The Making of Species 



CHAPTER III 



PAGE 



VARIATION . . . . . .52 



The assumption of Darwin and Wallace that variations are haphazard 

 in origin and indefinite in direction If these assumptions be 

 not correct Natural Selection ceases to be the fundamental factor 

 in evolution Darwin's views regarding variation underwent 

 modification He eventually recognised the distinction between 

 definite and indefinite variations, and between continuous and 

 discontinuous variations Darwin attached but little importance 

 to either definite or discontinuous variations Darwin's views on 

 the causes of variations Criticism of Darwin's views Variations 

 appear to occur along certain definite lines There seems to be 

 a limit to the extent to which fluctuating variations can be 

 accumulated De Vries' experiments Bateson on "discon- 

 tinuous variation" Views held by De Vries Distinction 

 between continuous and discontinuous variations The work of 

 De Vries Advantages enjoyed by the botanist in experimenting 

 on the making of species Difficulties encountered by the 

 animal breeder Mutations among animals The distinction 

 between germinal and somatic variations The latter, though 

 not transmitted to offspring, are often of considerable value to 

 their possessor in the struggle for existence. 



CHAPTER IV 



HYBRIDISM . . . . . .in 



The alleged sterility of hybrids a stumbling-block to evolutionists 

 Huxley's views Wallace on the sterility of hybrids Darwin on 

 the same Wallace's theory that the infertility of hybrids has 

 been caused by Natural Selection so as to prevent the evils of 

 intercrossing Crosses between distinct species not necessarily 

 infertile Fertile crosses between species of plants Sterile plant 

 hybrids Fertile mammalian hybrids Fertile bird hybrids 

 Fertile hybrids among amphibia Limits of hybridisation 

 Multiple hybrids Characters of hybrids Hybridism does not 

 appear to have exercised much effect on the origin of new 

 species. 



CHAPTER V 



INHERITANCE . . . . . 133 



Phenomena which a complete theory of inheritance must explain 

 In the present state of our knowledge it is not possible to 

 formulate a complete theory of inheritance Different kinds of 

 inheritance Mendel's experiments and theory The value and 



xiv 



