314 OUTLINES OF EVOLUTIONARY BIOLOGY 



considerably elongated, apparently serving as a support for the 

 lengthening proboscis. 



In Tetrabelodon angustidens, from European Miocene formations, 

 .this elongation of the mandible is much more marked, so that 

 I the lower jaw is much longer than the upper one and the short 

 _ lower tusk comes to project almost as 



far forward as the long upper one (Fig. 

 159, 3). From this time onwards, how- 

 ever, the chin shortens, thereby allowing 

 greater flexibility to the proboscis, so that 

 in the lower Pliocene we find Tctrabelodon 

 longirostris (Fig. 159, 4) with the lower 

 jaw only a little longer than the upper, 

 leading the way to the mastodons and 

 true elephants (Elephas), which also 

 appeared in Pliocene times and in which 

 the tusks have entirely disappeared from 

 the greatly abbreviated mandibles while 

 the cheek teeth have become enormously 

 enlarged and complicated (Fig. 159 ; 5). 



We have here a wonderfully perfect 

 series of connecting links between the 

 most primitive known ungulate mammals 

 and the elephants. Only forms which 

 appear to lie in or near the direct line of 

 descent have been mentioned in the above 

 brief account. Other modifications of 

 the proboscidean type arose as lateral 

 oft'shoots from this main stem. One of 

 the most remarkable of these is Dino- 

 therium, with its great, downwardly 

 directed lower tusks (Fig. 160), which 

 appeared in Europe in the Pliocene period. 

 In the case of the Cetacea, a group which includes the whales, 

 porpoises and dolphins, we have as yet only a much more frag- 

 mentary pedigree, but still quite sufficient to justify, on the 

 palaeontological side, the conclusion, already arrived at on ana- 

 tomical grounds, that these extremely aberrant forms are the 

 f descendants of typical terrestrial mammals which have become 

 re-adapted to an aquatic life and in accordance therewith have 

 re-acquired a superficial resemblance to their much more remote 



FIG. 158. a, Fore Foot 

 and />, Hind Foot of 

 Plioliippuspernix, X |. 

 (From Lull.) 



