The Evidence from Variation. 155 



Natural Selection cannot produce Race Modification 

 unless the Same Part tends to vary in a Number of 

 Individuals at the Same Time. 



This argument, which seems to me to be the most im- 

 portant one which has ever been adduced against the 

 theory of natural selection, was first advanced by a writer 

 in the North British Review in June, 1876. 



The author points out that since the chance of sur- 

 vival of any particular individual which is born is very 

 slight indeed, the birth of an individual with any par- 

 ticular slight advantage, and its consequent superiority 

 over its fellows, would not be sufficient to over-balance 

 the chance of its destruction. The objection, which is 

 purely logical, and not experimental, will be stated at 

 length in another place. At present the fact that those 

 who are best qualified to judge, Darwin among them, 

 have acknowledged its great weight, will suffice to show 

 that it is a real and valid objection, and that the foot- 

 hold of the theory of natural selection would be greatly 

 strengthened if we could show that the causes which 

 produce variation act in such a way as to cause the same 

 part to vary at the same time in great numbers of in- 

 dividuals. 



According to our theory of heredity, this will gener- 

 ally be the case. We suppose that an unfavorable change 

 in the environment of a particular cell causes this cell to 

 throw off gemmules. It is plain that a change in the 

 external world, which unfavorably affects any partic- 

 ular cell or group of cells in one individual, will usually 

 affect the corresponding cells of other individuals of the 

 species at the same time. When any particular cell is 

 prolific of gemmnles in one individual of a species, the 

 same thing will usually be true of the same cell in other 



