CONTENTS V 



Chapter XI — Continued. 



Section 1 — Continued. 



Principles of proportion — Continued. Page 



Harmonic allometry exceptional 826 



Dolichorhinus: adaptation of the lengthened head to the supposed habit of grazing 827 



Dolichocephalic increments of Mesaiirhinus and Dolichorhinus 827 



Slow but differential evolution in breadth of skull of Dolichorhinus 828 



Differential evolution in the grinding teeth of Dolichorhinus 828 



Contrasts of differential evolution in brachycephalic and dolichocephalic skulls 828 



Summary of harmonic and differential aUometrons in the skulls and feet and an interpretation of the phylogeny 



of the titanotheres, by W. K. Gregory 828 



History of research 828 



Application of the proportional reversal principle to the titanotheres 830 



Irreversible and reversible evolution of allometrons 833 



Separability and correlation of biocharacters 833 



Separabilitj' of allometrons in heredity 833 



Correlation, coordination; compensation of rectigradations and allometrons 834 



Theoretic causes of the evolution of new characters and new proportions 834 



Theories advanced to explain the origin of rectigradations and allometrons -^ 834 



Analysis of the evidence on the modes of origin of variation as considered in Darwinism, Lamarckism, and 



tetrakinesis 835 



Observed principle of tetraplastic development of body form; theoretic principle of the tetrakinetic evolution 



of the germ 835 



Quantitative increment of the four separable factors in development and evolution 837 



Analysis of the modes of variation; theoretic importance of initiation 838 



Bearing of saltation versus continuity on the Lamarckian, Darwinian, and tetrakinetic theories 839 



Darwin's hypothesis of fortuitous saltation and fluctuation 839 



Most saltations in mammals abnormal 840 



MendeUan discontinuity in heredity 841 



Truth and error in Johannsen's pure line saltation principle 842 



Experiments in the artificial selection of variations of proportion 842 



Theoretic and experimental causes of the evolution of allometrons 843 



Germinal allometrons arising by continuous or gradual change 843 



Germinal allometrons arising by sudden changes (saltations); interaction theory .! 844 



Certain germinal allometrons uninfluenced by the direct action of environment 844 



Germinal allometrons apparently influenced by direct action of environment 845 



Germinal allometrons, fluctuating and nonfluctuating 845 



Certain germinal allometrons of high survival selection value 845 



Other germinal allometrons of apparently no survival selection value; predetermination 846 



Ontogenetic allometrons experimentally influenced by changes of habit 846 



Harmonic or conflicting influence of the four factors of evolution 846 



Ontogenetic allometrons experimentally influenced by changes of environment 847 



Germinal allometrons: Prenatal, adolescent, adult 847 



Ontogenetic allometrons influenced by glandular internal secretions, enzymes, and other organic catalyzers. 848 



Germinal (?) allometrons influenced by organic catalyzers 849 



Summary of theoretic causes of evolution 849 



Evidence against the Lamarckian principle 849 



Evidence against the theory of selection of minute variations 849 



Evidence favorable to the selection of certain fluctuations 849 



Unknown causes of the origin of rectigradations 849 



Necessity of experiment on the tetrakinetic principle 849 



Bibliography of literature relating to the theories of evolution cited in section 1 850 



Section 2. Natural selection in mammals; causes of the extinction of the titanotheres and other quadrupeds 852 



Extinction of faunas in the age of mammals 852 



Gradual or sudden extinction 853 



Extinction of both the adaptive and the inadaptive 853 



Phases of extinction 854 



Multiple causes of extinction 854 



The law of natural selection 855 



History of opinion 856 



Cataclysmal hypotheses 856 



Unif ormitarian theories 857 



Lyell on extinction 857 



Balance of nature (Lyell, Darwin, Wallace) 858 



Environmental causes of extinction 860 



The physical environment 860 



Physiographic changes 860 



Changes of climate 861 



