The questions proposed to themselves by all students who would characterize Families should be these: What are, throughout the Animal Kingdom, the peculiar patterns of form by which Families are distinguished? and on what structural features are these patterns based? Only the most patient investigations can give us the answer, and it will be very long before we can write out the formulae of these patterns with mathematical precision, as I believe we shall be able to do in a more advanced stage of our science. But while the work is in progress, it ought to be remembered that a mere general similarity of outline is not yet in itself evidence of identity of form or pattern, and that, while seemingly very different forms may be derived from the same formula, the most similar forms may belong to entirely different systems, when their derivation is properly traced. Our great mathematician, in a lecture delivered at the Lowell Institute last winter, showed that in his science, also, similarity of outline does not always indicate identity of character. Compare the different circles,—the perfect circle, in which every point of the periphery is at the same distance from the centre, with an ellipse in which the variation from the true circle is so slight as to be almost imperceptible to the eye; yet the latter, like all ellipses, has its two foci by which it differs from a circle, and to refer it to the family of circles instead of the family of ellipses would be overlooking its true character on account of its external appearance; and yet ellipses may be so elongated, that, far from resembling a circle, they make the impression of parallel lines linked at their extremities. Or we may have an elastic curve in which the appearance of a circle is produced by the meeting of the two ends; nevertheless it belongs to the family of elastic curves, in which may even be included a line actually straight, and is formed by a process entirely different from that which produces the circle or the ellipse.
But it is sometimes exceedingly difficult to find the relation between structure and form in Families, and I remember a case which I had taken as a test of the accuracy of the views I entertained upon this subject, and which perplexed and baffled me for years. It was that of our fresh-water Mussels, the Family of Unios. There is a great variety of outline among them,—some being oblong and very slender, others broad with seemingly square outlines, others having a nearly triangular form, while others again are almost circular; and I could not detect among them all any feature of form that was connected with any essential element of their structure. At last, however, I found this test-character, and since that time I have had no doubt left in my mind that form, determined by structure, is the true criterion of Families. In the Unios it consists of the rounded outline of the anterior end of the body reflected in a more or less open curve of the shell, bending more abruptly along the lower side with an inflection followed by a bulging, corresponding to the most prominent part of the gills, to which alone, in a large number of American Species of this Family, the eggs are transferred, giving to this part of the shell a prominence which it has not in any of the European Species. At the posterior end of the body this curve then bends upwards and backwards again, the outline meeting the side occupied by the hinge and ligament, which, when very short, may determine a triangular form of the whole shell, or, when equal to the lower side and connected with a great height of the body, gives it a quadrangular form, or, if the height is reduced, produces an elongated form, or, finally, a rounded form, if the passage from one side to the other is gradual. A comparison of the position of the internal organs of different Species of Unios with the outlines of their shells will leave no doubt that their form is determined by the structure of the animal.
A few other and more familiar examples may complete this discussion. Among Climbing Birds, for instance, which are held together as a more comprehensive group by the structure of their feet and by other anatomical features, there are two Families so widely different in their form that they may well serve as examples of this principle. The Woodpeckers (Picidae) and the Parrots (Psittacidae), once considered as two Genera only, have both been subdivided, in consequence of a more intimate knowledge of their generic characters, into a large number of Genera; but all the Genera of Woodpeckers and all the Genera of the Parrots are still held together by their form as Families, corresponding as such to the two old Genera of Picus and Psittacus. They are now known as the Families of Woodpeckers and Parrots; and though each group includes a number of Genera combined upon a variety of details in the finish of special parts of the structure, such as the number of toes, the peculiarities of the bill, etc., it is impossible to overlook the peculiar form which is characteristic of each. No one who is familiar with the outline of the Parrot will fail to recognize any member of that Family by a general form which is equally common to the diminutive Nonpareil, the gorgeous Ara, and the high-crested Cockatoo. Neither will any one, who has ever observed the small head, the straight bill, the flat back, and stiff tail of the Woodpecker, hesitate to identify the family form in any of the numerous Genera into which this group is now divided. The family characters are even more invariable than the generic ones; for there are Woodpeckers which, instead of the four toes, two turning forward and two backward, which form an essential generic character, have three toes only, while the family form is always maintained, whatever variations there may be in the characters of the more limited groups it includes.
