ERUPTION OF THE PERMANENT TEETH 145 



during development. A person of 13 to 14 may appear to be 20 

 to 25 years of age. If later in life, hypothyroidism causes simple 

 goiter, and hyperthyroidism causes exophthalmic goiter. Ex- 

 ophthalmic goiter, or hyperthyroidism occurs invariably in hyper- 

 ontomorphs, and cretins are hypoontomorphs, therefore the 

 hyperactivity of the thyroid gland may have something to do 

 with the hypermorphism of Europeans, and the hypoactivity of 

 the thyroid gland may have something to do with the hypomor- 

 phism of the Filipinos and other peoples (see p. 148). Climatic 

 conditions, habits, food, water, and animal parasites may ac- 

 count in part for the differences. 



The sexual activity of a people may have a profound effect upon 

 their bodily form and mental condition through the interactivi- 

 ties of the secretions of the sexual glands and the other glands of 

 internal secretions. A great deal more has yet to be done on the 

 effect of the internal secretions before definite results can be 

 assured, but the indications are that the secret of multiple activi- 

 ties resides in them and the explanation of many phenomena of 

 development may be there, as well. Factors of selection, of 

 evolution and involution, of progression or regression, of pro- 

 gressive metamorphosis or of retrograde metamorphosis should 

 be mentioned here. The third molars and canines are retro- 

 grading, more in Americans than Filipinos, more in Germans 

 than Indians. 



The process of selection in evolution cannot be passed over as a 

 factor in the eruption of the teeth. Biting and chewing, or gnaw- 

 ing and grinding, are the essentials in man, rather than holding 

 and tearing, therefore the incisors and first molars develop early 

 and are larger, and the canines develop late and are small. It is 

 to be presumed that in prehistoric times those individuals in 

 whom the incisors and molars developed early would be better 

 fitted in the struggle for existence, and their kind would be 

 propagated to a greater extent than those who had the incisors 

 and molars developed late. The third molars are at present 

 undergoing retrograde metamorphosis among Europeans; they 

 appear late, and in some cases fail to appear. It is probable 

 that the second molars erupt in man later than they once did. 



THE AMERICAN JOURNAL Of ANATOMY, VOL. 17, NO. 1 



