244 DARWIN AND PALEONTOLOGY 



servations and measurements. Thus the evo- 

 lution of the group has been traced from a small, 

 hornless animal, of the size of a sheep, to a 

 gigantic horned quadruped little inferior to an 

 elephant. I have realized that the origin of all 

 changes which are discovered in the skeleton must 

 be credited to one of the four factors which take 

 part in evolution, namely: — 



Heredity. 

 Ontogeny. 



Environment. 

 Selection. 



Thus new characters which can not be credited 

 immediately to selection, to ontogeny, to environ- 

 ment, must by exclusion be attributed to heredity, 

 these are the mutations or rectigradations. 



METHOD OF EVOLUTION (TITANOTHERJES) 



An interpretation of the evolution of a family. 

 In picturing the evolution of this great family 

 of quadrupeds, the titanotheres, through a long 

 period of time and with an unique sequence of 

 material, may we not interpret the facts by imag- 

 ining a continuous interoperation of the four 

 chief factors, and analyze what we see somewhat 

 as follows? 



First, these animals are all of the titanothere 

 kinship and of the larger perissodactyl kinship. 

 The ordinal kinship and the family consanguinity 