The Turtles and Terrapins form another good illustration of family characters. They constitute together a natural Order, but are distinguished from each other as two Families very distinct in general form and outline. Among Fishes I may mention the Family of Pickerels, with their flat, long snout, and slender, almost cylindrical body, as contrasted with the plump, compressed body and tapering tail of the Trout Family. Or compare, among Insects, the Hawk-Moths with the Diurnal Butterfly, or with the so-called Miller,—or, among Crustacea, the common Crab with the Sea-Spider, or the Lobsters with the Shrimps,—or, among Worms, the Leeches with the Earth-Worms,—or, among Mollusks, the Squids with the Cuttle-Fishes, or the Snails with the Slugs, or the Periwinkles with the Limpets and Conchs, or the Clam with the so-called Venus, or the Oyster with the Mother-of-Pearl shell,—everywhere, throughout the Animal Kingdom, difference of form points at difference of Families.
There is a chapter in the Natural History of Animals that has hardly been touched upon as yet, and that will be especially interesting with reference to Families. The voices of animals have a family character not to be mistaken. All the Canidae bark and howl: the Fox, the Wolf, the Dog have the same kind of utterance, though on a somewhat different pitch. All the Bears growl, from the White Bear of the Arctic snows to the small Black Bear of the Andes. All the Cats miau, from our quiet fireside companion to the Lions and Tigers and Panthers of the forest and jungle. This last may seem a strange assertion; but to any one who has listened critically to their sounds and analyzed their voices, the roar of the Lion is but a gigantic miau, bearing about the same proportion to that of a Cat as its stately and majestic form does to the smaller, softer, more peaceful aspect of the Cat. Yet, notwithstanding the difference in their size, who can look at the Lion, whether in his more sleepy mood as he lies curled up in the corner of his cage, or in his fiercer moments of hunger or of rage, without being reminded of a Cat? And this is not merely the resemblance of one carnivorous animal to another; for no one was ever reminded of a Dog or Wolf by a Lion. Again, all the Horses and Donkeys neigh; for the bray of the Donkey is only a harsher neigh, pitched on a different key, it is true, but a sound of the same character,—as the Donkey himself is but a clumsy and dwarfish Horse. All the Cows low, from the Buffalo roaming the prairie, the Musk-Ox of the Arctic ice-fields, or the Jack of Asia, to the Cattle feeding in our pastures. Among the Birds, this similarity of voice in Families is still more marked. We need only recall the harsh and noisy Parrots, so similar in their peculiar utterance. Or take as an example the web-footed Family,—do not all the Geese and the innumerable host of Ducks quack? Does not every member of the Crow Family caw, whether it be the Jackdaw, the Jay, the Magpie, the Rook in some green rookery of the Old World, or the Crow of our woods, with its long, melancholy caw that seems to make the silence and solitude deeper? Compare all the sweet warblers of the Songster Family,—the Nightingales, the Thrushes, the Mocking-Birds, the Robins; they differ in the greater or less perfection of their note, but the same kind of voice runs through the whole group. These affinities of the vocal systems among animals form a subject well worthy of the deepest study, not only as another character by which to classify the Animal Kingdom correctly, but as bearing indirectly also on the question of the origin of animals. Can we suppose that characteristics like these have been communicated from one animal to another? When we find that all the members of one zoological Family, however widely scattered over the surface of the earth, inhabiting different continents and even different hemispheres, speak with one voice, must we not believe that they have originated in the places where they now occur with all their distinctive peculiarities? Who taught the American Thrush to sing like his European relative? He surely did not learn it from his cousin over the waters. Those who would have us believe that all animals have originated from common centres and single pairs, and have been distributed from such common centres over the world, will find it difficult to explain the tenacity of such characters and their recurrence and repetition under circumstances that seem to preclude the possibility of any communication, on any other supposition than that of their creation in the different regions where they are now found. We have much yet to learn in this kind of investigation, with reference not only to Families among animals, but to nationalities among men also. I trust that the nature of languages will teach us as much about the origin of the races as the vocal systems of the animals may one day teach us about the origin of the different groups of animals. At all events, similarity of vocal utterance among animals is not indicative of identity of Species; I doubt, therefore, whether similarity of speech proves community of origin among men.
