animal kingdom, and there mention only some of the cases which have yielded well-detailed proof of continuous differentiation. It is obvious that the parts of animals most likely to have been preserved in such a continuous series of fossils as the present line of evidence requires, would have been the hard parts. These are horns, bones, teeth, and shells. Therefore I will consider each of these four classes of structures separately. * * * * * Horns wherever they occur, are found to be of high importance for purposes of classification. They are restricted to the Ruminants, and appear under three different forms or types--namely solid, as in antelopes; hollow, as in sheep; and deciduous, as in deer. Now, in each of these divisions we have a tolerably complete palæontological history of the evolution of horns. The early ruminants were altogether hornless (Fig. 60). Then, in the middle Miocene, the first antelopes appeared with tiny horns, which progressively increased in size among the ever-multiplying species of antelopes until the present day. But it is in the deer tribe that we meet with even better evidence touching the progressive evolution of horns; because here not only size, but shape, is concerned. For deer's horns, or antlers, are arborescent; and hence in their case we have an opportunity of reading the history, not only of a progressive growth in size, but also of an increasing development of form. Among the older members of the tribe, in the lower Miocene, there are no horns at all. In the mid-Miocene we meet with two-pronged horns (_Cervus dicrocerus_, Figs. 61, 62, 1/5 nat. size). Next, in the upper Miocene (_C. matheronis_, Fig. 63, 1/8 nat. size), and extending into the Pliocene (_C. pardinensis_, Fig. 64, 1/18 nat. size), we meet with three-pronged horns. Then, in the Pliocene we find also four-pronged horns (_C. issiodorensis_, Fig. 65, 1/16 nat. size), leading us to five-pronged (_C. tetraceros_). Lastly, in the Forest-bed of Norfolk we meet with arborescent horns (_C. Sedgwickii_, Fig. 66, 1/35 nat. size). The life-history of existing stags furnishes a parallel development (Fig. 67), beginning with a single horn (which has not yet been found palæontologically), going on to two prongs, three prongs, four prongs, and afterwards branching. [Illustration: FIG. 60.--Skull of _Oreodon Culbertsoni_. (After Leidy.)] [Illustration: FIGS. 61-66. The series is reduced from Gaudry's illustrations, after Farge, Croizet, Jobert and Boyd Dawkins.] [Illustration: FIG. 67.--Successive stages in the development of an existing Deer's Antlers. (After Gaudry, but a better illustration has already been given on p. 100.)] * * * * *