GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



59 



and that in quite respectable numbers, among 

 the fossil remains af the New. 



Let us speak first of the Rhinoceroses. At 

 the present day we have about eight or nine 

 species, half of which inhabit tropical Africa, 

 while the other half are confined to India and 

 the Su nda I slands. All the African rhinoceroses 

 have two horns ; in Asia there are both one- 

 horned and two-horned species. All these 

 species approach one another so closely that 

 they can hardly be grouped with propriety 

 into sub-genera. 



The distribution during the Quaternary 

 period was quite different from the present. 

 Rhinoceroses were then found everywhere; 

 in the extreme north as well as in the tropics, 

 on the highest mountains (16,000 feet above 

 sea-level in Tibet) as well as on low-lying 

 plains, in the Old as well as in the New 

 World. We know about twenty species from 

 Quaternary, Pliocene, and Miocene strata, 

 and we can trace their descent backwards to 

 the time of the Upper Eocene. Step by step 

 we can demonstrate the gradual modifications 

 by which the old rhinoceroses have arrived 

 at their present forms. The old types (Acera- 

 therium) had no horns, and the development 

 of the horns, which have mostly got separated 

 from the fossil skulls, can be traced on the 

 nasal bones, which at last come to be streng- 

 thened by a bony partition, to enable them to 

 carry the enormous outgrowths by which they 

 are surmounted. But in this case also we 

 can follow out in the two hemispheres two 

 different independent lines, derived from dif- 

 ferent stocks, which gradually approach nearer 

 to one another, and which in Europe pass 

 from the Palseotheria through the hornless 

 forms (Aceratherium) to the true rhinoceroses; 

 while in America the original genera are called 

 Colonoceras, Diceratherium, and Amycodon, 

 and are totally different from the European 

 Stem-forms. But in America there were only 

 hornless forms, which die out with the Plio- 

 cene; while in the Old World the type is 

 continued down to the present time, though 



getting gradually more restricted in the area 

 of its domain, which in Quaternary times 

 was far more extensive. A rhinoceros with 

 enormous horns and a bony nasal septum 

 (Rh. tichorhinus) was the faithful companion 

 of the mammoth, and, like this elephant, had 

 a thick fleece as a protection against the 

 severe cold of the Polar Regions. 



The Wild Horses finally have a pretty 

 simple distribution in the present-day fauna. 

 The "tiger-horses" are inhabitants of tropical 

 and sub-tropical Africa. The asses with a 

 coat of uniform colour hail from the steppes 

 and deserts of Asia, and the wild ass distributed 

 over the western shores of the Red Sea forms 

 the connecting link. But in this family we see 

 astonishing circumstances in relation to the 

 origin. 



One of the most beautiful discoveries of 

 the palaeontology of the present day is that 

 of the two parallel lines in which the horse 

 type has gradually developed in the Old and 

 the New World. In the latter have been 

 found small five-toed animals of the size of a 

 fox {Eohippus Phenacodus) in strata belonging 

 to the Lower Eocene, and from this all the 

 different stages up to the Quaternary horse 

 iyEquus curvidens) have been discovered with- 

 out the omission of one. Every geological 

 series of strata has revealed a separate genus 

 different from that of the preceding group, 

 and these successive genera approach the 

 modern horse step by step through the in- 

 crease in the size of the body, through the 

 multiplication and increasing development 

 and complexity of the enamel folds in the 

 cheek-teeth, and through the gradual reduc- 

 tion in the number and size of the toes. In 

 the Lower Eocene genera the tendency to- 

 wards a reduction in the five toes present is 

 already manifest. The middle toe is the 

 longest and strongest ; the second and fourth 

 digits are equal in length, and though some- 

 what shorter than the middle one, still fur- 

 nished with broad hoofs, which, without doubt, 

 touched the ground. The fifth digit is much 



