Form and Function | Page 4

suckers while Octopus has two, Argonauta, Nautilus, Sepia, and apparently Loligo media (= his Teuthis) and L. vulgaris(or forbesii) which seems to be his Teuthos. He had a grasp of the principles which should be followed in judging of the natural affinities of species. For example, he knew that the cuckoo resembles a hawk. "But," he says, "the hawk has crooked talons, which the cuckoo has not, nor does it resemble the hawk in the form of its head, but in these respects is more like the pigeon than the hawk, which it resembles in nothing but its colour; the markings, however, upon the hawk are like lines, while the cuckoo is spotted" (Hist. Anim., Cresswell's trans., p. 147, London, 1862).
The groups he distinguished were--man, viviparous quadrupeds, oviparous quadrupeds, birds, fishes, Cetacea, Cephalopoda, Malacostraca (= higher Crustacea), Insecta (= annulose animals), Testacea (= molluscs, echinoderms, ascidians). A class of Acaleph?, including sea-anemones and sponges, was grouped with the Testacea. The first five groups were classed together as sanguineous, the others as exsanguineous, from the presence or absence of red blood.
Besides these classes "there are," he says, "many other creatures in the sea which it is not possible to arrange in any class from their scarcity" (Creswell, loc. cit., p. 90).
(3) Aristotle's greatest service to morphology is his clear recognition of the unity of plan holding throughout each of the great groups.
He recognises this most clearly in the case of man and the viviparous quadrupeds, with whose structure he was best acquainted. In the Historia Animalium he takes man as a standard, and describes his external and internal parts in detail, then considers viviparous quadrupeds and compares them with man. "Whatever parts a man has before, a quadruped has beneath; those that are behind in man form the quadruped's back" (Cresswell, loc. cit., p. 26). Apes, monkeys, and Cynocephali combine the characteristics of man and quadrupeds. He notices that all viviparous quadrupeds have hair. Oviparous quadrupeds resemble the viviparous, but they lack some organs, such as ears with an external pinna, mamm?, hair. Oviparous bipeds, or birds, also "have many parts like the animals described above." He does not, however, seem to realise that a bird's wings are the equivalent of a mammal's arms or fore-legs. Fishes are much more divergent; they possess no neck, nor limbs, nor testicles (meaning a solid ovoid body such as the testis in mammals), nor mamm?. Instead of hair they have scales.
Speaking generally, the Sanguinea differ from man and from one another in their parts, which may be present or absent, or exhibit differences in "excess and defect," or in form. Unity of plan extends to all the principal systems of organs. "All sanguineous animals have either a bony or a spinous column. The remainder of the bones exist in some animals; but not in others, for if they have the limbs they have the bones belonging to them" (Cresswell, loc. cit., p. 60). "Viviparous animals with blood and feet do not differ much in their bones, but rather by analogy, in hardness, softness, and size" (Cresswell, loc. cit., p. 59). The venous system, too, is built upon the same general plan throughout the Sanguinea. "In all sanguineous animals, the nature and origin of the principal veins are the same, but the multitude of smaller veins is not alike in all, for neither are the parts of the same nature, nor do all possess the same parts" (Cresswell, loc. cit., p. 56). It will be noticed in the first and last of these three quotations that Aristotle recognises the fact of correlation between systems of organs,--between limbs and bones, and between blood-vessels and the parts to which they go.
Sanguineous animals all possess certain organs--heart, liver, spleen, kidneys, and so on. Other organs occur in most of the classes--the oesophagus and the lungs. "The position which these parts occupy is the same in all animals [sc. Sanguinea]" (Cresswell, loc. cit., p. 39).
Unity of plan is observable not only in the Sanguinea, but also within each of the other large groups. Aristotle recognises that all his cuttlefish are alike in structure. Among his Malacostraca he compares point by point the external parts of the carabus (Palinurus), and the astacus (Homarus), and he compares also the general internal anatomy of the various "genera" he distinguishes. As regards Testacea, he writes, "The nature of their internal structure is similar in all, especially in the turbinated animals, for they differ in size and in the relations of excess; the univalves and bivalves do not exhibit many differences" (Cresswell, loc. cit., p. 83). There is an interesting remark about "the creature called carcinium" (hermit-crab), that it "resembles both the Malacostraca and the Testacea, for this in its nature is similar to the animals that are like carabi, and it is born