SUMMARY OF CHAPTERS xi 



PAGES 



the latter are not produced by consanguinity — Difference 

 between parthenogenesis and consanguinity — Hermaphroditism 

 — Mode of reproduction in each case an adaptation to the con- 

 ditions of life — ^Heterogeny of the Daphnids — Development of 

 exogamous and endogamous institutions in the human species 

 — ^Remarkable case of human consanguinity — Consanguioity 

 among other animal species - - 109-119 



CHAPTER VIII 

 Hybridism and Inteecrossing 

 Quatrefages on the social value of intercrossing — Adverse opinion of 

 Gobineau — Hybridism and intercrossing beyond the limits of 

 the species — Sterility of hybrids — Influence of domestication on 

 the fertility of hybrids — Degrees of fertiUty among hybrids — • 

 Causes of sterility among hybrids — Crossing of distinct species and 

 crossing of variations — ^Degeneracy of human hybrid races 120-128 



CHAPTER IX 



The Origin and Extinction of Species 



1. Variation, selection, and heredity the three principal factors in the 



evolution of species — The land-snakes of the Celebes — Spatial 

 evolution— Species are complexes of adaptation — -The " phyletic 

 force " of NageU — Insufficiency of the theory of Nageli — The 

 mutations theory of De Vries — Difficulty of ascertaining the 

 biological value of a part — ^The shell of the Celebes land-snails 

 — Secondary characters determined by germinal selection — • 

 Primary characters determined by natural selection completing 

 germinal selection — Reconciliation of the theories of Nageli and 

 Darwin. 



2. Biologically indifferent variations as characteristics of a species 



— Such variations can be produced by the action of the 

 environment — ^The alleged hereditary transmission of modifica- 

 tions acquired under the influence of environing conditions— 

 The case of gaUs on plants — ^Modifications effected by the action 

 of the environment which remain purely somatic — Important 

 modifications of the soma are not transmitted — Experiments of 

 Nageli on Hieracium — Influence of temperature on the pupa — 

 Experiments of Weismann on Polyommatus fhlceas — ^Experi- 

 ments of Standfuss and Merrifield on Vanessa — Aberrations 

 caused by the influence of temperature are in a certain degree 

 hereditary — The limits of adaptation. 



