230 THE PHILOSOPHY OF BIOLOGY 



stable race or " variety." Nevertheless some effect 

 is produced, and this may be accounted for by suppos- 

 ing that the observed variations are really of two 

 kinds fluctuating variations, which are not inherited, 

 and mutations, which are inherited. The small ob- 

 served effect is due to the selection of the mutations 

 alone : it is a real effect of selection, an undoubted 

 transmutation of the specific form, but experimental 

 and statistical investigations seem to show that selec- 

 tion from the variations that we usually observe is 

 too slow a process to account for the existing forms 

 of life. 



Natural selection acts, therefore, on mutations. 

 Now it seems that we are forced to recognise the exist- 

 ence of two categories of mutations, (i) those stable 

 modifications of an " unit-character " which we term 

 " Mendelian characters," and (2) those groups of stable 

 modifications to which de Vries applied the term 

 mutations. It seems at first difficult to see how per- 

 manent modifications of the specific form can be 

 brought about by the transmission of Mendelian char- 

 acters, for these characters are always transmitted in 

 pairs. Let us take a concrete case that of a man 

 who has six fingers on his right hand, and let us 

 suppose that this was a real, spontaneously appearing 

 character or mutation which had not previously 

 occurred in the ancestry of the man. Two contrasting 

 characters would then be transmitted, (i) the normal 

 five-fingered hand, and (2) the six-fingered hand. 

 Both of these characters are supposed to be present at 

 the same time in the organisation of the men and women 

 of the family originating in this individual, but one 

 of them is always latent or recessive. There would, 

 however, be individuals in which only one of the char- 

 acters would be present either the normal or abnormal 



