That one man should have discovered so many truths, and formed so sure a base for Zoology, is less surprising than the fact that for about eighteen centuries a science which seemed to offer particular attractions to men gifted with power of observation, was no farther advanced. Yet this is the case. Aristotle’s disciples, as well as his successors, remained satisfied to be his copiers or commentators, and to collect fabulous stories or vague notions. With very few exceptions (such as _Ausonius_, who wrote a small poem, in which he describes from his own observations the fishes of the Mosel) authors entirely abandoned original research. And it was not until about the middle of the sixteenth century that Ichthyology made a new step in advance by the appearance of _Belon_, _Rondelet_, and _Salviani_, who almost simultaneously published their grand works, by which the idea of species was established definitely and for all times. * * * * * [Sidenote: Belon.] P. BELON travelled in the countries bordering on the eastern part of the Mediterranean, in the years 1547–50; he collected rich stores of positive knowledge, which he deposited in several works. The one most important for the progress of Ichthyology is that entitled “De aquatilibus libri duo” (Paris 1553; small 4to.) Belon knows about 110 fishes, of which he gives rude, but generally recognisable, figures. In his descriptions he pays regard to the classical as well as vernacular nomenclature, and states the outward characteristics, sometimes even the number of fin-rays, frequently also the most conspicuous anatomical peculiarities. Although Belon but rarely gives definitions of the terms used by him, it is generally not very difficult to ascertain the limits which he intended to assign to each division of aquatic animals. He very properly divides them into such as are provided with blood, and into those without it: two divisions, called in modern language Vertebrate and Invertebrate aquatic animals. The former are classified by him according to sizes, the further subdivisions being based on the structure of the skeleton, mode of propagation, number of limbs, form of the body, and on the physical character of the localities inhabited by fishes. This classification is as follows:-- I. The larger fishes or Cetaceans. _A._ Viviparous Cetaceans with bony skeletons (= Cetacea). _B._ Viviparous Amphibians. 1. “With four limbs: Seals, Hippopotamus, Beaver, Otter, and other aquatic Mammalia. 2. With two limbs: Mermaids, etc. _C._ Oviparous Amphibians (= Reptiles and Frogs). _D._ Viviparous Cartilaginous fishes. 1. Of an oblong form (= Sharks). 2. Of a flat form (= Rays and Lophius). _E._ Oviparous Cartilaginous fishes (= Sturgeons and Silurus).