CHAPTER VI. 



CERTAIN LAWS OF VARIATION. 



Effect of environment on growth diminishes rapidly from time of 

 impregnation onwards Reaction of an organism to environment 

 dependent on nature of organism Rapidly diminishing rate of 

 growth in man and in the guinea-pig with progress in develop- 

 ment Variability also diminishes with growth Effect on growth 

 once produced, probably never eradicated Increased variability 

 of sparrow and of periwinkle in America Relation between vari- 

 ability and want of adaptation to environment Variability of 

 migratory and non-migratory birds Does domestication increase 

 variability? 



BEFORE entering on the discussion of the causes of 

 so-called somatogenic variations, i. e., of acquired char- 

 acters, it will be well to examine at some little length 

 certain more or less general laws and conditions which 

 control their acquisition and retention. This is the 

 more necessary, as the matter has received such very 

 little attention hitherto. It seems to have been more 

 or less tacitly assumed that external conditions act 

 equally powerfully at all periods in the growth of a de- 

 veloping organism, whilst the persistence or otherwise 

 of any effect, once produced, has scarcely been debated 

 at all. 



In what way, then, does a developing organism react 

 in its growth to the conditions of its surroundings? It 

 would probably be concluded that any given change of 

 environmental condition would produce more effect in 



