CHAPTER XIV FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY Fundamental difficulties and objections--Mr. Herbert Spencer's factors of organic evolution--Disuse and effects of withdrawal of natural selection--Supposed effects of disuse among wild animals--Difficulty as to co-adaptation of parts by variation and selection--Direct action of the environment--The American school of evolutionists--Origin of the feet of the ungulates--Supposed action of animal intelligence--Semper on the direct influence of the environment--Professor Geddes's theory of variation in plants--Objections to the theory--On the origin of spines--Variation and selection overpower the effects of use and disuse--Supposed action of the environment in imitating variations--Weismann's theory of heredity--The cause of variation--The non-heredity of acquired characters--The theory of instinct--Concluding remarks CHAPTER XV DARWINISM APPLIED TO MAN General identity of human and animal structure--Rudiments and variations showing relation of man to other mammals--The embryonic development of man and other mammalia--Diseases common to man and the lower animals--The animals most nearly allied to man--The brains of man and apes--External differences of man and apes--Summary of the animal characteristics of man--The geological antiquity of man--The probable birthplace of man--The origin of the moral and intellectual nature of man--The argument from continuity--The origin of the mathematical faculty--The origin of the musical and artistic faculties--Independent proof that these faculties have not been developed by natural selection--The interpretation of the facts--Concluding remarks LIST OF ILLUSTRATIONS