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Chapter XXXIII. Dividing Factors. Isolation 430 



Introductory 430 



"Isolation" Used in the Broadest Sense 431 



Isolation and Inbreeding 43 1 



Isolation and Selection 43 2 



The Various Categories of Isolation 432 



a) Geographic Isolation 43 2 



b) Isolation Due to Sheer Distance Apart 435 



c) Climatic Isolation 436 



d) Biotic Isolation 436 



e) Reproductive Isolation 436 



/) Psychic Isolation 438 



PART IV. EUGENICS 



Chapter XXXIV. Introduction to Eugenics 441 



Definitions of Eugenics 441 



The Scope of Eugenics 442 



The Aims and Ideals of Eugenics 443 



Methods of Research in Eugenics 444 



Chapter XXXV. Human Heredity as Revealed by Pedigrees . 446 



Weaknesses of the Pedigree Method 446 



Some Hereditary Traits in Man 447 



Some Pedigrees of Dominant Characters 450 



A Typical Pedigree of a Recessive Character 453 



Heredity of Mental Traits in Man 456 



The Heredity of Feeble-mindedness 458 



Insanity and Heredity 461 



Pedigrees of Royal Families 462 



Chapter XXXVI. The Statistical Study or Heredity in Man . 470 



Introduction 470 



Galton's Statistical Studies of Heredity in Man 471 



Criticism of Galton's Work 474 



Galton's Data in the Light of Genetics 475 



Chapter XXX VII . Twins and Heredity 476 



I. The Biology of Twins 476 



Various Kinds of Twins 476 



The Origin of Identical Twins 478 



II. The Twin Method of Studying Heredity in Man .... 480 



III. Crime and Destiny 484 



IV. Twins and the Relative Potency of Heredity and Environment 485 



