In the lower Pleiocene two kinds of ape, the Macaque and Semnopithecus (Fig 18), make their appearance, as well as the genus bear; while in the Upper Pleiocene period the marmots (Arctomys), voles (Arvicola), elephants (Elephas), oxen (Bos), and dog family (Canis), also appear for the first time. The hippopotamus is the first living species of placental mammal of which we have any record in the European strata.[1] It must also be remarked that the oxen (Bos etruscus) were sometimes devoid of horns, as may be seen in a specimen pointed out to me by Dr. Forsyth Major in the Museum at Florence. It seems very likely that horns were originally a mere sexual character peculiar to the males, and transferred ultimately, like other sexual characters, to the females. This was brought about before the beginning of the Pleistocene age, since all the oxen of that era possessed horns. If this view of the origin of horns be accepted, it is easy to explain the singular ease with which, in a comparatively short time, the horns have been bred out of some of the domestic cattle, by selection[2] carried on through a few generations, and our polled cattle may be looked upon as a reversion to an ancestral type. The small size also of the tusks of the domestic hogs, as compared with those of the wild boar, may be explained in the same manner.
In Fig. 18 five upper Pleiocene animals are grouped together, two deer respectively of extinct and living types, the big-nosed rhinoceros, the southern elephant,
- ↑ It is by no means certain that some of the Pleiocene deer of the section Axeidæ are not specifically identical with the Axis and Cervus taivanus of southern and eastern Asia. See Quart. Journ. Geol. Soc. xxxiv. 402.
- ↑ On this point see a letter of the Earl of Selkirk, published in my Preliminary Treatise, British Pleistocene Mammalia. Palæont. Soc. 1878, p. xiv.
