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.
It would, of course, be quite inappropriate to present here any general revision of these groups; but I may present a few instances to illustrate the principle of their classification, and to show on what characters they are properly based. Among Reptiles, we find, for instance, that the Genera of our fresh-water Turtles differ from each other in the cut of their bill, in the arrangement of their scales, in the form of their claws, etc. Among Fishes, the different Genera included under the Family of Perches are distinguished by the arrangement of their teeth, by the serratures of their gill-covers, and of the arch to which the pectoral fins are attached, by the nature and combination of the rays of their fins, by the structure of their scales, etc. Among Insects, the various Genera of the Butterflies differ in the combination of the little rods which sustain their wings, in the form and structure of their antennae, of their feet, of the minute scales which cover their wings, etc. Among Crustacea, the Genera of Shrimps vary in the form of the claws, in the structure of the parts of the mouth, in the articulations of their feelers, etc. Among Worms, the different Genera of the Leech Family are combined upon the form of the disks by which they attach themselves, upon the number and arrangement of their eyes, upon the structure of the hard parts with which the mouth is armed, etc. Among Cephalopods, the Family of Squids contains several Genera distinguished by the structure of the solid shield within the skin of the back, by the form and connection of their fins, by the structure of the suckers with which their arms are provided, by the form of their beak, etc. In every Class, we find throughout the Animal Kingdom that there is no sound basis for the discrimination of Genera except the details of their structure; but in order to define them accurately an extensive comparison of them is indispensable, and in characterizing them only such features should be enumerated as are truly generic; whereas in the present superficial method of describing them, features are frequently introduced which belong not only to the whole Family, but even to the whole Class which includes them.