366 IF. B. Scott — Variations and Mutations. 



After these general statements, we may examine Bateson's 

 conclusions with regard to the teeth somewhat more in detail. 

 (1) The effects of domestication are of course not significant 

 in the present connection. (2) Dental variation is more com- 

 monly unsymmetrical on the two sides of the same jaw and 

 with reference to the upper and lower jaws. The mammalian 

 phyla show no such lack of symmetry in dental reduction, 

 though the upper and lower jaws do not necessarily keep pace 

 w 7 ith each other in this respect. ' At any stage in the phylum 

 we may find individuals with such unsymmetrical dentition, 

 but they are always rare, the normal form having almost inva- 

 riably an equal number of teeth on the two sides of tlie mouth. 

 The only exception to this rule, if it can fairly be called so, is 

 to be found in the minute incisors of Tit another ium, which 

 are on the point of disappearing altogether and in many indi- 

 viduals are present in different numbers on the two sides of 

 the jaws, both upper and lower, but whether this asymmetry 

 characterizes the normal of any species has not yet been 

 determined. 



(3) While in variation two teeth in the variant may repre- 

 sent one in the normal, being formed by the division of a 

 single tooth-germ, no such process is of phylogenetic signifi- 

 cance in any of the heterodont phyla, except, perhaps, the 

 true whales. As a variation however, it is not unknown 

 among fossils; specimens of Amphicyon have been described 

 by Schlosser, in which a fourth upper molar is preserved. 



(4) Numerical variations in the dentition usually affect the 

 ends of series ; this is likewise true of phylogenetic reductions 

 in the number of teeth, but certainly does not apply to such 

 cases of increase as occur among the Cetacea and Edentata. 



(5) and (6) The conclusions as to the character of new teeth 

 and their effects upon adjacent teeth have no application to the 

 phylogenetic history of any of the heterodont phyla whose his- 

 tory is known, because, as we have already seen, such new 

 teeth rarely, if ever, arise. 



(7) In no heterodont group has any such remodelling of the 

 dentition occurred as to obscure the individuality of the sepa- 

 rate teeth. 



(8) While the facts of variation seem to show " that indi- 

 viduality should not be attributed to members of a series which 

 has normally a definite number of such members," the facts of 

 palaeontology lead to a diametrically opposite conclusion, so far 

 as the mammals are concerned, though there is no reason to 

 maintain that such individuality is fixed beyond possibility of 

 change. It further seems likely that such individuality is 

 more or less due to differentiation and is less distinctly marked 



