea of descent is no more essential in the one case than it is in the other; they are simply different ways of expressing the same thing, the relationship of living things, and neither of them involves more than the other any particular interpretation of the word relationship; nor can it be said that, while the one method assumes that the larger trunks of the system have at some time been embodied in actual organisms, the second method allows us to believe that these groups are purely ideal, for, although this latter conception may have been defensible to some extent in the early days of morphological science, the progress of discovery has shown that even nowadays animals exist in which the characteristics of a branch or of a class or order are exhibited in their simplicity, and uncomplicated by the presence of the characteristics of any of the minor divisions of the group: thus amphioxus is a generalized or diagrammatic vertebrate, with structural features which are common to the whole group, and none of the distinctive marks of any of the great subdivisions of the group. In a phylogenetic tree such a form would be represented by a line running almost directly upward from a point where great branches diverge from a common stem; and the fact that these generalized forms are much more often found among fossil than among recent animals is very suggestive.
This short analysis is sufficient to show that the essential similarity between a system of classification based on homology and a phylogenetic tree is so great that all objections to the one method of generalizing from the facts must apply to the other. If phylogenetic speculations retard science, speculations upon homology must do the same thing, and the only way to avoid danger will be to stick to facts, and, stripping our science of all that renders it worthy of thinking men, to become mere observing machines.
As it is plain that the strictest construction of the proper scope and limits of scientific thought does not make any such demands as this, we may feel at liberty to speculate upon phylogeny with such a basis as is furnished by the comparative study of the systematic relationship of living animals, but we are not able to go very far in this direction, for nearly all living animals fall into a few great well defined groups, and generalized types are the exception. Our conclusions must, therefore, be very vague and general, and we turn to the paleontological record for more exact data; but here, again, we soon meet with limitations which prevent any very exact and definite generalizations. One of these limitations, the imperfection of the record, we have already examined, and another is due to the fact that most of the main stems of the phylogenetic tree, and the minor branches of many of them, were established before the time of the oldest fossiliferous rocks, and we can not hope to find fossil remains of the unspecialized ancestors from which they were derived. This renders it hopeless for us to attempt actual proof of the more deep-seated phylogenetic relationships; and another consideration, which we will now examine, renders the discovery of the exact relationship of smaller groups of genera and species almost equally hopeless, even when the most ample series of fossils is discovered.
All that we know of variation indicates that it is not induced in the adult, but that it is congenital, and the effect of some force which has acted upon or through the parents. Hence it happens that, when a new variety is produced, it is not usually by the addition of something new to the characteristics of a mature animal, but by a divergence which shows itself before maturity is reached. When we compare two closely related species or varieties of birds, we do not find that one is like the other with additions, but we find that the young are alike up to a certain point, where the divergence of the adults begins. If the letter A, in diagram 2, represents an adult animal, and the series of dots in the vertical line a few of the stages of its development between birth and maturity, the relationship between A and a closely allied species, B, would not usually be of such a kind that it could be expressed by the line A B, P but by a line B c, running back to a younger stage of A. Since the duration of mature life is usually much longer than the period of development, adults are generally more numerous than young individuals, and, as their hard parts are in most cases more fully developed, there is a much greater chance of finding adult than young specimens of fossil species. Even if we should find in a recent geological formation the adult fossil ancestor A of a recent species B, the agreement between the two would be inexact, and could not be fully perceived until we had compared a number of stages in the development of B with corresponding stages in the development of A; and a reference to diagram 2 will show that the actual relationship of B to still another species, D, might be just as close as its relationship to A, although B might be much more different from D than from A. The complexity of the case would be still further increased if D were met with in a more recent stratum than that which yielded A, and in this case the preservation, discovery, and identification of three distinct sets of young forms would be necessary before the relationships of A, B, and D could be unraveled. In nature few cases are so simple as this, and, where a dozen species are involved, the complexity would be so great that no one who has had any practical acquaintance with the difficulty of identifying immature animals with certainty, without rearing them, could have any hope of complete, exact, and definite evidence of phytogeny from fossils.
