In the genus _Lepidosiren_, also a Dipnoan, the pectoral limb has the same axial structure, but is without fin-rays, although in the breeding season the posterior limb or ventral fin in the male is covered with a brush of fine filaments. This structure, according to Prof. J. G. Kerr,[3] is probably without definite function, but belongs to the "category of modifications so often associated with the breeding season (cf. the newts' crest) commonly called ornamental, but which are perhaps more plausibly looked upon as expressions of the intense vital activity of the organisms correlated with its period of reproductive activity." Professor Kerr, however, thinks it not unlikely that this brush of filaments with its rich blood-supply may serve in the function of respiration, a suggestion first made by Professor Lankester. FOOTNOTES: [3] Philos. Trans., Lond., 1900. CHAPTER V MORPHOLOGY OF THE FINS =Origin of the Fins of Fishes.=--One of the most interesting problems in vertebrate morphology, and one of the most important from its wide-reaching relations, is that of the derivation of the fins of fishes. This resolves itself at once into two problems, the origin of the median fins, which appear in the lancelets, at the very bottom of the fish-like series, and the origin of the paired fins or limbs, which are much more complex, and which first appear with the primitive sharks. In this study the problem is to ascertain not what theoretically should happen, but what, as a matter of fact, has happened in the early history of the fish-like groups. That these structures, with the others in the fish body, have sprung from simple origins, growing more complex with the demands of varied conditions, and then at times again simple, through degeneration, there can be no doubt. It is also certain that each structure must have had some element of usefulness in all its stages. In such studies we have, as Hæckel has expressed it, "three ancestral documents, paleontology, morphology, and ontogeny"--the actual history as shown by fossil remains, the sidelight derived from comparison of structures, and the evidence of the hereditary influences shown in the development of the individual. As to the first of these ancestral documents, the evidence of paleontology is conclusive where it is complete. But in very few cases are we sure of any series of details. The records of geology are like a book with half its leaves torn out, the other half confused, displaced, and blotted. Still each record actually existing represents genuine history, and in paleontology we must in time find our final court of appeal in all matters of biological origins.