VII.
I come now to an obscure part of my subject, very difficult to present in a popular form, and yet so important in the scientific investigations of our day that I cannot omit it entirely. I allude to what are called by naturalists Collateral Series or Parallel Types. These are by no means difficult to trace, because they are connected by seeming resemblances, which, though very likely to mislead and perplex the observer, yet naturally suggest the association of such groups. Let me introduce the subject with the statement of some facts.
There are in Australia numerous Mammalia, occupying the same relation and answering the same purposes as the Mammalia of other countries. Some of them are domesticated by the natives, and serve them with meat, milk, wool, as our domesticated animals serve us. Representatives of almost all types, Wolves, Foxes, Sloths, Bears, Weasels, Martens, Squirrels, Rats, etc., are found there; and yet, though all these animals resemble ours so closely that the English settlers have called many of them by the same names, there are no genuine Wolves, Foxes, Sloths, Bears, Weasels, Martens, Squirrels, or Rats in Australia. The Australian Mammalia are peculiar to the region where they are found, and are all linked together by two remarkable structural features which distinguish them from all other Mammalia and unite them under one head as the so-called Marsupials. They bring forth their young in an imperfect condition, and transfer them to a pouch, where they remain attached to the teats of the mother till their development is as far advanced as that of other Mammalia at the time of their birth; and they are further characterized by an absence of that combination of transverse fibres forming the large bridge which unites the two hemispheres of the brain in all the other members of their class. Here, then, is a series of animals parallel with ours, separated from them by anatomical features, but so united with them by form and external features that many among them have been at first associated together.
This is what Cuvier has called subordination of characters, distinguishing between characters that control the organization and those that are not essentially connected with it. The skill of the naturalist consists in detecting the difference between the two, so that he may not take the more superficial features as the basis of his classification, instead of those important ones which, though often less easily recognized, are more deeply rooted in the organization. It is a difference of the same nature as that between affinity and analogy, to which I have alluded before, when speaking of the ingrafting of certain features of one type upon animals of another type, thus producing a superficial resemblance, not truly characteristic. In the Reptiles, for instance, there are two groups,—those devoid of scales, with naked skin, laying numerous eggs, but hatching their young in an imperfect state, and the Scaly Reptiles, which lay comparatively few eggs, but whose young, when hatched, are completely developed, and undergo no subsequent metamorphosis. Yet, notwithstanding this difference in essential features of structure, and in the mode of reproduction and development, there is such an external resemblance between certain animals belonging to the two groups that they were associated together even by so eminent a naturalist as Linnaeus. Compare, for instance, the Serpents among the Scaly Reptiles with the Caecilians among the Naked Reptiles. They have the same elongated form, and are both destitute of limbs; the head in both is on a level with the body, without any contraction behind it, such as marks the neck in the higher Reptiles, and moves only by the action of the back-bone; they are singularly alike in their external features, but the young of the Serpent are hatched in a mature condition, while the young of the type to which the Caecilians belong undergo a succession of metamorphoses before attaining to a resemblance to the parent. Or compare the Lizard and the Salamander, in which the likeness is perhaps even more striking; for any inexperienced observer would mistake one for the other. Both are superior to the Serpents and Caecilians, for in them the head moves freely on the neck and they creep on short imperfect legs. But the Lizard is clothed with scales, while the body of the Salamander is naked, and the young of the former is complete when hatched, while the Tadpole born from the Salamander has a life of its own to live, with certain changes to pass through before it assumes its mature condition; during the early part of its life it is even destitute of legs, and has gills like the Fishes. Above the Lizards and Salamanders, highest in the class of Reptiles, stand two other collateral types,—the Turtles at the head of the Scaly Reptiles, the Toads and Frogs at the Lead of the Naked Reptiles. The external likeness between these two groups is perhaps less striking than between those mentioned above, on account of the large shield of the Turtle. But there are Turtles with a soft covering, and there are some Toads with a hard shield over the head and neck at least, and both groups are alike distinguished by the shortness and breadth of the body and by the greater development of the limbs as compared with the lower Reptiles. But here again there is the same essential difference in the mode of development of their young as distinguishes all the rest. The two series may thus be contrasted:—
Naked Reptiles. Toads and Frogs, Salamanders, Caecilians.
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.