While this is true in every case, its truth is most obvious where animals have become adapted to new conditions of life, not by the acquisition of new specializations of structure, but by the loss of old ones. The occurrence of unquestionable cases of simplification, or what is usually called degradation, is well known to naturalists, but, as these cases are not so well known to the unscientific public as their significance in any general theory of life demands, a short account of one of the less complex instances will not be out of place.
Entoconcha is an extremely simple, worm-like animal, which lives, as a parasite, inside the body of an holothurian. It is fastened, by a button-like head, into a perforation in the wall of the digestive tract of the holothurian in such a way that, while its mouth opens into the digestive cavity, its long, contorted body hangs in the body cavity of its host, so that it is bathed by its fluids, and protected by its bodywall. As the digested food passes by its mouth the animal sucks it into its rudimentary stomach, and, as all its wants are thus provided for, the conditions of its life are extremely simple, and its bodily structure exhibits a corresponding simplicity, and it may be described as a long, cylindrical, worm-like animal, with a simple, pouch-like stomach which opens by the mouth at the anterior end, and occupies about one half the length of the body, while the other half is filled by the equally simple organs of reproduction. It is as lowly organized as the simplest of parasitic worms, and it is only by a study of its development that we learn it is not a worm at all, but a gasteropod mollusk, which has become degraded or simplified to adapt it to a parasitic life. The ordinary gasteropods, the snails, conchs, etc., are animals of quite high organization. They are usually provided with a protecting shell, and their organs of locomotion are well developed. In connection with a specialized muscular system, they possess a well-marked and complex nervous system, as well as sense-organs, such as eyes, tentacles, and hearing organs. The digestive organs are quite highly specialized, and consist of parts which bear a close physiological resemblance to those of a vertebrate. The food is masticated by a very complicated system of jaws and teeth, and after it has been mixed with a secretion, which is poured into the mouth by salivary glands, it passes through a long muscular œsophagus into the stomach, where it is acted upon by fluids furnished by a liver and other glands, before it passes into the long, convoluted intestine, where the nutritive portions are absorbed into the blood, while the waste products are discharged from the body through the anus. The nutritive matter is driven to all parts of the body, with the blood, through a system of arteries and veins, by a pulsating heart, and during a part of its course the blood passes through respiratory organs, where it is aerated by contact with the air or water. The waste products are excreted from the blood by renal organs, and the organs of reproduction are extremely complicated and highly specialized.
The animal which has been seen to hatch from the egg of entoconcha is a young gasteropod, which, like the young of ordinary forms, has a spiral shell, a muscular locomotor foot, tentacles, eyes, hearing organs, and a nervous system like that of other gasteropods at the same stage of growth. The digestive tract is divided into regions, like those of an ordinary gasteropod, and the young entoconcha has no peculiarities to indicate that the structure and habits of the adult are to be in any way strange or unusual; but after a time it finds its way, in some manner which has not been clearly made out, into the body of an holothurian, and this change in its habits is accompanied by a most marvelous change in its organization. It now has no shell, since the body-wall of its host affords ample protection, and, as it no longer needs to change its position in search of food, or to escape its enemies, its foot and specialized muscles have disappeared. Its organs of sense are wanting, and the nervous system is so rudimentary that no traces of it can be discovered. It has no need of organs to capture, masticate, or digest its food, since it sucks this, already digested, from the stomach of its host, and its digestive system has accordingly become reduced to a simple pouch, with only one opening—the mouth—and, as the whole surface of the body is bathed by the blood which is aërated in the respiratory organs of the holothurian, it has no need for gills, or heart, or blood-vessels, and, so far as our knowledge goes, these organs are entirely lacking.
Of the highly specialized organs of a gasteropod only the simple stomach, the reproductive organs, and the slightly muscular body-wall remain, and no person who is not acquainted with the fact that young snails with spiral shells have been seen to come from its eggs would suspect that entoconcha is a mollusk.
Such cases, which modern research has proved to be by no means infrequent, show that the comparative study of adult animals can not furnish a complete key to their past history, and they also illustrate how little is to be hoped for from paleontology.
