CHAP, iv.] IMPORTANT CHARACTERS OF FAUNA. 87 



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), ele- 

 phants (Elepkas), 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 etriiscus) were some- 

 times 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, 



1 It is by no means certain that some of the Pleiocene deer of the 

 section Axddce are not specifically identical with the Axis and Cervus 

 taivanus of southern and eastern Asia. See Quart. Journ. Geol. Soc. 

 xxxiv. 402. 



2 On this point see a letter of the Earl of Selkirk, published in my Pre- 

 liminary Treatise, British Pleistocene Mammalia. Palseont. Soc. 1878, p. xiv. 



