48 VARIATION 



Meristic variation in teeth. As Bateson remarks, "Teeth 

 arise by special differentiation at points along the jaw, as mammae 

 arise by special differentiation at points along the mammary line," 

 and we shall see that with teeth as with mammae these points 

 of special differentiation may frequently lie outside the normal 

 region, that they are subject to increase or decrease in number, 

 and that the increase may be due either to the addition of a mem- 

 ber to the series, to the interpolation of a member, or to the 

 division of a normal member. 



Before considering special cases it is well to note that the 

 similarity between the right and left jaws is that of ordinary 

 bilateral symmetry, but that there is also a kind of symmetry, 

 not very close but still marked, between the dentition of the 

 upper and that of the lower jaw. It should be further noted 

 that in many animals, as in the shark, alligator, etc., the denti- 

 tion constitutes a series in which the separate teeth differ from 

 one another mainly in size. But mammals for the most part are 

 heterodont ; that is, the series is broken up into groups which 

 differ among themselves, though the members of the separate 

 groups resemble one another closely. Thus the incisors are 

 quite different from the canines, which in turn differ from the 

 premolars and the molars. The different incisors, however, 

 are very much alike, and the same is true of the canines and the 

 various molars and premolars. Meristic variation in a heteroge- 

 nous series like this is manifestly much more complicated than 

 in a simple series like the mammae or the rii>s. With this intro- 

 duction attention will be called to a few special examples quoted 

 from the 237 cases that have been collected by Bateson. 1 



1. One hundred and fifty-two adult skulls of anthropoid apes showed 

 twelve cases of extra teeth. One was an incisor, one was anomalous, and the 

 others were molars. This is nearly 8 per cent abnormal, as against 425 

 Old World monkeys that showed but two cases of extra teeth, less than 

 one half of I per cent. 



2. Adult orang, with an additional posterior molar on both sides above 

 and on the left side below. No trace of extra molar on right side was dis- 

 covered, " though there is almost as much room for it as on the left side." 

 Extra molars perfect but slightly smaller than the normal. 



1 Bateson, Materials, etc., pp. 195-273. 



