12 CONTENTS 



PAGE 



inent, to certain elements in which marked responses may be made, 

 though (4) generally speaking sessile organisms are far more responsive 

 to differences in the environment than are free-living organisms. 

 (5) Among free-living species in a state of nature closely related to 

 man, modifications are not of great importance in producing variations. 



CHAPTEE XV 



THE INFLUENCE OF THE ENVIRONMENT UPON MAN . . 336 



(1) Among men there is no normal environment. (2) The effect 

 of exercise, (3) of general conditions, (4) and of climate. (5) Himting- 

 ton's theory of optimum climatic conditions. (6) The effect of food, 

 altitude, &c. (7) Statistical results. (8) Effect of disease. (9) 

 Evidence derived from twins. (10) Conclusion that modifications, 

 with the exception of those produced by disease, are not of great 

 importance, though of more importance than in the case of species 

 closelv allied to man. 



CHAPTEE XVI 



HEREDITY IN MAN 356 



(1) Recent work in heredity leads to the conclusion that at the 

 basis of all human characters lies a vast number of unit-factors ; 

 but few have yet been recognized, and (2) we have to depend upon 

 statistical data, which show that all mental and physical characters 

 are inherited. Further discussion of the inheritance of (3) disease, 

 (4) temperament, (5) instinct, and (6) intellect. (7) Conclusion. 



CHAPTEE XVII 



THE EVOLUTION OF PHYSICAL CHARACTERS . . .368 



(1) Preliminary notice of certain difficulties — cause of mutation, 

 strength of selection. (2) During the intermediate period great 

 changes took place, but (3) in general dm^ing the first and second 

 periods the tendency vras towards the evolution and preservation 

 of certain types, (4) which is comprehensible when the conditions 

 among primitive races are studied. (5) In the third period more varia- 

 tions have been allowed to persist ; disease becomes of importance. 

 (6) At all times warfare, migration, and crossing have influenced 

 physical characters. (7) Factors of possible importance other than 

 selection. (8) Conclusion. 



