246 DARWIN AND PALEONTOLOGY 



of variations in proportion which coincide with 

 the needs of the phylum/ 



'No abrupt variations (mutations) have been 

 observed in the evolution of the titanotheres, but 

 this in no way renders it inconceivable that skel- 

 etal mutations in the De Vries sense have pro- 

 duced new races in certain phyla. The addition 

 or loss of a vertebra in the sacral region, which 

 appears to distinguish certain titanothere phyla, 

 may be a case of such sudden inheritable muta- 

 tions. 



Independently in four or five Eocene branches 

 of the titanothere stock the horn rudiments very 

 gradually arise, apparently through hereditary 

 predisposition or family kinship, as rectigrada- 

 tions, at the junction of the nasal and frontal 

 bones. As in the case of the cusps, the shape of 

 these horn rudiments is from the first conditioned 

 by the respective breadth (brachycephaly) or 

 length (dohchocephaly) of the skull. 



The branches or sub-phyla become more and 

 more sharply distinguished from each other 

 by increasing brachycephaly or dohchocephaly, 

 brachypody or dolichopody, apparently through 

 congenital variations of proportion accumulated 

 by selection and guided by ontogeny through 

 " organic selection." The animals belonging to 



' It is important to note, on the authority of Professor Castle, 

 that proportions of the skeleton and probably of the teeth are not 

 inherited as distinct "unit characters." Inheritance of bone size 

 and shape seems to be as a rule regularly blended by interbreed- 

 ing and without subsequent Mendelian splitting. 



