202 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



of size up to the Chasmotherium Cartieri of the Upper 

 Bartonian of Robiac, with which it becomes de- 

 finitely extinct. The rapidity of the increase in 

 size of this branch is slight, the last mutation being 

 hardly twice as large as the first. 



The three branches of the Lophiodons are later 

 than that of the Chasmotherium as regards the 

 uniformity of structure in the pre-molars and 

 molars ; the two mutations of their branches, 

 Lophiodon lautricense and Lophiodon Thomasi, 

 hardly beginning this at the very moment when 

 they become extinct. The three branches obey, 

 however, the law of increase in size, but with very 

 unequal rates of speed. The first grows with an 

 extreme rapidity ; starting with the small Lophio- 

 don remense, of which the size is about that of a 

 Tapir, it rapidly reaches huge forms like the Lophio- 

 don of Issel, and winds up with a giant mutation, 

 the Lophiodon of Lautrec, which exceeds the 

 dimensions of the largest existing Rhinoceros. 

 The second branch evolves much less quickly ; 

 starting, no doubt, from a point common to the 

 preceding branch, it reaches, in the Lophiodon 

 Thomasi, the size of an animal hardly half as big as 

 the Lophiodon lautricense. Finally, the third branch 

 is composed of quite small forms, for the reason that 

 its evolution of growth is very slow ; it starts, so 

 far as we know, by a mutation, the Lophiodon sub- 

 pyrvenaicum, smaller than the initial mutation of 

 the two other branches, and terminates with the 

 Lophiodon leptorhynchum, which is hardly larger 

 than the Lophiodon remense. 



Thus the four phyletic branches known at the 



