196 MERISTIC VARIATION. [part I. 



also ; and thus, as in the case of vertebra?, for instance, in any 

 given example of a numerical change qualitative changes must be 

 looked for too."] 



As a preliminary to the consideration of evidence relating to 

 the Variation of teeth it may be useful to call attention to certain 

 peculiarities of teeth considered as a Meristic Series. In the 

 Introduction, Section V, it was pointed out that in order to get 

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

 the fact of this Repetition must be recognized, and it must be 

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

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

 simplest case to which this principle applies is that of organs 

 paired about the middle line, and in the steps by which such parts 

 have taken on a given form it is clear that similar variations must 

 have occurred on the two sides. In the absence of evidence it 

 might be supposed either that such variations had occurred little 

 by little on the two sides independently, or on the other hand, that 

 Variation had come in symmetrically and simultaneously on the 

 two sides. Upon the answer given to this question the success of 

 all attempts to form a just estimate of the magnitude of the 

 integral steps of Variation depends. In many examples already 

 given it has now been shewn that though in the case of paired organs 

 Variation may be asymmetrical, yet it is not rarely symmetrical, 

 and in part the question 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 

 established that when the organs stand in bilateral symmetry, that 

 is to say, as images on either side of a middle line, their Varia- 

 tion may be similar and symmetrical. 



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

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

 teeth we have to consider not only the steps by which the right 

 and left sides of 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 those 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 copy of those in the lower, but nevertheless there is 

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

 have to consider the bond or kinship between the upper and 

 lower teeth whereby they have become similar or remained so. 

 For it may be stated at once that there is some evidence that the 

 teeth in the upper and lower jaws may vary similarly and simul- : 

 taneously, though such cases are decidedly rare, especially in 

 numerical Variation, and are much less common than symmetrical 

 Variation on the two sides of the same jaw. 



