OTHER POSSIBLE GUIDING FACTORS 427 



their outstanding peculiarities consists of a unique type of horn on the 

 tip of the nose, a horn branching at the base and diverging laterally 

 into two flat prongs. Osborn distinguishes at least four divergent 

 lines of evolution among these mammals. Line I retained the front 

 teeth (incisors) and remained heavy bodied. Line II lost the incisors 

 and became longer limbed, lighter bodied, speedier, and went in for 

 grazing. In neither of these lines did the horns grow very large. Line 

 III, the first of the large-horned lines, lost the incisors and remained 

 relatively small bodied. Line IV, to which the giant Brontotherium 

 belongs, retained the incisors, grew extremely bulky, and went in for 

 slow, leisurely browsing. 



In all these four lines that are known to have separated early and 

 to have remained genetically independent throughout their careers, 

 the evolution of the horns has run parallel courses. All four lines 

 started hornless, each developed horns independently, and the steps 

 in horn evolution ran the same course in all the four lines, going some- 

 what farther in the larger types than in the smaller. In the last sur- 

 viving representatives of all four lines the horns had reached the same 

 characteristic form, being attached to the same bones of the skull and 

 having a shape quite unlike that known for any other group of mam- 

 mals. 



The inferences that have been drawn from this situation are as 

 follows : The original ancestral titanotheres must have had something 

 in the germ plasm that was predestined to vary along certain definite 

 lines and to produce certain definite structures, in spite of any differ- 

 ences in environment or habits of the different lines of descendants. 

 The horns were compelled to appear at a certain stage in the evolution 

 of each of the four lines, though they may have had little or no adap- 

 tive value. Osborn has no very satisfactory theory as to the mechan- 

 ism involved in this remarkable orthogenesis, but Julian Huxley has 

 offered a purely physiological explanation of horn evolution in titano- 

 theres, an explanation that may apply equally well to many other cases. 

 He has discovered that in horned animals in general, the larger the 

 individual is, the larger are the horns in proportion to the body size. 

 Now, as is true of most phylogenetic series, there was a steady increase 

 in size in all four lines of titanotheres. Thus the earliest titanotheres 

 were too small to have horns at all, the first horned types were of 

 moderate size, the most elaborately horned types were the giant end- 

 products of each of the four lines, and especially was this the case for 

 'he great Brontotherium. 



