LAW OF SIZE IN THE PHYLETIC BRANCHES 199 



allows us to predict the future discovery of smaller 

 ancestors in the Devonian and in the Silurian. 

 But these animals, already of a respectable size, 

 are greatly surpassed by the Triassic Labyrintho- 

 dons, the Capitosaurus, the Metopias, and, above 

 all, by the gigantic Mastodonsaurus, whose skull 

 measured nearly a metre in length. 



But it is above all in the mammals that the law 

 of increase in size presents itself most clearly, to 

 the degree of being utilizable by modern palaeon- 

 tologists as a veritable criterion for the reconstruc- 

 tion of phyletic branches. We must here limit our- 

 selves to a few examples among those most re- 

 markable. I have pointed out above the pro- 

 gressive evolution of size in the branch of the 

 Brachyodus from the Catodus Rutimeyeri (the size 

 of a small hare), up to the gigantic Brachy- 

 odus onoidens, which attains that of a large 

 Rhinoceros. By referring to the table which in- 

 dicates the evolution of the branches of the Pro- 

 boscidians we may also note a very regular pro- 

 gression in size in several branches : that of the 

 Dinotheria, from the small Dinotherium Cuvieri 

 of the Orleanais up to the Dinotherium gigantissi- 

 mum of Roumania, the giant form of the whole 

 group. In the same way the branch of the Masto- 

 dons with molars composed of rounded mounds 

 begins in the Oligocene of Egypt with the Palceo- 

 mastodon Beadnelli, of which the size, relatively 

 small for a Proboscidian, does not exceed that of 

 a tiny Rhinoceros ; then comes the Burdigalian, 

 the pygmceus, a mutation of the Mastodon angus- 

 tidens ; next the normal type of this species in 



