Not all of the abnormalities which thus suddenly appear, we know not how or wherefore, are new. Many recall characters in lower or older groups, and may reasonably be interpreted as 'reversions.' Thus the horse's leg shown in Fig. 6 bears a well-developed side toe, in place of the small vestige that is normally present. Horses with this peculiarity have occurred with some frequency, probably before, and certainly since, the most famous of their kind, which Julius Cæsar rode. It seems reasonable to regard this peculiarity as a return to the old ancestral condition illustrated before, in which the side toes were well developed. The same applies to the instance of a persistent tail and persistent gill slits in man (Figs. 7 and 8), and to many other instances that might be quoted. One must indeed deal carefully with such cases, for it is always difficult to say what changes are new departures, and what are returns to ancestral types. There is danger of arguing in a circle--of supposing the ancestry from the abnormality, and of terming the latter a reversion because it suggests the supposed ancestry. Nevertheless, when variations occur, suggesting characters which are believed, on other grounds, to be ancestral, they must tend to strengthen the other evidence as to the evolution of the type in question. [Illustration: FIG. 9. (a) The blind-gut of a kangaroo (_bl_), and (b) the corresponding reduced structure, the vermiform appendix in man (_bl_).] [Illustration: FIG. 10.--Skeleton of Cassowary, showing reduced wing-bones (a piece of black paper is placed under them). From Dendy's _Outlines of Evolutionary Biology_ (Constable).] Another and a very strong evidence of Evolution is to be found in what are termed vestigeal structures, two of which are illustrated in Figs. 9 and 10. They are, for the most part, obviously useless, and their occurrence has never been satisfactorily explained except by supposing them to be remnants of organs that were functional in the past history of their possessors' race. The appendix of man, for instance, is not only useless, but is frequently a source of danger. But its presence is readily explained by supposing that it represents the blind-gut, which is large and functional in many of the lower animals. Again, how should we account for the presence of small functionless wing-bones in the cassowary, unless by supposing that its ancestors were accustomed to fly like ordinary birds? How should we explain the bones which represent the hind limbs of the whale, unless by regarding the whale as descended from an animal which had functional hind limbs, or the representatives of eyes in animals that live in the dark, unless by supposing that these are descended from ancestors which saw? It has been well said that the bodies of many animals are veritable antiquarian museums, filled with relics of their own ancestors.