No one who accepts the doctrine of descent with modification, and is familiar with the embryology of the gasteropods, can doubt that, if we were able to trace back the pedigree of entoconcha, we should be led to a remote ancestor which was an ordinary gasteropod; not necessarily a species exactly like any we know, but a form with general gasteropod characteristics at least: nor can we doubt that, if we were able to study the embryology of this ancestral form, which we may represent by the letter A in diagram 3, we should find its life, from the egg to maturity, to be made up of a series of stages, a, b, c, d, e, f, etc., substantially like stages in the life of ordinary gasteropods, B and C. The relation between the entoconcha of the present day, D, and this gasteropod ancestor, is shown by the curved line to D. Starting with the egg, the young entoconcha passes through a series of stages, a, b, c, d, e, f, g, h, which are like stages in the development of an ordinary gasteropod, and therefore like stages in the life of its gasteropod ancestor, A; hut, after reaching a certain point, it takes the back track shown by the unbroken line, and, gradually losing the structural complexity which has been acquired, becomes an adult, D, which has reproductive organs, but is, in other respects, as unspecialized as an ordinary gasteropod at one of its earliest embryonic stages, b.
It is obvious that paleontology can give us little help in tracing out such a life-history as this, and we turn to the remaining source of evidence, embryology, to examine how far the facts furnished by this department of life-science can afford a basis for phylogenetic generalizations.
The case which we have just examined shows that the embryology of two related forms may be essentially the same, since both of them have inherited the greater part of their life-history from a common parent, and it would seem at first sight as if all that we need, to enable us to trace out the relationship of all living animals, is a complete acquaintance with the embryology and metamorphosis of each one of them. A comparison of all the stages in the life of one species with all the stages in the life of another species of the same genus ought to show essential identity; and a comparison of the stages of development of the species of one genus with those of the species of a related genus ought to show how far their history has been the same: the common features in the embryology of two allied families should show how far the history of the species in one of them has been the same as that of the species in the other, and so on, each wider and wider comparison showing broader and broader relationships, until the features which are common to the embryos of all animals unite them into one great group.
As this may be clearer in a more abstract shape, I will try to state it in the form of a diagram (see Fig. 4).
Suppose that, in studying the development of four species, 1, 2, 3, 4, we find that 1 passes through a series of stages, a, b, c, d, e, f, g, h; that 2 presents the series a, b, c, d, e, f, i, j, k; while 3 passes through the stages a, b, c, l, m, n, o, p, q; and 4 through the stages a, b, c, l, m, r, s, t, u. A comparison of these four life-histories would indicate that their common relationships are such as are represented by the four branched tree shown in Fig. 4. We have already seen that it is perfectly possible that n or c or e may not have been an adult animal, but simply a stage in the development of an unknown adult, x; so there would not be much chance of finding m or e or c as a fossil, and the embryonic record would not show us what the common ancestor of 1 and 2 or the more remote ancestor of all four species actually was, but would simply show that they are related in this way; but it would show this relationship as conclusively as the vertebrate relationship of birds and mammals is shown by the presence of a backbone in each of them. It will be seen that the facts in this imaginary case belong to the same category with the facts of homology, but that they
furnish a much more complete index of relationship, since they cover the whole life of the animal, instead of its adult form alone.
