Chap. XXII. CAUSES OF VABIABILITY. 259 



the whole organisation, or on certain parts alone ; other 

 variations appear to be induced indirectly through the re- 

 productive system being affected, as we know is often the 

 case with various beings, which when removed from their 

 natural conditions become sterile. The causes which induce 

 variability act on the mature organism, on the embryo, and, 

 probably, on the sexual elements before impregnation has 

 been effected. 



