270 



THE ORIGIN AND EVOLUTION OF LIFE 



tribution in the northern hemisphere and in the equatorial 

 regions. 



Among the giant tribes which issued from this ancient con- 

 tinent the evolution of the proboscidians gives us an instance 

 of the most extreme divergence of a terrestrial type from a 

 related family, the sirenians, which evolve into the aquatic, 



fluviatile, and littoral t>'pe of 

 the existing sea-cows and man- 

 atees. 



In the transformation of 

 PalcEomastodon (Fig. 131) into 

 Elephas there are notable 

 changes of proportion as well 

 as the loss of many characters, 

 as seen in the disappearance of 

 the lower tusks, the enlarge- 

 ment and curvature of the up- 

 per tusks, the elongation of the 

 proboscis, the abbreviation of 

 the skull, the elongation of the 

 limbs, the relative abbreviation 

 of the vertebrae of the neck and of the backbone, the reduction 

 of the tail. The limbs become of the weight-bearing type, the 

 hind limbs attaining proportions which converge toward those 

 of the titanothere BrontotJicrium (Fig. 127). The final numerical 

 loss of characters as witnessed in the very gradual reduction of 

 the lower tusks affords an instance of the leisurely methods of 

 nature, for the process requires 2,000,000 years in the elephant 

 line while in the mastodon line the lower tusks were still pres- 

 ent at the time of the comparatively recent extinction of this 

 animal, which occurred since the final glaciation of North 

 America. The loss of parts through retardation is also seen 



Fig. 132. The Ice-Fii: i,1)> (jf the 

 Fourth Glaciation. 



Southward extension of the ice-fields 

 over the northeastern United States 

 during the period of the fourth glacia- 

 tion. After studies of Chamberlain. 

 Modelled by Howell. 



