Scaly Reptiles. Turtles, Lizards, Serpents.
Such corresponding groups or parallel types, united only by external resemblance, and distinguished from each other by essential elements of structure, exist among all animals, though they are less striking among Birds on account of the uniformity of that class. Yet even there we may trace such analogies,—as between the Palmate or Aquatic Birds, for instance, and the Birds of Prey, or between the Frigate Bird and the Kites. Among Fishes such analogies are very common, often suggesting a comparison even with land animals, though on account of the scales and spines of the former the likeness may not be easily traced. But the common names used by the fishermen often indicate these resemblances, —as, for instance, Sea-Vulture, Sea-Eagle, Cat-Fish, Flying-Fish, Sea-Porcupine, Sea-Cow, Sea-Horse, and the like. In the branch of Mollusks, also, the same superficial analogies are found. In the lowest class of this division of the Animal Kingdom there is a group so similar to the Polyps, that, until recently, they have been associated with them,—the Bryozoa. They are very small animals, allied to the Clams by the plan of their structure, but they have a resemblance to the Polyps on account of a radiating wreath of feelers around the upper part of their body: yet, when examined closely, this wreath is found to be incomplete; it does not, form a circle, but leaves an open space between the two ends, where they approach each other, so that it has a horseshoe outline, and partakes of the bilateral symmetry characteristic of its type and on which its own structure is based. These series have not yet been very carefully traced, and young naturalists should turn their attention to them, and be prepared to draw the nicest distinction between analogies and true affinities among animals.
VIII.
After this digression, let us proceed to a careful examination of the natural groups of animals called Families by naturalists,—a subject already briefly alluded to in a previous chapter. Families are natural assemblages of animals of less extent than Orders, but, like Orders, Classes, and Branches, founded upon certain categories of structure, which are as distinct for this kind of group as for all the other divisions in the classification of the Animal Kingdom.
That we may understand the true meaning of these divisions, we must not be misled by the name given by naturalists to this kind of group. Here, as in so many other instances, a word already familiar, and that had become, as it were, identified with the special sense in which it had been used, has been adopted by science and has received a new signification. When naturalists speak of Families among animals, they do not allude to the progeny of a known stock, as we designate, in common parlance, the children or the descendants of known parents by the word family; they understand by Families natural groups of different kinds of animals, having no genetic relations so far as we know, but agreeing with one another closely enough to leave the impression of a more or less remote common parentage. The difficulty here consists in determining the natural limits of such groups, and in tracing the characteristic features by which they may be defined; for individual investigators differ greatly as to the degree of resemblance existing between the members of many Families, and there is no kind of group which presents greater diversity of circumscription in the classifications of animals proposed by different naturalists than these so-called Families.
It should be remembered, however, that, unless a sound criterion be applied to the limitation of Families, they, like all other groups introduced into zoölogical systems, must forever remain arbitrary divisions, as they have been hitherto. A retrospective glance at the progress of our science during the past century, in this connection, may perhaps help us to solve the difficulty. Linnaeus, in his System of Nature, does not admit Families; he has only four kinds of groups,—Classes, Orders, Genera, and Species. It was among plants that naturalists first perceived those general traits of resemblance which exist everywhere among the members of natural families, and added this kind of group to the framework of their system. In France, particularly, this method was pursued with success; and the improvements thus introduced by the French botanists were so great, and rendered their classification so superior to that of Linnaeus, that the botanical systems in which Families were introduced were called natural systems, in contradistinction especially to the botanical classification of Linnaeus, which was founded upon the organs of reproduction, and which received thenceforth the name of the sexual system of plants. The same method so successfully used by botanists was soon introduced into Zoölogy by the French naturalists of the beginning of this century,—Lamarck, Latreille, and Cuvier. But, to this day, the limitation of Families among animals has not yet reached the precision which it has among plants, and I see no other reason for the difference than the absence of a leading principle to guide us in Zoölogy.
