ERUPTION OF THE PERMANENT TEETH 



157 



American boys have 1 hyper- to 3 hj^po-s at 8 years, and 12 

 hyper-s to 1 hypo- at 16 years. 



7. The laiv of alternation of development 



There are one or more periods of acceleration alternating with 

 periods of retardation in the development of the structures of the 

 body. Each organ or structure has a critical period when it is 

 developing most rapidly, and when it is probably most susceptible 

 to its environment. The periods of acceleration in the develop- 

 ment of one structure are synchronous with the periods of retar- 

 dation in the development of another, and the two may be called 

 complementary structures. 



The relative number of hj^per-onto-morphs is greatest among 

 the Americans, least among the Filipinos, and nearly as great 

 among the Germans as among the Americans. Hypo-morphism 

 decreases with age, and hyper-morphism increases, so that whereas 

 among the Filipinos there are 15.2 hypo-s to 1 hyper- between the 

 ages of 5 and 16, there are only 3.8 hypo-s to 1 hyper- from 16 to 

 30 years of age. Hypo-morphism is a condition of less maturity 

 than hyper-morphism. Apparently the Filipinos mature more 

 slowly than the Americans and Germans in morphologic form. 