As a matter of fact we do find in nature something like this hypothetical case, and it is universally recognized that an acquaintance with all the stages in the growth of an animal is the greatest aid to the discovery of its true affinities; as is well shown by the case of entoconcha, and by the barnacles which were classed among the mollusca until a knowledge of their development showed that they are Crustacea. When descriptive embryology was in its infancy, it so frequently happened that a knowledge of younger stages threw a flood of light upon the affinities of doubtful forms, that naturalists felt a growing hope that here was the true key to the natural system of classification, and that all they needed for reading the riddle was a thorough knowledge of the whole course of development of each form of life. If the embryology of each animal were a fixed quantity, this purely descriptive knowledge would undoubtedly furnish such a basis for phylogenetic generalizations; but the great advances which have been made in this field within the last twenty years show conclusively that this is not the case, but that the early stages in the life of an animal may undergo modifications which are quite independent of any which may meanwhile take place in the adult, so that, while the paleontological record is faulty through incompleteness, that of embryology is faulty through distortion and secondary modification. In general we do find the young stages of related species more like each other than the adults, and the distinctive characteristics of each are usually acquired gradually during the process of development; but this rule is by no means universal, and there are very many cases in
|Fig. 6.||Fig. 5.|
which the adults of related species are more alike than the young, and in some cases the difference between the young forms is so great that their close relationship would hardly be suspected until each had been traced to its adult form; and we have the converse of the case of entoconcha where the true affinities of a greatly modified adult are shown by its younger stages.
I give four figures to illustrate one such instance, which is not by any means exceptional or extreme; and in certain groups of animals, such as the Crustacea, such cases are quite numerous.
Fig. 5 is a magnified side-view of a crustacean, Sergestes, which is not quite full grown, but still essentially like the adult; and Fig. 6
is a similar view of a closely related form, Lucifer, in a similar stage of development. Their close relationship is obvious at a glance, and their resemblances, which are much more conspicuous than their differences, are rendered more obvious by careful study; but the case is quite different when the younger stages are compared, and at first sight no one would suspect that the Sergestes larva (Fig. 7) and the Lucifer larva (Fig. 8) are corresponding stages in the development of two animals as similar to each other as those shown in Figs. 5 and 6. Not only do we find animals whose young stages differ more than the adults, but we also meet cases—and they are very numerous indeed—where the order of appearance of organs and features of the greatest taxonomic importance differs in the embryos of closely related forms.
To take a particular instance, it is plain that, since the features which all the two-gilled cephalopods have in common, and which are characteristic of the group as a whole, must have been inherited from the common ancestor of the whole group, they ought, unless the embryonic history of the different recent species has undergone secondary modifications, to appear in the same order in the embryos of all the existing forms; and, if they do not, it is clear that descriptive embryology alone can not furnish a key to systematic affinity.
As a matter of fact, each one of the three species of two-gilled cephalopods with whose embryology we are most familiar differs from, both the others in the order in which such significant organs as the arms, the shell, the eyes, the siphon, the gills, and the mouth make their appearance; and it must be obvious that, unless we have some means of analyzing these three life-histories, and determining which of them gives the true ancestral order, we can not make use of their embryology as a key to phylogeny. One who is not familiar with the whole field of life-science may fairly ask how it is possible to discover the relationships of animals from the study of their embryology if it is true that the early stages in the life of closely related species may differ so greatly, and if it is true that the order and manner in which structures make their appearance in the embryo are not alike in all cases. Before answering this question, we must call the attention of the unscientific reader to a familiar fact which will throw great light upon the matter.
The animals with which we are most familiar, the mammals and birds, are born in substantially the form which they will have when they reach maturity. They breathe the same medium; they employ the same organs of locomotion, in the same way; they require the same or nearly the same kind of food, and their habits and surroundings are the same as they will be during mature life, or at least the differences are slight and insignificant, and the adult is little more than the young animal grown to its full size, and with sexual characteristics, and able to reproduce its kind. But we must recollect that, in the greater part of the animal kingdom, this is not the case. In by far the greater part of the species of animals the rule is that the newly born young is very different, in structure, in habits, surroundings, and needs, from the adult, and its passage to the adult form is not simply a process of growth, but a process of great change in every particular.
The young butterfly crawls over the plant on which it is born, and finds an abundant supply of proper food in the green leaves, which it cuts to pieces with its strong, scissor-like jaws. Its capacious digestive tract is fitted for dealing with great quantities of bulky but very slightly nutritious food; and its enemies, dangers, and means of defense are very different from those of the adult winged insect, which is furnished with highly developed sense-organs, and flies from place to place in search of the highly concentrated liquid food adapted for sucking up through the proboscis which has replaced the cutting jaws of the young; and we must recollect that the life-history of the butterfly, so far as its great changes are concerned, is a type of the life of numbers of other animals, for nearly all the invertebrates pass through a metamorphosis.
