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CHAPTER IX Hybridism Distinction between the sterility of first crosses and of hybrids— Sterility various in degree, not universal, affected by close inter- breeding, removed by domestication—Laws governing the ster- ility of hybrids—Sterility not a special endowment, but incidental on other differences, not accumulated by natural selection— Causes of the sterility of first crosses and of hybrids—Parallel- ism between the effects of changed conditions of life and of crossing—Dimorphism and Trimorphism—Fertility of varieties when crossed, and of their mongrel offspring not universal— Hybrids and mongrels compared independently of their fertility —Summary. THE view commonly entertained by naturalists is that species, when intercrossed, have been specially en- dowed with sterility, in order to prevent their con- fusion. This view certainly seems at first highly probable, for species living together could hardly have been kept dis- tinct had they been capable of freely crossing. The subject is in many ways important for us, more especially as the sterility of species when first crossed, and that of their hybrid offspring, cannot have been acquired, as I shall show, by the preservation of successive profitable degrees of sterility. It is an incidental result of differences in the reproductive sys- tems of the parent-species. In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been con- founded; namely, the sterility of species when first crossed, and the sterility of the hybrids produced from them. Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the formative organs themselves are 298
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The Origin of Species
Title
The Origin of Species
Author
Charles Darwin
Publisher
P. F. Collier & Son
Location
New York
Date
1909
Language
English
License
PD
Size
10.5 x 16.4 cm
Pages
568
Keywords
Evolutionstheorie, Evolution, Theory of Evolution, Naturwissenschaft, Natural Sciences
Categories
International
Naturwissenschaften Biologie

Table of contents

  1. EDITOR'S INTRODUCTION 5
  2. AN HISTORICAL SKETCH of the Progress of Opinion on the Origin of Species 9
  3. INTRODUCTION 21
  4. Variation under Domestication 25
  5. Variation under Nature 58
  6. Struggle for Existence 76
  7. Natural Selection; or the Survival of the Fittest 93
  8. Laws of Variation 145
  9. Difficulties of the Theory 178
  10. Miscellaneous Objections to the Theory of Natural Selection 219
  11. Instinct 262
  12. Hybridism 298
  13. On the Imperfection of the Geological Record 333
  14. On the Geological Succession of Organic Beinss 364
  15. Geographical Distribution 395
  16. Geographical Distribution - continued 427
  17. Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs 450
  18. Recapitulation and Conclusion 499
  19. GLOSSARY 531
  20. INDEX 541
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