SUMMARY OF CAUSES 105 



The experiments of Prof. W. L. Tower, in particular, suggest that 

 important external changes may provoke changes in the germ-cells 

 without necessarily affecting the parental body. He subjected full- 

 grown potato-beetles (Leptinotarsa) to peculiar conditions of tem- 

 perature and humidity during the time when the eggs were maturing, 

 and found that " mutations " occurred in a certain proportion of the 

 offspring. The parents were not affected, having passed the plastic 

 stage; and some of 1h^ eggs were not affected at all. Moreover, 

 the same environmental peculiarity did not always produce the 

 same mutation in the offspring. But what Tower's experiments 

 forcibly suggest is this : that deeply saturating environmental changes 

 may serve to pull the trigger of germinal variability. 



2. It is also to be remembered that if the chromosomes stand 

 I in some definite causal relation to heritable qualities, as seems 



practically certain, then the maturation reduction of the chromo- 

 j somes to one half their original number offers an opportunity 

 |, for variation. 



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

 I and orderly union of two sets of hereditary contributions which 



have often had very different histories — will promote variation. 

 j • 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 con- 

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

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



I 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 

 lof 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. 



