Fig. 300. Family tree of Ranunculaceae, illustrating the evolution of the group as provisionally suggested in the following text.

Fig. 300.-Family tree of Ranunculaceae, illustrating the evolution of the group as provisionally suggested in the following text.

Our vertical diagram thus indicates that of all the living forms of Ranunculaceae, Caltha most nearly resembles the ancestor of the family. According to this view the story of the family's evolution would be somewhat as follows. Among the descendants of Calthalike moisture-loving plants which grew in a remote geologic age, some retained for innumerable generations the characteristics which fitted them for living under the comparatively uniform conditions afforded by protected moist situations, and are represented to-day by our marsh-marigolds. In these have survived a most primitive type of flower, hypogynous, with many-ovuled, distinct carpels, numerous stamens of ordinary form, and a regular perianth of almost leaf-like sepals; and a very simple form of palmate leaf and herbaceous stem. Certain other descendants of the primitive stock showed in very early times a modification of the torus and perianth (resulting in perigynous flowers with the perianth differentiated into a well-defined calyx and corolla), also an advance in complexity of leaf-form, and more or less woodiness of stem. The plants of this branch were thus able to thrive under the more exacting conditions of open fields, and have become the highly developed sturdy peonies of to-day. Very early in the development of what we may call the hypogynous branch of the family, there appeared some more or less Caltha-like forms, in which the carpels did not open at maturity, but separated like seeds from the parent plant. This change of habit made it unnecessary for more than one seed to mature in each carpel, so the result was an achenial instead of a follicular fruit. Those descendants which retained rudiments of several ovules became in time either forms of Anemone or of Clematis according as they developed the peculiarities of this or that genus; while similarly those in which the reduction to one ovule was complete, gave rise to such plants as buttercups and mouse-tails. In much the same way we find those descendants which comprise the more primitive subbranch with its many-seeded carpels becoming differentiated into forms retaining the original follicular fruit, and forms in which the fruit is indehiscent and fleshy, as in Actaea. Along the lines with follicular fruit the appearance of forms with certain of the stamens changed into more or less conspicuous nectaries give rise to a departure from the Caltha-like forms, which in turn became differentiated into those in which the corolla remained Tegular and those in which more or less irregularity was shown, as in monkshood. Finally, the regular-flowered forms with staminodes developed such peculiarities as the spurs of Aquilegia, and the curious follicular capsule of Nigella, by which the modern genera of this subgroup are now distinguished.1

1 It might fairly be asked whether entirely different lines of descent from those here given would not as well explain the modern forms upon which our reasonings have been based. Undoubtedly this is true. Thus, instead of supposing the progenitor of the family to have been an herb like the marsh-marigold we might perhaps with more probability assume it to have been a shrub or tree resembling a woody peony or a magnolia; for in other families there is much evidence in favor of the view that herbaceous seed-plants have had woody ancestors. But in this family we have no direct proof that its earliest members were woody, and for us to make the assumption would complicate our reasoning without rendering any clearer the general principle we are trying to make plain. Let the student, therefore, take our crowfoot family tree simply as representing one out of many possible ways of accounting for the facts at hand, and as liable to modification whenever new light appears on the problem. It is merely intended to illustrate the kind of reasoning that biologists employ in the absence of evidence from fossil remains, and the kind of conclusions they reach by such means. That such conclusions are at best far from satisfactory goes without saying; but herein lies a challenge to do better, and a spur to further study.

If we trace the evolution of a given form backward to its beginning we come to more and more primitive conditions. Mouse-tails, for example, we should derive from ancestors resembling buttercups in having a shorter torus, more stamens, spurless staminodes, and broader leaves. It seems reasonable to suppose that these mouse-tail-buttercups left no unmodified descendants, and so were not exactly like any living form of buttercup, although we might fairly presume that they showed more of the relatively primitive buttercup structure than of the peculiar mouse-tail features. These buttercup-like ancestors would be traced back to progenitors more nearly resembling anemonies in which rudimentary ovules still bore witness to a previous many-ovuled stage, when also certain of the outer stamens were showing only rudimentary anthers and were secreting nectar on broadened filaments. The step from this form, from which both mouse-tails and anemonies descended, to the marsh-marigold-like ancestors of the family, requires us to imagine little more than a previous full development of the ovules and anthers which later became reduced, and a more ordinary form of filaments and carpels.

In the above examples of the way an evolutionist conceives the changes in a group to have progressed, innumerable details of the process have necessarily been omitted for the sake of simplicity. Many branches would have to be interpolated if all living genera of the family were to be represented, and there is no knowing how many dead branches should be shown to make the family tree complete. It must, of course, be frankly admitted that the conclusions are largely guesswork supported only by circumstantial evidence, for no remains of the very perishable ancestral forms of the crowfoot family have come to light. Our examples, however, are typical of evolutionary schemes in general, and so may help our understanding of the theory of evolution. One prevalent misconception which it should correct is the notion that forms now living are supposed to have been derived from ancestors just like other recent forms. On the contrary, evolutionists believe that more or less modification is the rule among all the descendants of a given form, and that only rarely has it happened that ancestral forms have persisted for ages unchanged. Thus, in regard to the last examples, the most we should be willing to say is that mouse-tails were probably derived from plants something like the more primitive forms of recent buttercups, such as the ditch crowfoot (Fig. 209); that these ancestral crowfoots came from plants something like our marsh-marigold; and so on. It is thought that single traits or a few traits in combination, are more apt to survive through long periods than the many peculiarities characterizing a complex structure.

Closely connected with the erroneous view above mentioned is the notion that all the modern representatives of a group can be arranged in a single series beginning with the most primitive, and passing on to the most highly evolved through survivals of the intermediate stages or "connecting links." From what has been said it will be obvious that such an arrangement would be possible only on condition that all the forms which had ever appeared were actually represented by living descendants and that modification had occurred only in one direction. As a matter of fact, gaps, often great, between related forms are continually met with. Indeed, if the "connecting links" of the past, represented by the dead branches of our tree, had not disappeared there would be no possibility of classifying living forms into groups and subgroups, for there could be no limits to any group. Sometimes within a group it is as if Nature had as yet done little or no pruning, and the result is most bewildering to those who attempt a classification of the forms. No two students of the group are likely to agree as to where lines of demarcation should be drawn. Such a group is that of the roses previously mentioned. But even here, for all the embarrassing wealth of connecting links, it is quite impossible to arrange the forms in a single series. There are many diverging series which branch again and again. Their relationships as inferred from their degrees of likeness can best be expressed by a branching system, and while of course the systematist must deal with his groups one after another in simple sequence, he cannot make such a sequence adequately express his idea of their kinship.

If we are warranted in supposing that all members of the crowfoot family are the descendants of plants like our marsh-marigold we may assume with scarcely less probability that plants closely similar to these primitive marsh-marigolds were the ancestors of all of the crowfoot order, and indeed that from plants having very much the same primitive characteristics came, in the course of geological ages, all of the dicotyls and perhaps all the monocotyls as well. Our mental images of ancestral forms are necessarily dim in proportion to the remoteness of the form conceived, and are likely to change as we receive new light. Yet these mental diagrams of things no longer to be seen help our seeing of the things about us. From the evolutionary point of view all life takes on a new significance. The possibility of gaining some glimpses of how the living world came to be as it is, makes all life more deeply interesting. Details of structure or behavior in a humble plant may lead us to some of the greatest truths of life.