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.
X.
There remains but one more division of the Animal Kingdom for our consideration, the most limited of all in its circumscription,—that of Species. It is with the study of this kind of group that naturalists generally begin their investigations. I believe, however, that the study of Species as the basis of a scientific education is a great mistake. It leads us to overrate the value of Species, and to believe that they exist in Nature in some different sense from other groups; as if there were something more real and tangible in Species than in Genera, Families, Orders, Classes, or Branches. The truth is, that to study a vast number of Species without tracing the principles that combine them under more comprehensive groups is only to burden the mind with disconnected facts, and more may be learned by a faithful and careful comparison of a few Species than by a more cursory examination of a greater number. When one considers the immense number of Species already known, naturalists might well despair of becoming acquainted with them all, were they not constructed on a few fundamental patterns, so that the study of one Species teaches us a great deal for all the rest. De Candolle, who was at the same time a great botanist and a great teacher, told me once that he could undertake to illustrate the fundamental principles of his science with the aid of a dozen plants judiciously selected, and that it was his unvarying practice to induce students to make a thorough study of a few minor groups of plants, in all their relations to one another, rather than to attempt to gain a superficial acquaintance with a large number of species. The powerful influence he has had upon the progress of Botany vouches for the correctness of his views. Indeed, every profound scholar knows that sound learning can be attained only by this method, and the study of Nature makes no exception to the rule. I would therefore advise every student to select a few representatives from all the Classes, and to study these not only with reference to their specific characters, but as members also of a Genus, of a Family, of an Order, of a Class, and of a Branch. He will soon convince himself that Species have no more definite and real existence in Nature than all the other divisions of the Animal Kingdom, and that every animal is the representative of its Branch, Class, Order, Family, and Genus as much as of its Species, Specific characters are only those determining size, proportion, color, habits, and relations to surrounding circumstances and external objects. How superficial, then, must be any one's knowledge of an animal who studies it only with relation to its specific characters! He will know nothing of the finish of special parts of the body,—nothing of the relations between its form and its structure,—nothing of the relative complication of its organization as compared with other allied animals,—nothing of the general mode of execution,—nothing of the plan expressed in that mode of execution. Yet, with the exception of the ordinal characters, which, since they imply relative superiority and inferiority, require, of course, a number of specimens for comparison, his one animal would tell him all this as well as the specific characters.
All the more comprehensive groups, equally with Species, have a positive, permanent, specific principle, maintained generation after generation with all its essential characteristics. Individuals are the transient representatives of all these organic principles, which certainly have an independent, immaterial existence, since they outlive the individuals that embody them, and are no less real after the generation that has represented them for a time has passed away than they were before.
From a comparison of a number of well-known Species belonging to a natural Genus, it is not difficult to ascertain what are essentially specific characters. There is hardly among Mammalia a more natural Genus than that which includes the Rabbits and Hares, or that to which the Rats and Mice are referred. Let us see how the different Species differ from one another. Though we give two names in the vernacular to the Genus Hare, both Hares and Rabbits agree in all the structural peculiarities which constitute a Genus; but the different Species are distinguished by their absolute size when full-grown,—by the nature and color of their fur,—by the size and form of the ear,—by the relative length of their legs and tail,—by the more or less slender build of their whole body,—by their habits, some living in open grounds, others among the bushes, others in swamps, others burrowing under the earth,—by the number of young they bring forth,—by their different seasons of breeding,—and by still minor differences, such as the permanent color of the hair throughout the year in some, while in others it turns white in winter. The Rats and Mice differ in a similar way: there being large and small Species,—some gray, some brown, others rust-colored,—some with soft, others with coarse hair; they differ also in the length of the tail, and in having it more or less covered with hair,—in the cut of the ears, and their size,—in the length of their limbs, which are slender and long in some, short and thick in others,—in their various ways of living,—in the different substances on which they feed,—and also in their distribution over the surface of the earth, whether circumscribed within certain limited areas or scattered over a wider range. What is now the nature of these differences by which we distinguish Species? They are totally distinct from any of the categories on which Genera, Families, Orders, Classes, or Branches are founded, and may readily be reduced to a few heads. They are differences in the proportion of the parts and in the absolute size of the whole animal, in the color and general ornamentation of the surface of the body, and in the relations of the individuals to one another and to the world around. A farther analysis of other Genera would show us that among Birds, Reptiles, Fishes, and, in fact, throughout the Animal Kingdom, Species of well-defined natural Genera differ in the same way. We are therefore justified in saying that the category of characters on which Species are based implies no structural differences, but presents the same structure combined under certain minor differences of size, proportion, and habits. All the specific characters stand in direct reference to the generic structure, the family form, the ordinal complication of structure, the mode of execution of the Class, and the plan of structure of the Branch, all of which are embodied in the frame of each individual in each Species, even though all these individuals are constantly dying away and reproducing others; so that the specific characters have no more permanency in the individuals than those which characterize the Genus, the Family, the Order, the Class, and the Branch. I believe, therefore, that naturalists have been entirely wrong in considering the more comprehensive groups to be theoretical and in a measure arbitrary, an attempt, that is, of certain men to classify the Animal Kingdom according to their individual views, while they have ascribed to Species, as contrasted with the other divisions, a more positive existence in Nature. No further argument is needed to show that it is not only the Species that lives in the individual, but that every individual, though belonging to a distinct Species, is built upon a precise and definite plan which characterizes its Branch,—that that plan is executed in each individual in a particular way which characterizes its Class,—that every individual with its kindred occupies a definite position in a series of structural complications which characterizes its Order,—that in every individual all these structural features are combined under a definite pattern of form which characterizes its Family,—that every individual exhibits structural details in the finish of its parts which characterize its Genus,—and finally that every individual presents certain peculiarities in the proportion of its parts, in its color, in its size, in its relations to its fellow-beings and surrounding things, which constitute its specific characters; and all this is repeated in the same kind of combination, generation after generation, while the individuals die. If we accept these propositions, which seem to me self-evident, it is impossible to avoid the conclusion that Species do not exist in Nature in any other sense than the more comprehensive groups of the zoological systems.
