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ARTIODACTYLS 



XXXI. 12 



fossil horses to arrange them in a number of series with side branches, 

 so that we feel that we can imagine the whole evolution of the group. 

 It is interesting that the sequence of evolution used as a type-specimen 

 for students is so often that of the horses and not of the bovids, 

 although we have very much more material for the latter, at least in 

 the later stages. It would be wise to study the two together and to learn 

 from the difficulty of recognizing clear-cut boundaries among the 



millions in the herds of intergrading 

 sheep, goats, oxen, and antelopes 

 that the most important result of the 

 discovery of evolutionary change 

 was the realization that our logic 

 and use of words can no longer 

 depend, as the ancients thought and 

 many backward-lookers still wish 

 today, on the recognition of a cer- 

 tain number of 'species', to one of 

 which every individual can be re- 

 ferred. 



In trying to classify the Bovidae 

 we may perhaps recognize a central 

 group of 'antelopes', but the term 

 is vague and certainly includes 

 several diverse lines; it is not even 

 possible to find criteria for saying 

 'this is an antelope, that a cow, 

 and this other a sheep'. 'Typical' 

 antelopes (Fig. 508) live in Eurasia and Africa, especially the latter. 

 They are rather tall and slender, with smooth hair and backward- 

 curving horns, living mostly on warm or tropical plains. The gazelles 

 may be taken as an example among many. The oxen are heavier 

 animals, often almost of graviportal structure, but with very high 

 thoracic spines; they live in cooler conditions on more northern plains 

 and have more and shaggier hair. Their horns curve forwards and are 

 not twisted. They originated in Eurasia. The domestic cattle and yaks, 

 Bos (Fig. 509), are good examples, and the Bison (Fig. 510) are related 

 creatures, now almost restricted to North America. There are animals, 

 however, that, with the criteria used, cannot be classed as either 

 'antelopes' or 'oxen'; for instance, the elands (Taurotragus) (Fig. 511) 

 of Africa are large and cow-like, but have backwardly directed and 

 twisted horns. Similarly the ovine (sheep and goat) section of the 



Fig. 508. Impala antelope, Aepyceros 

 (From life.) 



