LAW OF SIZE IN THE PHYLETIC BRANCHES 203 



present time of the Lophiodontidae all conform to 

 the law of increase in size, but with very unequal 

 intensity. We might even imagine a fifth branch, in 

 which the increase should be still slighter, or even 

 almost nil. 



And this is not simply an exceptional fact. I 

 might quote a great number of families of Tertiary 

 mammals, the Palaeotheridae, the Anthracother- 

 idae, the Rhinocerotidae, the Amphicyonidae, the 

 Viverridae, the Mustelidae, the Felidae, etc., etc., in 

 which the law of increase in size applies under 

 almost similar conditions to those of the Lophio- 

 dontidae, that is to say, that certain branches, en- 

 dowed with very great evolutionary power, rapidly 

 reach gigantism, while others adopt a moderate 

 pace, and others, again, remain almost without in- 

 crease in size. It is for this reason that we observe 

 almost constantly in the same natural family and 

 at the same epoch, large types of great size, medium, 

 and small or dwarf forms. Existing nature offers 

 us numerous examples in the Felines, the Stags, 

 the Antelopes, and, to generalize, in all groups of 

 the animal kingdom. 



There might be made, it is true, a somewhat 

 specious objection in principle to this interpretation. 

 Why should this differentiation in size, which is 

 observed in nearly all families of living or fossil 

 animals, be reached solely by the process stated 

 above, that is, of the growth at unequal speed of 

 several parallel branches, instead of being produced 

 by degeneration, that is, by the decrease of size 

 of certain branches ? Is it possible to establish, 

 in the present state of palaeontology, the existence 



