262 



CAUSES OF DEGENERATIVE EVOLUTION 



frequently it is absent, in the lower races in 



nineteen per cent, of 

 cases examined ; in the 

 higher races in forty-two 

 per cent. 



The reduction in num- 

 ber and size of these 

 teeth is due to a re- 

 duction in size of the 

 jaw-bones, a cause which 

 also frequently produces 

 a distortion in the 

 arrangement of the other 

 teeth. The rudiments 

 of the wisdom teeth 

 appear on the maxillary 

 tuberosity and on the 

 coronoid process ; it is 

 only after eruption that 

 they come into normal 

 connection with the jaw- 



FIG. 76. Tip of an car ofLolium perenne, 

 with two earlets, the superior bearing 

 two glumes, the lateral earlet with 

 only the superior glume. In the 

 lateral earlet the lower flower is open . 7 



and has two glumules ; all the other SUpeTlOT glume. AniOllg 



flowers are closed and exhibit only the , . -. 



inferior giumuie. plants there are few cases 



G I, inferior glume; GS, superior glume; , ,, 



gi, inferior glumule; gs, superior 01 atropny as a result 01 



lack of space. In grasses 



of the genus Lolium, the earlets are arranged in a 

 spike, but in such a fashion that only the terminal 

 earlet has space for both glumes (fig. 76). The 



2. Atrophy of the 



