14 ARE THE EFFECTS OF USE INHERITED f 



CROWDED TEETH. 



The too closely-packed teeth in the " decreasing " 

 jaws of modern men (p. I3) 1 are also suggestive 

 of other causes than use and disuse. Why is there 

 not simultaneous variation in teeth and jaws, if 

 disuse is the governing factor ? Are we to suppose 

 that the size of the human teeth is maintained 

 by use at the same time that the jaws are being 

 diminished by disuse ? Mr. Spencer acknowledges 

 that the crowding of bull-dogs' and lap-dogs' 

 teeth is caused by the artificial selection of shortened 

 jaws. If a similar change is really occurring in 

 man, could it not be similarly explained by some 

 factor, such as sexual selection, which might 

 affect the outward appearance at the cost of 

 less obvious defects or inconveniences ? 



Mr. Spencer points to the decay of modern teeth 

 as a sign or result of their being overcrowded 



1 References of course are to Factors of Organic Evolution. 



