Antiquity and Persistence in Character of the existing Races of Mankind. Theory of their Unity of Origin considered. Bearing of the Diversity of Races on the Doctrine of Transmutation. Difficulty of defining the Terms "Species" and "Race." Lamarck's Introduction of the Element of Time into the Definition of a Species. His Theory of Variation and Progression. Objections to his Theory, how far answered. Arguments of modern Writers in favour of Progression in the Animal and Vegetable World. The old Landmarks supposed to indicate the first Appearance of Man, and of different Classes of Animals, found to be erroneous. Yet the Theory of an advancing Series of Organic Beings not inconsistent with Facts. Earliest known Fossil Mammalia of low Grade. No Vertebrata as yet discovered in the oldest Fossiliferous Rocks. Objections to the Theory of Progression considered. Causes of the Popularity of the Doctrine of Progression as compared to that of Transmutation. CHAP. 21. ON THE ORIGIN OF SPECIES BY VARIATION AND NATURAL SELECTION. Mr. Darwin's Theory of the Origin of Species by Natural Selection. Memoir by Mr. Wallace. Manner in which favoured Races prevail in the Struggle for Existence. Formation of new Races by breeding. Hypotheses of definite and indefinite Modifiability equally arbitrary. Competition and Extinction of Races. Progression not a necessary Accompaniment of Variation. Distinct Classes of Phenomena which Natural Selection explains. Unity of Type, Rudimentary Organs, Geographical Distribution, Relation of the extinct to the living Fauna and Flora, and mutual Relations of successive Groups of Fossil Forms. Light thrown on Embryological Development by Natural Selection. Why large Genera have more variable Species than small ones. Dr. Hooker on the Evidence afforded by the Vegetable Kingdom in favour of Creation by Variation. Steenstrup on alternation of Generations. How far the Doctrine of Independent Creation is opposed to the Laws now governing the Migration of Species. CHAP. 22. OBJECTIONS TO THE HYPOTHESIS OF TRANSMUTATION CONSIDERED.