CONTENTS. 7 



ternal Cause co5perates. A Certain Definiteness in Variations. Mr. Darwin ad- 

 mits the Principle of Specific Stability in Certain Cases of Unequal Variability. 

 The Goose. The Peacock. The Guinea-fowl. Exceptional Causes of Variation 

 under Domestication. Alleged Tendency to Eeversion. Instances. Sterility of 

 Hybrids. Prepotency of Pollen of Same Species, but of Different Race. Mortality 

 in Young Gallinaceous Hybrids. A Bar to Intermixture exists somewhere. 

 Guinea-pigs. Summary and Conclusion . . . . . p. 127 



CHAPTER VI. 

 >. 



SPECIES AND TIME. 



Two Relations of Species to Time. No Evidence of Past Existence of Minutely- 

 intermediate Forms when such might be expected a priori. Bats, Pterodac- 

 tyls, Dinosauria, and Birds. Ichthyosauria, Chelonia, and Anoura. Horse An- 

 cestry. Labyrinthodonts and Trilobites. Two Subdivisions of the Second Rela- 

 tion of Species to Time. Sir William Thomson's Views. Probable Period re- 

 quired for Ultimate Specific Evolution from Primitive Ancestral Forms. Geo- 

 metrical Increase of Time required for Rapidly-multiplying Increase of Structural 

 Differences. Proboscis Monkey. Time required for Deposition of Strata neces- 

 sary for Darwinian Evolution. High Organization of Silurian Forms of Life. 

 Absence of Fossils in Oldest Rocks. Summary and Conclusion . . p. 142 



CHAPTEK VH. 



SPECIES AND SPACE. 



The Geographical Distribution of Animals presents Difficulties. These not Insur- 

 mountable hi themselves ; harmonize with other Difficulties. Fresh-water Fishes. 

 Forms common to Africa and India; to Africa and South America; to China 

 and Australia; to North America and China; to New Zealand and South 

 America; to South America and Tasmania; to South America and Australia. 

 Pleurodont Lizards. Insectivorous Mammals. Similarity of European and South 

 American Frogs. Analogy between European Salmon and Fishes of New Zea- 

 land, etc. An Ancient Antarctic Continent probable. Other Modes of accounting 

 for Facts of Distribution. Independent Origin of Closely-similar Forms. Con- 

 clusion . . . . . . . . . . p. 158 