The similarity of motion in Families is another subject well worth the consideration of the naturalist: the soaring of the Birds of Prey,—the heavy flapping of the wings in the Gallinaceous Birds,—the floating of the Swallows, with their short cuts and angular turns,—the hopping of the Sparrows,—the deliberate walk of the Hens and the strut of the Cocks,—the waddle of the Ducks and Geese,—the slow, heavy creeping of the Land-Turtle,—the graceful flight of the Sea-Turtle under the water,—the leaping and swimming of the Frog,—the swift run of the Lizard, like a flash of green or red light in the sunshine,—the lateral undulation of the Serpent,—the dart of the Pickerel,—the leap of the Trout,—the rush of the Hawk-Moth through the air,—the fluttering flight of the Butterfly,—the quivering poise of the Humming-Bird,—the arrow-like shooting of the Squid through the water, —the slow crawling of the Snail on the land,—the sideway movement of the Sand-Crab,—the backward walk of the Crawfish,—the almost imperceptible gliding of the Sea-Anemone over the rock,—the graceful, rapid motion of the Pleurobrachia, with its endless change of curve and spiral. In short, every Family of animals has its characteristic action and its peculiar voice; and yet so little is this endless variety of rhythm and cadence both of motion and sound in the organic world understood, that we lack words to express one-half its richness and beauty.
IX.
The well-known meaning of the words generic and specific may serve, in the absence of a more precise definition, to express the relative importance of those groups of animals called Genera and Species in our scientific systems. The Genus is the more comprehensive of the two kinds of groups, while the Species is the most precisely defined, or at least the most easily recognized, of all the divisions of the Animal Kingdom. But neither the term Genus nor Species has always been taken in the same sense. Genus especially has varied in its acceptation, from the time when Aristotle applied it indiscriminately to any kind of comprehensive group, from the Classes down to what we commonly call Genera, till the present day. But we have already seen, that, instead of calling all the various kinds of more comprehensive divisions by the name of Genera, modern science has applied special names to each of them, and we have now Families, Orders, Classes, and Branches above Genera proper. If the foregoing discussion upon the nature of these groups is based upon trustworthy principles, we must admit that they are all founded upon distinct categories of characters,—the primary divisions, or the Branches, on plan of structure, the Classes upon the manner of its execution, the Orders upon the greater or less complication of a given mode of execution, the Families upon form; and it now remains to be ascertained whether Genera also exist in Nature, and by what kind of characteristics they may be distinguished. Taking the practice of the ablest naturalists in discriminating Genera as a guide in our estimation of their true nature, we must, nevertheless, remember that even now, while their classifications of the more comprehensive groups usually agree, they differ greatly in their limitation of Genera, so that the Genera of some authors correspond to the Families of others, and vice versa. This undoubtedly arises from the absence of a definite standard for the estimation of these divisions. But the different categories of structure which form the distinctive criteria of the more comprehensive divisions once established, the question is narrowed down to an inquiry into the special category upon which Genera may be determined; and if this can be accurately defined, no difference of opinion need interfere hereafter with their uniform limitation. Considering all these divisions of the Animal Kingdom from this point of view, it is evident that the more comprehensive ones must be those which are based on the broadest characters,—Branches, as united upon plan of structure, standing of course at the head; next to these the Classes, since the general mode of executing the plan presents a wider category of characters than the complication of structure on which Orders rest; after Orders come Families, or the patterns of form in which these greater or less complications of structure are clothed; and proceeding in the same way from more general to more special considerations, we can have no other category of structure as characteristic of Genera than the details of structure by which members of the same Family may differ from each other, and this I consider as the only true basis on which to limit Genera, while it is at the same time in perfect accordance with the practice