Whenever young animals are left to shift for themselves, without parental protection, they are compelled to struggle for existence with a host of competitors and enemies; and in all cases where the structure and habits of the young differ from those of the adult, a variation in the young animal may be as important for the welfare of the species as one in the adult, and may, therefore, be seized upon and perpetuated by natural selection, and in this way the young stages of two closely related species may be modified in different directions until they become quite different from each other, while the adults may remain essentially alike; and as natural selection may act in such a way as to modify the life-history of an organism at any stage of its existence, there is no limit to the secondary changes which may thus be brought about. A larva may acquire new organs, or it may lose old ones; the order in which organs are acquired may be modified; stages of development may be dropped out of the series, or new ones may be added. A young form may become adapted to a new mode of life, or it may escape competition by seizing upon a new field; its enemies, dangers, and means of defense may change, and with these changes of habits corresponding changes of structure may occur, so that the primitive or ancestral record may become completely obscured by secondary changes.
The examination of the various kinds of modification which may be brought about in this way falls within the scope of a treatise on comparative embryology, but it would be out of place here, although one or two examples of the more common sorts of modification may be of interest.
The chrysalis stage of butterflies is an instance of the secondary acquisition of a new stage of development, which forms no part of the ancestral record.
It is obvious that the inactive pupa, which takes no food, and exhibits few of the ordinary activities of animal life, can not possibly have existed in the past as an adult ancestor of the butterflies, nor is it conceivable that any of the remote ancestry of this group bore a general resemblance to a pupa. While it is impossible to believe that the pupa stage is ancestral, we have good evidence to show the manner in which it has been acquired as a secondary modification.
Lubbock has pointed out that the least specialized or most primitive insects have mouth-parts which are indifferently adapted for either cutting or sucking, and that these insects do not undergo a metamorphosis, but are gradually converted into the adult form by a simple process of gradual development. He also shows good ground for believing that the common ancestors of all the groups of insects were like these forms in these particulars; and he holds that, as the stock which led to our present butterflies was evolved from this ancient stem-form, the young became adapted to a sedentary creeping life, and their indifferent mouth-parts became gradually converted into cutting jaws, while the adults became adapted to quite a different mode of life, and the same indifferent mouth-parts became gradually modified into a sucking proboscis. While the caterpillar and butterfly were thus diverging in two directions from the original unspecialized form, and the structure and habits of the larva were becoming more and more different from those of the adult, it is plain that the metamorphosis must at the same time have become more and more violent; and, according to Lubbock, one of the periods of slight activity which, in most insects, accompany the periodical molts, was seized upon, and gradually extended into a long resting or chrysalis stage, in order to enable the animal to exist while the highly specialized organs of the caterpillar are changing into the equally specialized but very different organs of the winged insect. The growing butterfly now passes through a resting or pupa stage which connects the two periods of great specialization, and bridges over the gap between them, and thus does away with a period of imperfect specialization to both modes of life.
As an instance of the opposite kind of modification, the simplification of an embryonic history by the loss of ancestral stages, we may take the life of the fresh-water crawfish. Young lobsters, and most of the other marine allies of the crawfish, leave the egg in a form which is quite unlike the adult, in structure as well as in habits, and the new-born young pass through a long series of stages of metamorphosis before the mature form is reached.
The larval stages of the marine long-tailed Crustacea bear such a resemblance to each other, and to certain lower Crustacea, that we must regard them as ancestral; and we must therefore believe that they were one time present in the life-history of the crawfish, although we find nothing of the kind now. These larval forms are adapted to a swimming life at the surface of the ocean, and we can understand that, when the ancestors of the crawfishes became adapted to a life in fresh water, the larval stages must either have been modified to correspond or else been got rid of, and, in the crayfish, the. latter has happened, and the new-born young is simply a very small image of the adult, the whole metamorphosis having been suppressed.
