ceeding from such analogies to stricter inquiries, other and firmer divisions were founded on the organs of motion, prehension, food, respiration and circulation, the organs of sense, and the internal arrangements of the nervous system. In the course of these inquiries we have detected, under many disguises, some general rules of structure and function -patterns or types so to speak to each of which a vast number of specific forms can be referred, as if they were so many examples of one general structure, modified in an almost infinite variety of ways to suit appointed habits of life. The general characters of agreement we often call typical, the special characters of difference are often called adaptive-terms which imply the recognition of law and modification, mind and choice-a Creator who works by rule, and who has provided for wonderful diversity in his works and for the persistence through them all of some firm general principles of construction, which can be traced out in their operation by the human mindprocess of observation, comparison, and inference. When reduced to the smallest number of great primary types, we find Plants to be ranked in two divisions: Phanerogamous or flowering and seed-bearing, and Cryptogamous or flowerless plants deficient of true seeds. These may be again subdivided so as to make four great groups: viz. Phanerogamous plants with stamens and pistils, spiral vessels, and seeds. Cryptogamous plants, without flowers, true stamens, pistils, spiral vessels, or seeds. 1. Dicotyledones. Embryo with two seed-lobes, stem growth outward, leaves reticulated, symmetry quinary or quaternary (as Ranunculaceӕ, Rosacece, Labiatæ, Conifera). 2. Monocotyledones. Embryo with one seed-lobe, stem growth internal, leaves usually with parallel veins, symmetry ternary (as Liliaceæ, Araceæ, Graminea). 3. Acrogens. Stem, with leaves and branches, mostly traversed by vessels (as Filices, Lycopodiacea, Equisetacea). 4. Thallogens. No distinct stem or leaves, or vessels, but cellular expansions (as Lichenes, Algæ, Fungi). In like manner Animals may be collected in four great groups: I. Vertebrata. Brain protected by bony (or cartilaginous) case, and, proceeding from it, a nervous trunk carried along the upper side of a chain of articulated bones (vertebræ) or a corresponding cartilaginous axis. Symmetry bilateral. II. Articulata. Body jointed or ringed across. A double nervous cord, ganglionated at intervals, proceeding along the body. Symmetry bilateral. III. Mollusca. Body not jointed or ringed across : nervous cord encircling the alimentary canal, and ramifying through the body. Symmetry bilateral or spiral. IV. Radiata. Nervous system absent or reduced to a ring round the alimentary canal with few radiating threads. Symmetry radiate round an axis. There are some forms of plants and animals so slightly adapted to special purposes of life, as far as we have yet discovered, that life may be thought to reside in them only in a general form; the type of which-if that can be called typical which seems rather to be marked by absence of all but elementary organization-may be called cellular or rudimentary. This group, however, is but provisional, and will probably be hereafter better divided according as the nutritive and reproductive systems are more surely analysed by the microscope. Under the great types of vegetable and animal structure which have been mentioned, naturalists find it convenient to adopt many classes, orders, families, genera, species and varieties. These are not so settled in any case as to be altogether free from change by fresh inquiries and discoveries: they have been augmented and modified by the discoveries of palæontology; which have filled several void spaces in the series of affinities, and illustrated modern types by ancient parallels. The adherence of plants and animals to the several leading types is sufficient in most cases to leave no doubt of the place of each; the typical peculiarity influences all parts of their structure, the leaves, flowers and fruit of a plant-the limbs, dermal covering, composition, colour and temperature of the blood in an animal. So that a single leaf will generally decide the place of a plant in the three great divisions-a single drop of circulating fluid, the eye, a bone, a shell or crust, that of an animal. As an exception among plants we may mention those of the family Smilaceæ which yield Sarsaparilla, whose leaves are veined with the net-work of Dicotyledones, but whose seeds are formed on the model of Monocotyledones. Among animals the Polyzoa or Bryozoa were long ranked as Radiate animals, and there is now a difference of opinion regarding the place of the conspicuous family of Echinodermata, which some place at the head of the Radiata, but others join with Annulose animals. Each great type comprehends several considerable subdivisions, in each of which a reigning idea may often be traced in the structure, and exemplified in the function. These are again subdivided in a manner suggestive of other ideas. For example, Mammalia are separated from all other animals by their mode of rearing their young. Among Mammalia quadrupedal motion on land appears to be the reigning idea, which determines the complete type; but the Cetacea, destined for aquatic life, are bipedal, and have their two legs altered to perform the work of pectoral fins, and the tail expanded to a propulsive instrument. Thus ideas are expressed which seem borrowed from fishes, to which in general form suited for easy motion in water the Cetaceans also correspond. So in early geological times we find Ichthyosaurus assuming that sort of conformity to fish-structure and form, which belongs to what Mac Leay calls analogy, while the real affinity of the whale is to ordinary Mammalia, and the real affinity of the Ichthyosaurus is to ordinary Reptilia. In a different manner the quadrupedal maınmal is modified for flight in the air. The Bat takes up the extended interdigital membrane, and the sternal keel, by analogy with birds-as in older periods of the world, the Pterodactyle spread its wings and worked its pectoral muscles under the same peculiarities1. Instances of this kind might be greatly multiplied, but these may be sufficient to shew that each great type or subtype of structure admits of variations of 1 See the evidence of the sternal keel of Pterodactylus in the fine collection of Green Sand Fossils in the Cambridge Museum. |