IV CONTENTS 



Chapter X — Continued. Tage 



Section 3. Summarj' of the cranial and skeletal evolution of the titanotheres 780 



General conclusions reached 780 



Disharmonic evolution in length and breadth of skull 783 



Evolution of the skull in correlation (coadaptation) with that of the teeth and horns 784 



Are the proportions of skull and teeth adaptive? 786 



Radiation and divergence in Eocene skuUs 786 



Evolution of the skull in Eocene time partly prophetic of that in Oligocene time 786 



Independent evolution of the skuU in Eocene time nonprophetic of that in Oligocene time 786 



Zygomatic indices 787 



Differential evolution in skull proportions in the several phj'la of titanotheres j__ 788 



Palate and shifting posterior nares 788 



Rudiments of horns arising independently in Eocene phyla 788 



Modes of origin of the horns of the titanotheres 790 



Phyletio divergence in time of origin of horns 790 



Accelerated direct evolution of horns in the "prophet horn " phylum 790 



Retarded evolution of horns in the Palaeosyopinae and Telmatheriinae 790 



Divergence in the form of horns of the Oligocene titanotheres 792 



Correlation (coadaptation) of horns with cranial and dental characters 792 



Correlation of horns with sex 793 



Evolution of the brain 793 



Dental mechanism 794 



Gradual change of diet 795 



Harmony of proportions of head and grinding teeth 795 



Arrested evolution in the teeth 796 



Foramina of the atlas vertebra in the titanotheres and other perissodactyls 797 



Progressive adaptations of the pectoral and pelvic arches, the limbs, and the feet of the titanotheres 798 



Ancestral titanotheres suboursorial 799 



The titanothere pes 800 



Progressive stages of the manus 800 



The titanothere magnum 800 



Evolution of the astragalus 801 



Section 4. Bibliography for Chapters VIII-X 802 



Chapter XI. Causes of the evolution and extinction of the titanotheres 805 



Section 1. Modes and causes of the origin and evolution of new adaptive characters (rectigradations) and new propor- 

 tions (allometrons) in old characters 805 



Distinction between modes and causes 805 



Speculation as to the causes of evolution 806 



Distinction between invisible germ evolution and visible bodily evolution in the titanotheres 806 



Principles of tetraplasy in individual development; ontogeny 807 



Principles of single character (biocharacter) evolution 808 



Separability and coordination of biocharacters 809 



Initiation of biocharacters : Is it environmental, ontogenetic, or germinal? 809 



Differing hereditary velocities; rates of motion of biocharacters 810 



Modes of biocharacter evolution actually observed 811 



Mammalian phyla distinguished by different ontogenetic and phylogenetic velocities in the same biochar- 

 acters 811 



Distinction between rectigradation biocharacters and allometron biocharacters 812 



Resemblances between rectigradations and allometrons as biocharacters 813 



Contrasts between rectigradations and allometrons as biocharacters 813 



Principles of rectigradation 814 



Horns arise as typical rectigradations — orthogenetic , continuous 814 



Phylogenesis and initiation of horns in titanotheres 816 



Dental rectigradations 817 



Rectigradations of Osborn contrasted with mutations of Waagen 818 



" Mutations" and "species" 819 



Heritage separabiUty of rectigradation dental cusps and folds 819 



Rectigradation cusps on titanothere teeth are unlike saltations 819 



Rectigradations influenced by degree of zoologic affinity 820 



Principles of proportion 820 



Proportion biocharacters (allometrons) 820 



Terminology indicating proportions of the skuU and skeleton , 822 



Proportional evolution in the typical brachycephal Palaeosyops 823 



Contrast between evolution and growth 823 



Differential (disharmonic) allometrons in Palaeosyops 823 



BrachycephaUc allometrons affect all biocharacters; harmonic 823 



Harmonic proportion trend constitutes the generic character of Palaeosyops 824 



Fourfold modes of allometric brachycephaly 824 



Differential allometrons of specific value 825 



