xxxi. iz BOVIDAE 761 



smaller and more closely knit groups being more suitable for forest 

 life. The placenta is cotyledonary. 



The Bovidae, with more than 100 genera, is much the largest 

 ungulate family. The original centre of evolution of the family was in 

 Eurasia, where they are now less common, whereas in Africa they are 

 particularly successful at present. A few types, such as the bison, reached 

 North America, but none entered South America until man showed 

 that they can flourish there, and indeed also in Australia. The fact that 

 we possess numerous fossil remains and that the group is still at the 

 height of its development makes classification very difficult. This is the 

 situation that we should expect, remembering that evolution consists 

 in the slow change of the characteristics of populations. At first thought 

 it may seem paradoxical that in a group so recently evolved and of 

 which we know so much it should be exceptionally difficult to trace 

 affinities and lines of descent. The fact is that the numerous remains of 

 fossil bovids from the Pliocene and Miocene are still quite insufficient 

 to enable us to reconstruct the changes in the populations. It is not 

 really to be expected that the relatively few specimens of these large 

 animals that can be collected and studied should show us the detailed 

 changes, extending over 20 million years or more, by which a popula- 

 tion of perhaps a million small creatures such as *Eotragus of the 

 Miocene, developed into the present bovid population of, say, a 

 thousand million animals, divisible into hundreds of non-interbreeding 

 populations that range in structure and habits from the gazelle to the 

 bison. An imaginative look at the details of evolutionary change reveals 

 a terrifyingly complicated system, which we can hardly hope to follow 

 in detail. The geological information can surely never be sufficient to 

 show us the necessary facts about the variation of such great popula- 

 tions, and their gradual changes, at least in the case of animals as large 

 and rarely preserved as Bovidae. We know hardly anything about 

 variation and heredity in our own cattle, so how can we hope to follow 

 the genetics of their ancestors ? Yet nothing less than a full view of the 

 gradual population changes will show us how the evolution of a group 

 has proceeded. 



Following types of organization over long geological periods gives a 

 deceptively simplified idea of the stages traversed. We recognize the 

 'stages' because, fortunately for us, only tiny remnants of the popula- 

 tions have been preserved, and perhaps some 'primitive' types remain 

 to the present day. Thus in the long history of the perissodactyls we 

 can refer all our modern and fossil forms to some 160 genera; the tapirs 

 are there to show us a very ancient condition, and we know just enough 