A person who is unfamiliar with morphology may fairly ask whether we are not entering upon treacherous ground, and why we are to regard the life-history of the lobster as the ancestral one, and that of the crawfish as a secondary modification, rather than the reverse. This feeling is not confined to unscientific thinkers, for many naturalists are inclined to reject this conception of the falsification of the embryonic record, and to say that, when we accept the evidence furnished by one species as a basis for phylogeny, and reject the evidence of a related species as of no taxonomic importance, we are actuated by mere caprice, or by a desire to establish some pet hypothesis, and that this method of reasoning can have no scientific value.
The most satisfactory answer to this objection would be a thorough analysis of a specific example, but this would involve technical comparisons and discussions which could not be adequately presented without a number of figures; and a sufficient answer for our present purpose may be found by a reference to the facts and conclusions of comparative anatomy.
A whale differs from all the ordinary mammals in quite a number of features in which it bears a close resemblance to fishes, and at the same time it differs from fishes in a number of points of resemblance to mammals. The attention of the earlier naturalists was attracted by the first set of resemblances and differences, and they placed the whale among the fishes; but later investigators have decided that the second set of resemblances alone give any evidence of systematic relationship, and that the whale is a mammal. Now, what reason is there for regarding one set of resemblances as of taxonomic importance rather than the other? The answer is plain. It is easy to show that all the features in which a whale resembles fishes are such as we should expect to find if the whale is a mammal, adapted to an aquatic life; but the features of resemblance to an ordinary mammal do not admit of any such explanation, and they must therefore be held to indicate the true relationship.
If the crawfish originally passed through stages somewhat similar to those of the growing lobster, we can see why they may have been suppressed to adapt it to a life in fresh water; but, if the life-history of the crawfish is ancestral, we can find no reason, in the life of the lobster, for the acquisition of larval stages which are like those of more distantly related macroura, and, in rejecting one life-history and accepting the other, we are simply carrying the accepted principles of homological reasoning into wider fields, and applying them to a new class of phenomena; and a thorough acquaintance with the facts will render our conclusions as thoroughly scientific in the one case as in the other.
Those who are unfamiliar with the status of modern morphology are still accustomed to regard systematic zoology as a science of observation, but our review of the subject shows that the attempt to trace out the natural system of classification of animals carries us far beyond the bare facts, and that the observed phenomena, although practically infinite in numbers, bear about the same relation to the generalizations of the science that the facts of mathematics or of astronomy do to the general laws of these sciences.
The facts are so numerous and so difficult to observe, and our acquaintance with the conditions of life is so slight, that our attempts at general conclusions must frequently be tentative and provisional, and in some cases future research may show that they are entirely wrong; but this is no valid objection to the use of such evidence as we have. There is no more justice in the assumption that, because they may possibly be wrong, phylogenetic speculations upon the basis of paleontology, comparative anatomy, and embryology are adverse to the best interests of science, than there would be in the assumption that the attempt to trace the relationship of animals from the facts of homology is unscientific, because Cuvier thought that he had discovered homologies between the barnacles and mollusks, or because Agassiz associated the vorticellas with the polyzoa.
The end of phylogenetic speculation is perfectly legitimate, but we must rid our minds of the belief that it can be reached by mere observation and description. The evidence is often so hard to read that the accounts of the best observers are contradictory, and in many cases it is so scanty and incomplete that it must be submitted to a severely critical process of comparison, analysis, cross-examination, and elimination, before a general conclusion can be reached. The field is so vast, the amount of evidence so great, and special features are so numerous, that the thorough discussion of the problem in all its bearings will furnish employment for the most acute and comprehensive powers of analysis for an indefinite period; but there is no reason to believe that the subject is beyond our grasp, or that it is not a perfectly proper field for intellectual activity.
We may fairly ask, though, whether, after all this is granted, it pays to spend our time in speculation upon circumstantial evidence, when all our conclusions may possibly be wrong, when they can not possibly be true unless they are put into a general form, and when the presumption in their favor is only a probability at best. Would it not be wise for us to spend all our time in the observation of nature, rather than to devote our energies to the discussion of general problems?