There is one question respecting Species that gives rise to very earnest discussions in our day, not only among naturalists, but among all thinking people. How far are they permanent, and how far mutable? With reference to the permanence of Species, there is much to be learned from the geological phenomena that belong to our own period, and that bear witness to the invariability of types during hundreds of thousands of years at least. I hope to present a part of this evidence in a future article upon Coral Reefs, but in the mean time I cannot leave this subject without touching upon a point of which great use has been made in recent discussions. I refer to the variability of Species as shown in domestication.
The domesticated animals with their numerous breeds are constantly adduced as evidence of the changes which animals may undergo, and as furnishing hints respecting the way in which the diversity now observed among animals has already been produced. It is my conviction that such inferences are in no way sustained by the facts of the case, and that, however striking the differences may be between the breeds of our domesticated animals, as compared with the wild Species of the same Genus, they are of a peculiar character entirely distinct from those that prevail among the latter, and are altogether incident to the circumstances under which they occur. By this I do not mean the natural action of physical conditions, but the more or less intelligent direction of the circumstances under which they live. The inference drawn from the varieties introduced among animals in a state of domestication, with reference to the origin of Species, is usually this: that what the farmer does on a small scale Nature may do on a large one. It is true that man has been able to produce certain changes in the animals under his care, and that these changes have resulted in a variety of breeds. But in doing this, he has, in my estimation, in no way altered the character of the Species, but has only developed its pliability to the will of man, that is, to a power similar in its nature and mode of action to that power to which animals owe their very existence. The influence of man upon Animals is, in other words, the action of mind upon them; and yet the ordinary mode of arguing upon this subject is, that, because the intelligence of man has been able to produce certain varieties in domesticated animals, therefore physical causes have produced all the diversities among wild ones. Surely, the sounder logic would be to infer, that, because our finite intelligence can cause the original pattern to vary by some slight shades of difference, therefore an infinite intelligence must have established all the boundless diversity of which our boasted varieties are but the faintest echo. It is the most intelligent farmer that has the greatest success in improving his breeds; and if the animals he has so fostered are left to themselves without that intelligent care, they return to their normal condition. So with plants: the shrewd, observing, thoughtful gardener will obtain many varieties from his flowers; but those varieties will fade out, if left to themselves. There is, as it were, a certain degree of pliability and docility in the organization both of animals and plants, which may be developed by the fostering care of man, and within which he can exercise a certain influence; but the variations which he thus produces are of a peculiar kind, and do not correspond to the differences of the wild Species. Let us take some examples to illustrate this assertion.
Every Species of wild Bull differs from the others in its size; but all the individuals correspond to the average standard of size characteristic of their respective Species, and show none of those extreme differences of size so remarkable among our domesticated Cattle. Every Species of wild Bull has its peculiar color, and all the individuals of one Species share in it: not so with our domesticated Cattle, among which every individual may differ in color from every other. All the individuals of the same Species of wild Bull agree in the proportion of their parts, in the mode of growth of the hair, in its quality, whether fine or soft: not so with our domesticated Cattle, among which we find in the same Species overgrown and dwarfish individuals, those with long and short legs, with slender and stout build of the body, with horns or without, as well as the greatest variety in the mode of twisting the horns,—in short, the widest extremes of development which the degree of pliability in that Species will allow.
A curious instance of the power of man, not only in developing the pliability of an animal's organization, but in adapting it to suit his own caprices, is that of the Golden Carp, so frequently seen in bowls and tanks as the ornament of drawing-rooms and gardens. Not only an infinite variety of spotted, striped, variegated colors has been produced in these Fishes, but, especially among the Chinese, so famous for their morbid love of whatever is distorted and warped from its natural shape and appearance, all sorts of changes have been brought about in this single Species. A book of Chinese paintings showing the Golden Carp in its varieties represents some as short and stout, others long and slender,—some with the ventral side swollen, others hunch-backed,—some with the mouth greatly enlarged, while in others the caudal fin, which in the normal condition of the Species is placed vertically at the end of the tail and is forked like those of other Fishes, has become crested and arched, or is double, or crooked, or has swerved in some other way from its original pattern. But in all these variations there is nothing which recalls the characteristic specific differences among the representatives of the Carp Family, which in their wild state are very monotonous in their appearance all the world over.
Were it appropriate to accumulate evidence here upon this subject, I could bring forward many more examples quite as striking as those above mentioned. The various breeds of our domesticated Horses present the same kind of irregularities, and do not differ from each other in the same way as the wild Species differ from one another. Or take the Genus Dog: the differences between its wild Species do not correspond in the least with the differences observed among the domesticated ones. Compare the differences between the various kinds of Jackals and Wolves with those that exist between the Bull-Dog and Greyhound, for instance, or between the St. Charles and the Terrier, or between the Esquimaux and the Newfoundland Dog. I need hardly add that what is true of the Horses, the Cattle, the Dogs, is true also of the Donkey, the Goat, the Sheep, the Pig, the Cat, the Rabbit, the different kinds of barn-yard fowl,—in short, of all those animals that are in domesticity the chosen companions of man.