Aristotle, whose great zoological work Pliny had closely studied, was certainly aware of the broad distinction between the Whales and Dolphins (the position of whose blow-holes he mentions), and Fishes.
Gesner separated the Whales from the Fishes, including them in a distinct order of marine animals. Aldrovandi separated them also, though they appear in the same volume, the title of which is De Piscibus Libri V.: De Cetis Liber Unus.' Johnston gives them a separate chapter at the head of his book 'De Piseibus.' Ray, in his Synopsis Methodica Piecium ' (1713), observes that the term fish' is extended, even by the learned of our country, to the bloodless aquatics, as they were then termed, Exanguia aquatica, such as Crustacea, Testacea, and Mania, or Shellless Mollusks. On the other hand, some, he remarks, not only exclude those Exanguia aquatica, but also the Cetacea (" Cetaceum genus, 'seu Balm Marime"), contending that no other animals can justly be termed fishes except those which breathe by means of gills, and have but one ventricle to the heart. With these last Ray agrees, and expresses his own opinion, that, if we speak properly and philosophically, the name of Fish should be restricted to such last-mentioned animals only, and points out the absence of any relationship of the "Pisces Cetacei dicti" with the true fishes ; adding, that with the exception of the place where they spend their lives, the external figure of their body, their hairless skin, and their natatory progression, the Cetacea have hardly anything in common with the true fishes, but in other respects agree with the viviparous quadrupeds.
Nevertheless, that he may avoid dissent from received opinions and the appearance of paradox, Ray declares that he will not inno vate, but consider the Cetaceous Animals as Fishes; and he proceeds to define what a fish is, thus : An aquatic animal having blood, wanting feet, swimming with fins, covered either with scales or with a naked, smooth, hairless skin, passing its life in the waters, and never voluntarily leaving it for the dry land.
The Cetaceous Fishes, or Belluce Marine, form his first section, and are immediately followed by the Cartilaginous Fishes, called /facixn by Aristotle. Of the Cetaceans he says, that they breathe, like quadrupeds, by means of lungs, copulate, bring forth their young alive, and nourish them with their milk, and in the structure and use of all their internal parts agree with those animals.
The following are the genera enumerated by Ray :— Bakena (2 species); Cete (1); Orca (2, but one not clearly defined); Albus ; Monoceros ; Ddphinus ; Phoccena. And he divides the Cetacei
generic Pisces, seu Balcence, into two great groups—the Toothed and Toothless ; the latter having horny lamina in the upper jaw.
The Toothed Whales are subdivided into those which have teeth in both jaws, and those which have teeth in the lower jaw ; and there are further subdivisions depending ou the absence or presence of the back-fin and the shape of the teeth.
The Toothless or Whalebone Whales are subdivided also with reference to the absence or presence of the back-fin, the presence of a blow-hole, or the employment of nostrils in respiration, the presence of plaits on the belly, and the width of the lower jaw.
Linnnus, in his last edition of the 'Systems Natures' (1766), defines the fulcra, or props, of his Mammalia to he 4 feet, with the exception of those Mammals which are merely aquatic, "in quibus pedes posteriores in calla pinnam compedes ;" in other words, in which the posterior limbs are manacled or conjoined, so as to form a The seven orders of Mammalia in this system are divided into three sections :-1, Unguiculata ; 2, Ungulaia ; 3, Mutica. The seventh and last order, Cete, is the only one belonging to the section Mutica.
The following is the Linnrean definition of the last-named order :— Pectoral fins in lieu of feet, and feet conjoined into a horizontal flattened fin in lieu of a tail. No claws. Teeth cartilaginous. Nose often a frontal pipe. Food, mollusks, fishes. Locality, the ocean.
Linnzeus then declares that he has separated these Cetaceans from the Fishes, and associated them with the Mammals, on account of their warm bilocular heart, their lungs, their moveable eyelids, their hollow ears, " penem iutrantem feminam mammis lactantem," and this, to use his own expressive words, "ex lege natural jure meri toque." Here then we find the decisive step taken, with the unflinching firmness of a master mind, relying upon the Philosophical principles that demanded the separation, and no longer yielding to popular prejudice by calling that a fish which he knew to he a mammiferous animal. Some parts of his definition—not much of it—may be open to criticism, as where he designates the teeth as cartilaginous, a term probably used to comprehend both the horny lamina of the Whale bone Whales and the true teeth of the other Cetaceans ; but the broad line of distinction is unassailable, and will ever remain so.