lf)<; MKIMSTK 1 VARIATION. [PART I. 



also; and \}\ns, AS in the case of vertebra?, for instance, in any 

 given example )f a numerical change qualitative changes must be 



|. PI iked t< 'I' t/OO, 



A- a preliminary to the consideration of evidence relating to 

 the Vaiiati.in .f te.'th it may be useful to call attention to cert.-iin 

 peculiarities of teeth considered as a Meristic Series. In the 

 lntr."lucti"ii. S.-cti.in V, it was pointed out that in order to get 

 auv conception of the Evolution of parts repeated in an animal, 

 tli.'- ta.t of this II. -petition must be recognized, and it must be 

 alwavs remembered that we are seeking for the mode in which not 

 one part but a series of similar parts has been produced. The 

 simple^ case to \\hi.-h this principle applies is that of organs 

 paiivd about th.- middli- line, and in the steps by which such parts 

 have taken .>n a gi\en f.-nn it is clear that similar variations must 

 have ..ccurred .,11 the- two sides. In the absence of evidence it 

 mi^ht I..- suppos.-d fit her that such variations had occurred little 

 l.\ little on thf two sides independently, or on the other hand, that 

 Variation had come in symmetrically and simultaneously on the 

 t\\o sides. Upon the answer given to this question the success of 

 all attempts t.. form a just estimate of the magnitude of the 

 int.-gral steps of Variation depends. In many examples already 

 gi\en it has now been -h.-wn that though in the case of paired organs 

 Variation ma\ be a-ymmet rical, yet it is not rarely symmetrical, 

 ami m part t In- quest ion has thus been answered. 



In the evidence that remains many more cases of such sym- 

 metrical variations will be described, and it may be taken as 

 e-tablished that \\lien the organs stand in bilateral symmetry, that 

 i- t<- say, as images <,n eitlier side of a middle line, their Varia- 

 tion ///'/// be similar and symmetrical. 



The teeth present this problem .if the Variation of parts stand- 

 ing as images, in an unusual and peculiar way. For in the case of 

 teeth we ha\e to consider not only the steps by which the right 

 and left sides i,t' each jaw have maintained their similarity and 

 symmetry, but in addition the further question as to the relation 

 of the teeth in the upper jaw to tho-e in the lower jaw. There 

 are many animals in which there is very great difference between 

 the upper and lower rows of teeth, and it must of course be 

 remembered that perhaps in no animal are the teeth in the upper 

 jaw an exact cnp\ ..f those in the lower, but nevertheless there is 

 often a substantial similarity between them, and in such cases we 

 have to consider the In, ml er kinship bet \\een the upper and 

 lo\\er teeth whereby they have become similar or remained so. 

 F"i- 11 may b.- state.! at .nice that there is s..me evidence that the 

 teeth iii the upper and lower jaws may vary similarly and shnul- 

 taneoiisl\. though such cases are decidedly rare, especially in 

 nuiiH-rieal \aiiati.m. and are much less c.iiiimon than symmetrical 

 Variation . .n the two sides of the same jaw. 



