xxix. 5 PARALLEL EVOLUTION 717 



been found in frozen soil and glaciers, allowing study of the soft parts 

 and contents of the stomach. 



Loxodonta, the African elephant, has straighter tusks and the sur- 

 face of the molars wears to a diamond-shaped pattern. In Elephas 

 the tusks are also nearly straight and the molar ridges are parallel. 

 There are, of course, numerous other differences between the two 

 modern elephants and it is not possible to trace out in detail the 

 ancestry of the two types and of the mammoths. In fact one lesson to 

 be learned from the study of elephant evolution is that mammalian 

 fossil remains are seldom sufficiently abundant to allow study of the 

 details of evolutionary history. There is usually some doubt about the 

 exact relationship of the bones and teeth that are found. Phyletic 

 lines are constructed by careful comparison of the characters of the 

 fossils, there is seldom direct evidence of the genetic relationship of 

 any two types. In the present instance it is probable that from the 

 gomphotheres of Miocene times onwards the lower jaw began to 

 shorten and the skull to achieve an elephant-like form in at least four 

 separate stocks (perhaps far more). *Anancus and the true elephan- 

 tines evolved faster in this direction than * Serridentinns and *Mammut, 

 both of which retained the 'mastodont' characters associated with 

 browsing even into the Pleistocene. The existence of parallel evolu- 

 tion may be regarded as established beyond reasonable doubt in this 

 case ; evidently there was some feature either in environmental change 

 or internal 'tendency', or in both, leading all these stocks to change in 

 similar ways, though at different times and rates. 



*Deinotheriam was a distinct type of elephant, separate from all 

 others from Miocene times or earlier. There were down-turned lower 

 tusks and probably also a trunk. There were several molars in the 

 tooth row and the full elephant specializations did not develop. The 

 animals remained similar in structure for a long period, but became 

 very large before they disappeared in the Pleistocene. This is a good 

 example of the development of different variants of a type; deinotheres 

 had the trunks but not the molars of elephants. 



5. *Order Pantodonta (Amblypoda) 



During the Palaeocene and Eocene the ungulate stock produced 

 various large herbivores and these may be referred to the paenungulate 

 group. The relationships of the numerous types discovered are still 

 obscure and classification is probably not yet final. The animals here 

 placed (following Simpson) in the order *Pantodonta were formerly, 

 with others, known as amblypods ('blunt feet'). The Palaeocene 



