xii PRfiCIS OF CONTENTS 



Difficulties raised by the theory, (i) Specific differences in colour and 

 structure ; (2) The problem of secondary sexual characters ; (3) The 

 origin of habits ; (4) Complex organs and co-adaptation. General 

 summary and conclusions. 



CHAPTER VIII 



OTHER THEORIES OF EVOLUTION 



(1) Lamarckism and ' the inheritance of induced modifications.' 



(2) Evolution by hybridism. Origin of new characters and character 

 combinations already discussed (Ch. II). These theories now reviewed 

 in their wider evolutionary application. Transformation of popula- 

 tions. Progressive modification. 



(3) ' Chance survival.' Survival of non-advantageous mutants. Elton's 

 theory. The occupation of new habitats. Rapid spread of introduced 

 species. 



(4) Orthogenesis. Use of the term. Historical. Parallel variation. 

 Main groups of evolutionary phenomena which are treated as ' ortho- 

 genetic' (a) Normal evolutionary series. Haldane's explanation. 

 (b) Recapitulatory series : (i) Haldane's explanation. (ii) The 

 racial life-cycle and the theory of racial senescence, (c) Abnormal 

 growth. Excessive size of parts distinguished from excessive com- 

 plexity. Various explanations, (i) The direct adaptive value of 

 excessive size. Haldane's theory. Certain special cases examined. 

 (ii) Huxley's explanation (heterogony and selection), (iii) Fisher 

 and Haldane's theory of the effect of selection of metrical characters 

 determined by numerous genes, (iv) Theory of an internal momentum. 

 Conclusion on the various explanations of ' orthogenetic ' phenomena. 



(5) Theories involving an internal impulse of a non-physiological nature, 

 (i) Bergson's theory, (ii) ' Psycho-biological ' theory of Russell and 

 others, (iii) Smuts's holistic theory. General conclusions. 



CHAPTER IX 



ADAPTATION 



Use of the term. (1) Useful characters. (2) Specialisation. (3) Statis- 

 tical adaptation. (4) The organismal concept of adaptation. The 

 adjustment of the organism to environmental pressure : (i) modi- 

 fication, (ii) compensation, and (iii) independence. Closeness 

 of adaptation. The conception of optimum conditions. Internal 

 optima. Optimum density. Self-regulation. Organisation and 

 development. Organisation and specialisation. Difficulty of ex- 

 plaining the origin of organisation by random mutations. Initiation 

 of variation by the organism itself. 