Families, as they exist in Nature, are based upon peculiarities of form as related to structure; but though a very large number of them have been named and recorded, very few are characterized with anything like scientific accuracy. It has been a very simple matter to establish such groups according to the superficial method that has been pursued, for the fact that they are determined by external outline renders the recognition of them easy and in many instances almost instinctive; but it is very difficult to characterize them, or, in other words, to trace the connection between form and structure. Indeed, many naturalists do not admit that Families are based upon form; and it was in trying to account for the facility with which they detect these groups, while they find it so difficult to characterize them, that I perceived that they are always associated with peculiarities of form. Naturalists have established Families simply by bringing together a number of animals resembling each other more or less closely, and, taking usually the name of the Genus to which the best known among them belongs, they have given it a patronymic termination to designate the Family, and allowed the matter to rest there, sometimes without even attempting any description corresponding to those by which Genus and Species are commonly defined.
For instance, from Canis, the Dog, Canidae has been formed, to designate the whole Family of Dogs, Wolves, Foxes, etc. Nothing can be more superficial than such a mode of classification; and if these groups actually exist in Nature, they must be based, like all the other divisions, upon some combination of structural characters peculiar to them. We have seen that Branches are founded upon the general plan of structure, Classes on the mode of executing the plan, Orders upon the greater or less complication of a given mode of execution, and we shall find that form, as determined by structure, characterizes Families. I would call attention to this qualification of my definition; since, of course, when speaking of form in this connection, I do not mean those superficial resemblances in external features already alluded to in my remarks upon Parallel or Collateral Types. I speak now of form as controlled by structural elements; and unless we analyze Families in this way, the mere distinguishing and naming them does not advance our science at all. Compare, for instance, the Dogs, the Seals, and the Bears. These are all members of one Order,—that of the Carnivorous Mammalia. Their dentition is peculiar and alike in all, (cutting teeth, canine teeth, and grinders,) adapted for tearing and chewing their food; and their internal structure bears a definite relation to their dentition. But look at these animals with reference to form. The Dog is comparatively slender, with legs adapted for running and hunting his prey; the Bear is heavier, with shorter limbs; while the Seal has a continuous uniform outline adapted for swimming. They form separate Families, and are easily recognized as such by the difference in their external outline; but what is the anatomical difference which produces the peculiarity of form in each, by which they have been thus distinguished? It lies in the structure of the limbs, and especially in that of the wrist and fingers. In the Seal the limbs are short, and the wrists are on one continuous line with them, so that it has no power of bending the wrist or the fingers, and the limbs, therefore, act like flappers or oars. The Bear has a well-developed paw with a flexible wrist, but it steps on the whole sole of the foot, from the wrist to the tip of the toe, giving it the heavy tread so characteristic of all the Bears. The Dogs, on the contrary, walk on tip-toe, and their step, though firm, is light, while the greater slenderness and flexibility of their legs add to their nimbleness and swiftness. By a more extensive investigation of the anatomical structure of the limbs in their connection with the whole body, it could easily be shown that the peculiarity of form in these animals is essentially determined by, or at least stands in the closest relation to, the peculiar structure of the wrist and fingers.
Take the Family of Owls as distinguished from the Falcons, Kites, etc. Here the difference of form is in the position of the eyes. In the Owl, the sides of the head are prominent and the eye-socket is brought forward. In the Falcons and Kites, on the contrary, the sides of the head are flattened and the eyes are set back. The difference in the appearance of the birds is evident to the most superficial observer; but to call the one Strigidae and the other Falconidae tells us nothing of the anatomical peculiarities on which this difference is founded.
These few examples, selected purposely among closely allied and universally known animals, may be sufficient to show, that, beyond the general complication of the structure which characterizes the Orders, there is a more limited element in the organization of animals, bearing chiefly upon their form, which, if it have any general application as a principle of classification, may well be considered as essentially characteristic of the Families. There are certainly closely allied natural groups of animals, belonging to the same Order, but including many Genera, which differ from each other chiefly in their form, while that form is determined by peculiarities of structure which do not influence the general structural complication upon which Orders are based, or relate to the minor details of structure on which Genera are founded. I am therefore convinced that form is the criterion by which Families may be determined. The great facility with which animals may be combined together in natural groups of this kind without any special investigation of their structure, a superficial method of classification in which zoölogists have lately indulged to a most unjustifiable degree, convinces me that it is the similarity of form which has unconsciously led such shallow investigators to correct results, since upon close examination it is found that a large number of the Families so determined, and to which no characters at all are assigned, nevertheless bear the severest criticism founded upon anatomical investigation.