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.