172 CHARLES DARWIN. 



in previous generations, and not, although it may 

 appear to do so, from the corresponding parts of 

 earlier stages. In the process of time certain parts 

 during certain stages may be affected by use or 

 disuse or surroundings, and the parts of subsequent 

 generations will be similarly affected, because formed 

 from correspondingly altered gemmules ; but this need 

 not affect the other stages of the same parts. 



Transposition and multiplication of parts. — The 

 cases of abnormal transposition or multiplication of 

 organs — for instance, the development of teeth in the 

 palate or of pollen in the edge of a petal — are to be 

 explained by supposing that the gemmules unite with 

 wrong cells instead of, or as well as, the right ones ; 

 " and this would follow from a slight change in their 

 elective affinities." Such slight changes are known 

 to occur ; for instance, certain plants " absolutely 

 refuse to be fertilised by their own pollen, though 

 abundantly fertile with that of any other individual 

 of the same species." Inasmuch as the cells of 

 adjoining parts will often have nearly the same 

 structure, we can understand that some slight change 

 in elective affinity may affect a large area. Hence 

 we can account for a crowd of horns on the head of 

 a sheep, or many spurs on the leg of a fowl, etc. 

 Frequently repeated parts are extremely liable to 

 vary in number ; in this case we have a large number 

 of closely allied gemmules and of points for their 

 union, and slight changes in elective affinity would 

 be specially apt to occur. 



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