CHAPTER IV 



SPONTANEOUS VARIATIONS 



Variations attributed to the direct action of the environment Some 

 variations at least arise otherwise The evidence that variations are 

 caused by external influences Reasons for believing they are not so 

 caused Variations said to be caused by bi-parental reproduction 

 The principal forms of bi-parental inheritance Evidence that bi- 

 parental reproduction is a cause of variations. 



64. THE Theory of Natural Selection as propounded by 

 Darwin is not a theory of heredity, but one of evolution only. 

 As a doctrine of heredity he propounded the hypothesis of 

 Pangenesis. But the two do not stand or fall together. On 

 the contrary, Pangenesis, since it implies the transmission of 

 acquirements, is distinctly incompatible with Natural Selection, 

 and is moreover quite irreconcilable with the facts of modern 

 embryology. His modern followers have accepted Natural 

 Selection while they have rejected Pangenesis, as they have 

 the Lamarckian and Bathmic doctrines of heredity. Instead 

 they have propounded several theories of heredity all of 

 which as must every theory of heredity centre round the 

 question of the causation of variations. 



65. These theories fall naturally into two categories. On 

 the one hand, it is supposed that all, or nearly all, variations 

 are due to the direct and immediate action of the environ- 

 ment on the germ-plasm. On the other hand, it is supposed 

 that all, or nearly all, variations arise " spontaneously " or 

 " fortuitously." By the word " spontaneous " it is of course 

 not intended to imply that variations arise without cause, 

 but only that the cause or causes of them do not proceed 

 from the environment, but are inherent in the germ-plasm 

 itself. In other words, it is supposed that just as the 

 somatic descendants of the fertilised ovum vary amongst them- 

 selves, some becoming nerve-, others skin-cells, and so forth, 

 so the germinal descendants vary, but not in the same regular 

 manner. Spontaneous variations, according to this hypo- 

 thesis, occur in a " haphazard " manner all round the specific 



38 



