The Osteology of the Reptiles/Chapter 6
PART II
THE CLASSIFICATION AND RANGE OF REPTILES
CHAPTER VI
THE PROBLEM OF CLASSIFICATION
One who has studied attentively the skeleton of reptiles cannot fail to be impressed with the fact that similar or even apparently identical structures have arisen in different orders. Procoelous vertebrae, for instance, occur in crocodiles, pterodactyls, lizards, and frogs when it seems impossible that all should have been evolved from the same common ancestor with procoelous vertebrae. Snakes, some lizards, and certain Stegocephalia have a peculiar mode of articulation of the vertebrae, called zygosphenal, but their evolution from a common ancestor is impossible. For such resemblances the convenient term homoplasy has been proposed. Did they occur rarely in organisms they would not trouble us much; but they are everywhere in nature, and the problem of all classification is to distinguish between them and those characters due to heredity. Until we have learned to distinguish them our classification must remain more or less artificial.
The true end of all classification is genealogy. Some time in the Carboniferous period there was but a single kind of reptile, differing very slightly from its ancestors, and from this reptile has descended all the kinds that have ever lived. In the adaptation of its progeny to various provinces and modes of life they have divided into innumerable branches. Many of these branches were feeble and of short duration; others have continued to modern times, but none has ever reunited with another branch, even though small. Our object in classification is to determine these branches, and especially the early or primary ones. The twigs we call species, the lesser branches genera and families, the limbs orders, and the main boughs subclasses. It is easy enough relatively to distinguish the twigs and smaller branches, but it is often very difficult to determine where the limbs united with the boughs and where the boughs joined the trunk. A perfect classification would be dichotomous, each bough, limb, and branch dividing first into two, and each division again into two; but an approximation even to such a classification cannot be attained, and we must often treat groups of organisms as though radiating from a common center. And it is also evident that such divisions occurred rapidly. Many of the first groups of species that branched as twigs from the common stem were the ancestors of orders, for they held, all of them, possibilities of great developments; succeeding species became more and more restricted in their potentialities.
Our chief object, then, in classification is to trace the history of each species, genus, family, and order to its separation from allied forms, and to give to each minor and major group a name and place. And our chief difficulty in doing this is to determine whether the resemblances that they show to each other have been due to descent and common heritage, or have been the result of common environmental influences. The problems are hard and always will be hard because actual proofs of heredity must ultimately rest on the facts of paleontology, and paleontological history is and always will be imperfect. In all probability the earth since remote ages has always been as densely populated with living organisms as it is at the present time, and rapidly or slowly in different kinds of organisms evolution and extinction have replaced the faunas and floras many times. There are to-day living upon the earth about twenty thousand species of air-breathing vertebrate animals, and doubtless there has been no time since the first general invasion of land by air-breathers that the number has been less; it may have been greater, since man has exerted a powerful influence upon them. As the only air-breathers of paleozoic times were amphibians and reptiles, there must have been, during the time that they reigned supreme,—from the Mississippian to the Jurassic, millions of years,—scores of thousands of their kinds; we know but a few hundreds. Had we records of all that have lived, the major problems would be much easier, the minor ones greatly increased.
Nevertheless, in tracing the genealogies of organisms, that is, in classifying them, we are aided by general laws which have obtained recognition among students of extinct animals. First of all, by the law that evolution is irreversible, that organs or functions once lost can never be regained by descendants; similar organs or similar functions often, but never the original ones. By the general law that there has been a continuous loss of parts; we can trace, for instance, probably every bone of the human skull back to the primitive reptilian or amphibian skull, but there were twice or thrice as many bones in the older forms as there are in recent ones. And there has been in general an increase in bodily size in every phylum. The largest animals have always lived at or near the end of their race, and a race of small animals has never been evolved from a race of large animals. Furthermore, horns, spines, protuberances, and excrescences occur only in the later history of any race, never at its beginning.
The two chief factors of evolution have been environment and heredity. There is more or less impulse due to heredity, a sort of vis a tergo that seems to influence evolution along parallel lines in related forms, though we are never sure how much is due to it and how much to similar environmental influences.
The chief problem, then, in any classification is the relative importance of structural characters in the absence of the actual connecting links insensibly uniting different forms, that is, the determination of the more conservative hereditary characters of the skeleton, those which have been influenced less by environmental conditions. Some parts of the skeleton are very variable even in nearly related forms. The number of vertebrae in the spinal column, we have seen, may vary extraordinarily within an order. Chameleon lizards have only about sixty vertebrae; other lizards may have a hundred and ninety-four, while snakes of the same order may have as many as four hundred and fifty. The number is seldom of more than generic value, and sometimes perhaps not more than specific. It would be absurd, for instance, to unite in the same group a lizard and a turtle because each happens to have eight cervical vertebrae.
And the teeth of reptiles, unlike those of mammals, have little value as criteria of relationships, so adaptable are they in shape and number to food habits, though their location may be more conservative. The pectoral and pelvic girdles have been influenced less by environmental conditions; the structure of the feet still less in adaptation to life conditions. More conservative is the arrangement and mode of articulation of the ribs. Most conservative of all has been the structure of the cranial region of the skull, that surrounding the brain, and in consequence it furnishes the most reliable characters for the discrimination of the larger groups, the subclasses or superorders.
The most primitive reptiles that we know had no less than thirty-seven pairs and four single bones in the skull. The crocodiles have but twenty-four pairs and six unpaired bones in the adult; the turtles have twenty-two or twenty-three pairs and five or six unpaired bones; the lizards have at the most twenty-nine pairs and five single bones. But not all are the same. The crocodiles have three or four pairs that have been lost in the turtles; the turtles, one pair that is fused in the crocodiles; the lizards, several bones lost or fused in the turtles, and so on. All reptiles since Triassic times have lost four bones in the pectoral girdle, and all have lost some bones of the feet. The persistence or loss of bones furnishes many certain evidences of relationships and descent. Each order must have descended from ancestors that had the persistent bones; they could by no possibility have regained them when once lost.
The relative importance of all such characters in classification is, however, largely a matter of the classifier's personal opinion. No two persons see them from the same viewpoint and consequently no two persons whose opinions deserve consideration ever wholly agree as to the value of characters in classification. It is only in the gradual crystallization of opinions that stability finally results, and this crystallization is never complete. So long as science endures, new facts will be discovered to influence our opinions. Any system of classification, then, merely represents the present state of our knowledge and the consensus of the opinions of those best qualified to decide as to their value, more or less influenced by the classifier's individual opinions. No classification will ever be perfect, for perfection postulates complete knowledge. Fortunately, however, the increase of knowledge affects less and less the major principles, and more and more subordinate details.
Fig. 160. Phylogeny and distribution of Amphibians and Reptiles.