From Bateson's Materials for the Study of Variation (Macmillan).
Fig. 7.—Persistent Coccyx in Man.
Fig. 8.—Persistent Gill Slits in Man.
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.
Fig. 9.
(a) The blind-gut of a kangaroo (bl), and (b) the corresponding reduced structure, the vermiform appendix in man (bl)
