Let us now endeavour to find some cases of true linear types, or forms which are intermediate between others because they stand in a direct genetic relation to them. It is no easy matter to find clear and unmistakable evidence of filiation among fossil animals; for, in order that such evidence should be quite satisfactory, it is necessary that we should be acquainted with all the most important features of the organisation of the animals which are supposed to be thus related, and not merely with the fragments upon which the genera and species of the palaeontologist are so often based. M. Gaudry has arranged the species of _Hyoenidoe, Proboscidea, Rhinocerotidoe_, and _Equidoe_ in their order of filiation from their earliest appearance in the Miocene epoch to the present time, and Professor Rütimeyer has drawn up similar schemes for the Oxen and other _Ungulata_--with what, I am disposed to think, is a fair and probable approximation to the order of nature. But, as no one is better aware than these two learned, acute, and philosophical biologists, all such arrangements must be regarded as provisional, except in those cases in which, by a fortunate accident, large series of remains are obtainable from a thick and widespread series of deposits. It is easy to accumulate probabilities--hard to make out some particular case in such a way that it will stand rigorous criticism. After much search, however, I think that such a case is to be made out in favour of the pedigree of the Horses. The genus _Equus_ is represented as far back as the latter part of the Miocene epoch; but in deposits belonging to the middle of that epoch its place is taken by two other genera, _Hipparion_ and _Anchitherium_;[2] and, in the lowest Miocene and upper Eocene, only the last genus occurs. A species of _Anchitherium_ was referred by Cuvier to the _Paloeotheria_ under the name of _P. aurelianense_. The grinding-teeth are in fact very similar in shape and in pattern, and in the absence of any thick layer of cement, to those of some species of _Paloeotherium_, especially Cuvier's _Paloeotherium minus_, which has been formed into a separate genus, _Plagiolophus_, by Pomel. But in the fact that there are only six full- sized grinders in the lower jaw, the first premolar being very small; that the anterior grinders are as large as, or rather larger than, the posterior ones; that the second premolar has an anterior prolongation; and that the posterior molar of the lower jaw has, as Cuvier pointed out, a posterior lobe of much smaller size and different form, the dentition of _Anchitherium_ departs from the type of the _Paloeotherium_, and approaches that of the Horse.