of the most eminent modern zoologists. It is in this way that Cuvier has distinguished the large number of Genera he has characterized in his great Natural History of the Fishes, in connection with Valenciennes. Latreille has done the same for the Crustacea and Insects; and Milne Edwards, with the coöperation of Haime, has recently proceeded upon the same principle in characterizing a great number of Genera among the Corals. Many others have followed this example, but few have kept in view the necessity of a uniform mode of proceeding, or, if they have done their researches have covered too limited a ground, to be taken into consideration in a discussion of principles. It is, in fact, only when extending over a whole Class that the study of Genera acquires a truly scientific importance, as it then shows in a connected manner, in what way, by what features, and to what extent a large number of animals are closely linked together in Nature. Considering the Animal Kingdom as a single complete work of one Creative Intellect, consistent throughout, such keen analysis and close criticism of all its parts have the same kind of interest, in a higher degree, as that which attaches to other studies undertaken in the spirit of careful comparative research. These different categories of characters are, as it were, different peculiarities of style in the author, different modes of treating the same material, new combinations of evidence bearing on the same general principles. The study of Genera is a department of Natural History which thus far has received too little attention even at the hands of our best zoologists, and has been treated in the most arbitrary manner; it should henceforth be made a philosophical investigation into the closer affinities which naturally bind in minor groups all the representatives of a natural Family.
Genera, then, are groups of a more restricted character than any of those we have examined thus far. Some of them include only one Species, while others comprise hundreds; since certain definite combinations of characters may be limited to a single Species, while other combinations may be repeated in many. We have striking examples of this among Birds: the Ostrich stands alone in its Genus, while the number of Species among the Warblers is very great. Among Mammalia the Giraffe also stands alone, while Mice and Squirrels include many Species. Genera are founded, not, as we have seen, on general structural characters, but on the finish of special parts, as, for instance, on the dentition. The Cats have only four grinders in the upper jaw and three in the lower, while the Hyenas have one more above and below, and the Dogs and Wolves have two more above and two more below. In the last, some of the teeth have also flat surfaces for crushing the food, adapted especially to their habits, since they live on vegetable as well as animal substances. The formation of the claws is another generic feature. There is a curious example with reference to this in the Cheetah, which is again a Genus containing only one Species. It belongs to the Cat Family, but differs from ordinary Lions and Tigers in having its claws so constructed that it cannot draw them back under the paws, though in every other respect they are like the claws of all the Cats. But while it has the Cat-like claw, its paws are like those of the Dog, and this singular combination of features is in direct relation to its habits, for it does not lie in wait and spring upon its prey like the Cat, but hunts it like the Dog.
While Genera themselves are, like Families, easily distinguished, the characters on which they are founded, like those of Families, are difficult to trace. There are often features belonging to these groups which attract the attention and suggest their association, though they are not those which may be truly considered generic characters. It is easy to distinguish the Genus Fox, for instance, by its bushy tail, and yet that is no true generic character; the collar of feathers round the neck of the Vultures leads us at once to separate them from the Eagles, but it is not the collar that truly marks the Genus, but rather the peculiar structure of the feathers which form it. No Bird has a more striking plumage than the Peacock, but it is not the appearance merely of its crest and spreading fan that constitutes a Genus, but the peculiar structure of the feathers. Thousands of examples might be quoted to show how easily Genera may be singled out, named, and entered in our systems, without being duly characterized, and it is much to be lamented that there is no possibility of checking the loose work of this kind with which the annals of our science are daily flooded.