Evolution and Adaptation

by Thomas Hunt Morgan

In so far as this criticism of Fleischmann’s applies to the difficulties of determining the past history of the horse, it may be granted that he has scored a point against those who have pretended that the evidence is simple and conclusive; but we should not fail to remember that this difficulty has been felt by paleontologists themselves, who have been the first to call attention to the complexity of the problem, and to the difficulties of finding out the actual ancestors of the living representative of the series. And while we may admit that the early enthusiasts exaggerated, unintentionally, the importance of the few forms known to them, and went too far in supposing that they had found the actual series of ancestors of living horses, yet we need not let this blind us to the importance of the facts themselves. Despite the fact that it may be difficult and, perhaps, in most cases, impossible, to arrange the fossil forms in their relations to one another and to living forms, yet on an unprejudiced view it will be clear, I think, that so far as the evidence goes it is in full harmony with the theory of descent. This is especially evident if we turn our attention to a part of the subject that is almost entirely ignored by Fleischmann, and yet is of fundamental importance in judging of the result. The series of forms beginning with the five-toed horses and ending with those having a single toe has not been brought together haphazard, as Fleischmann’s comparison might lead one to suppose, but the five-fingered forms are those from the older rocks, and the three-toed forms from more recent layers. The value of this kind of evidence might have been open to greater doubt had the series been made up of forms found scattered over the whole world, for it is well known how difficult it is to compare in point of time the rocks of different continents. But in certain parts of the world, especially in North America, series of fossil horses have been found in sedimentary deposits that appear to be perfectly continuous. This series, by itself, and without regard to the point as to whether in other parts of the world other series may exist, shows exactly those results which the theory of descent postulates, and we find here, in all probability, a direct line of descent. While it may be freely admitted that no such series can demonstrate the theory of descent with absolute certainty, yet it would be folly to disregard evidence as clear as this.

In regard to the other point raised by Fleischmann concerning the large number of species of fossil horses that have existed in past times, it is obvious that while this greatly increases the difficulty of the paleontologist it is not an objection to the descent theory. In fact, our experience with living species would lead us to expect that many types have been represented at each geological period by a number of related species that may have inhabited the same country. On the descent theory, one species only in each geological period could have been in the line of descent of the present species of horse. The difficulty of determining which species (if there were several living in a given epoch) is the ancestor of the horse is increased, but this is not in itself an objection to the theory.

The descent of birds from flying reptiles is used by Fleischmann as another point of attack on the transmutation theory. The theory postulates that the birds have come from ancestors whose fore-legs have been changed into highly specialized wings. The long vertebrated tail of the ancestral form is supposed to have become very short, and long feathers to have grown out from its stump which act as a rudder during flight. Flying reptiles with winged fore-legs and a long vertebrated tail have been actually found as fossil remains, as seen in the pterodactyls and in the famous archæopteryx. The latter, which is generally regarded either as the immediate ancestor of living birds, or at least as a closely similar form, possessed a fore-leg having three fingers ending in claws, and feathers on the forearm similar to those of modern birds. It had a long tail, like that of a lizard, but with well-developed feathers along its sides. It had pointed teeth in the horn-covered jaws. Fleischmann proceeds to point out that the resemblance of the hand of archæopteryx to that of the reptiles is not very close, for two fingers are absent as in modern birds. The typical form of the foot is that of the bird, and is not the simple reptilian type of structure. Feathers and not scales cover the body, and give no clew as to how the feathers of birds have arisen. He concludes, therefore, that archæopteryx, having many true bird-like characters, such as feathers, union of bones in the foot, etc., has other characters not possessed by living birds, namely, a long, vertebrated tail, a flat breastbone, biconcave vertebræ, etc. Therefore, it cannot be regarded as an intermediate form. Fleischmann does not point out that it is just these characters that would be postulated on the descent theory for the ancestor of the birds, if the latter arose from reptiles. Even if it should turn out that archæopteryx is not the immediate forefather of living birds, yet the discovery that a form really existed intermediate in many characters between the reptiles and the birds is a gain for the transmutation theory. It is from a group having such characters that the theory postulates that the birds have been evolved, and to have discovered a member of such a group speaks directly and unmistakably in favor of the probability of the transmutation theory.

Fleischmann again fails to point out that the geological period in which the remains of archæopteryx were found, is the one just before that in which the modern group of birds appeared, and, therefore, exactly the one in which the theory demands the presence of intermediate forms. This fact adds important evidence to the view that looks upon archæopteryx as a form belonging to a group from which living birds have arisen. That a number of recent paleontologists believe archæopteryx to belong to the group of birds, rather than to the reptiles, or to an intermediate group, does not in the least lessen its importance, as Fleischmann pretends it does, as a form possessing a number of reptilian characters, such as the transmutation theory postulates for the early ancestors of the birds.