In matters where we are called upon to act we must weigh the probabilities, and form the best judgment we can according to our evidence; but this is necessary because we must act in any case, and it is no reason for carrying scientific thought into similar fields. In life it is often wise to act against the probabilities, as when an old man denies himself to make provision for a prolonged life, which he has very little chance of enjoying; but it does not follow that it is wise to form a scientific conclusion against the probabilities, and, if the analogy of actual life will not justify this, how can it justify a scientific conclusion which is based upon probabilities in the absence of demonstrative evidence?
If science were a pure abstraction, standing by itself, this objection might have weight; but no part of the phenomenal world does stand by itself, and nothing in nature is so independent of human interests that broader knowledge does not conduce to wiser judgments and actions: nor is the past history of life a remote subject, bearing so slightly upon human interests that it may properly be left to occupy the time and energy of future generations.
It has the same importance to us as living things that the history of the human race has to us as human beings. The future history of our race will be a continuation of the one line as well as of the other, or, rather, one is included by the other. The end of the study of history is not the discovery of what has been in the past, but the discovery of general laws and causes that shall enable us to foresee what is to take place in the future; but this sort of historical knowledge, the wisdom of history, does not come from observation, but from reflection upon the inner relations, the causes and effects of phenomena—from a weighing of the probabilities between one interpretation and another; and the wisdom which leads us to accept and act upon these probable conclusions, as the best available basis for the guidance of conduct, equally requires us to accept, in the same way, the results of the study of our prehistoric life-history. Our conclusions may be wrong, but, so long as they are the best, we can not regard them as abstractions. We must welcome them as something more than knowledge—as an increase of wisdom in its widest sense.
If this justification of morphological speculation seems vague and indefinite, we need not seek far for more concrete reasons for encouraging this kind of thought, one of the most important features of which is its value as an intellectual discipline.
We can hardly overestimate the value of the power to reach logical conclusions, for this power is the basis of all wise conduct, and that education which aids in its acquisition has pre-eminent claims upon our attention. In almost every case where we are called upon to form a judgment, and to act upon it, the premises are so uncertain, the conditions of the problem so numerous and so little known, and its relations to other things admit of so much variation, that our conclusion can be nothing but a probability; and it is of the greatest importance that the mind should be trained in such a way as to fit it for forming wise judgments upon this class of complicated and indefinite problems.
Now, the questions which are presented for solution in the more exact physical sciences differ from the questions of morphology in the same way that they differ from the problems upon which we are constantly called to decide and act in ordinary life, but the degree of difference is less.
While the number of factors involved in a morphological problem is vastly greater than that of those which bear upon any problem into which life does not enter, and while the relations between these factors vary, in closely allied cases, in a way which has no parallel outside life-science, the problems of general morphology are still vastly simpler than those of society or of morality, or of almost any other department of human conduct, although they are like them in kind, and supply the same sort of evidence. The attempt to trace the mode of action of a constantly changing environment upon a form of life inheriting from an unknown series of ancestors a constitution that has been modified by a series of changes that can not be repeated, is no bad training for the attempt to foresee the working of a social reform that admits of no experiment, but must be tried once for all; which involves so many side-issues that no exact parallel to it can be found, and which is so complicated that it is impossible to foresee or follow out its results in detail.
The problems of the physical sciences are too definite and simple to afford much intellectual training in this most important field, and the problems of human life and society are too involved, too diversified, and too changeable to afford a proper field for studying the logical basis of our opinions; but in morphology we find what is needed, a field midway between the two.
The discipline which is to be obtained by the careful mastery of such a work as Claus's speculation upon the origin of the Crustacea, or by the critical study of the problem of the vertebrate skull, or by the study of the literature upon the affinity between the vertebrates and the annelids, is a better training in that logic of probabilities which is the basis of all conduct than can be found in the study of the other sciences; and from this point of view it is plain that good may result from honest but erroneous attempts at morphological speculation, for the logical restrictions of sound reasoning are often studied to the best advantage in the errors of acute thinkers.