SUMMARY OF CAUSES 105 



reduction of the chromosomes to one half their original number. 

 This offers an opportunity for variation. 



3. It is likely that fertilisation or amphimixis — the intimate 

 and orderly union of two sets of hereditary contributions which 

 have often had very different histories — will promote variation. 

 It is difficult to believe that it does not bring about new permuta- 

 tions and combinations. 



4. It is possible that variations may also arise in a less 

 conceivable fashion — " bathmically," as the phrase goes — for un- 

 known internal reasons. It is not absurd to suppose that the 

 germ-plasm grows from generation to generation, and, in growing, 

 changes — because it is its nature so to do. 



Apart from variation of internal origin and positive modifica- 

 tion of external origin, we must remember that the offspring may 

 differ from its parents through non-expression of certain items 

 of its inheritance, the non-expression being due to the absence 

 of the appropriate liberating stimulus. This kind of deviation 

 may of course be obliterated next generation, when the full en- 

 vironment allows the latent character to re-express itself. 