The origin of the mammalian phylum serves as the text for another attack on the transmutation theory. Fleischmann points out that the discovery of the monotremes, including the forms ornithorhynchus and echidna, was hailed at first as a demonstration of the supposed descent of the mammals from a reptilian ancestor. The special points of resemblance between ornithorhynchus and reptiles and birds are the complete fusion of the skull bones, the great development of the vertebræ of the neck region, certain similarities in the shoulder girdle, the paired oviducts opening independently into the last part of the digestive tract (cloaca), and the presence of a parchment-like shell around the large, yolk-bearing egg. These are all points of resemblance to reptiles and birds, and were interpreted as intermediate stages between the latter groups and the group of mammals. In addition to these intermediate characters, ornithorhynchus possesses some distinctive, mammalian features—mammary glands and hair, for instance. Fleischmann takes the ground, in this case, that there are so many points of difference between the monotremes and the higher mammals, that it is impossible to see how from forms like these the higher groups could have arisen, and that ornithorhynchus cannot be placed as an intermediate form, a link between saurians and mammals, as the followers of the transmutation theory maintain. He shows, giving citations, that anatomists themselves are by no means in accord as to the exact position of ornithorhynchus in relation to the higher forms.

In reply to this criticism, the same answer made above for archæopteryx may be repeated here, namely, that because certain optimists have declared the monotremes to be connecting forms, it does not follow that the descent theory is untrue, and not even that these forms do not give support to the theory, if in a less direct way. I doubt if any living zoologist regards either ornithorhynchus or echidna as the ancestral form from which the mammals have arisen. But on the other hand it may be well not to forget that these two forms possess many characters intermediate between those of mammals and reptiles, and it is from a group having such intermediate characters that we should expect the mammals to have arisen. These forms show, if they show nothing else, that it is possible for a species to combine some of the characters of the reptiles with those of the mammals; and the transmutation theory does no more than postulate the existence at one time of such a group, the different species of which may have differed in a number of points from the two existing genera of monotremes.

The origin of lung-bearing vertebrates from fishlike ancestors, in which the swim-bladder has been changed into lungs, has been pointed to by the advocates of the transmutation theory as receiving confirmation in the existence of animals like those in the group of dipnoan fishes. In these animals both gills and a swim-bladder, that can be used as a lung, are present; and through some such intermediate forms it is generally supposed that the lung-bearing animals have arisen. Fleischmann argues, however, that, on account of certain trivial differences in the position of the duct of the swim-bladder in living species, the supposed comparison is not to the point; but the issue thus raised is too unimportant to merit further discussion. Leaving aside also some even more doubtful criticisms which are made by Fleischmann, and which might be added to indefinitely without doing more than showing the credulity of some of the more ardent followers of the transmutation theory, or else the uncertainty of some of the special applications of the theory, let us pass to Fleischmann’s criticism of the problem of development.[^[4]]

[4]. The long argument of Fleischmann in regard to the origin of the fresh-water snails, as illustrated by the planorbis series, and also the origin of the nautiloid group, has been recently dealt with fully by Plate, and, therefore, need not be considered here.

With fine scorn Fleischmann points to the crudity of the ideas of Oken and of Haeckel in regard to the embryology (or the ontogeny) repeating the ancestral history (or the phylogeny). We may consider briefly (since we devote the next chapter almost entirely to the same topic) the exceptions to this supposed recapitulation, which Fleischmann has brought together. The young of beetles, flies, and butterflies creep out of the egg as small worm-like forms of apparently simple organization. They have a long body, composed of a series of rings; the head is small and lacks the feelers, and often the faceted eyes. The wings are absent, and the legs are short. At first sight the larva appears to resemble a worm, and this led Oken to conclude that the insects appear first in the form of their ancestors, the segmented worms. If we examine the structure of the larva more carefully, we shall find that there are a great many differences between it and the segmented worms; and that even the youngest larva is indeed a typical insect. The tracheæ, so characteristic of the group of insects, are present, the structure of the digestive tract with its Malpighian tubes, the form of the heart, the structure of the head, as well as the blastema of the reproductive organs, show in the youngest larva the type of the insects. In other words the body of the caterpillar is formed on exactly the same fundamental plan as that of the butterfly.

In regard to the larval forms of other groups we find the same relations, as, for example, in the amphibians. The young of salamanders, toads, and frogs leave the egg not in the completed form, but as small tadpoles adapted to life in the water. A certain resemblance to fish cannot be denied. They possess a broad tail, gills (rich in blood vessels) on each side of the neck, and limbs are absent for a long time. These are characters similar to those of fish, but a more careful anatomical examination destroys the apparent resemblance. The superficial resemblances are due to adaptation to the same external conditions.