272 MERISTIC VARIATION. [part i. 



Variation of unusual amplitude may be seen also in the molars 

 of Bettongia Nos. 389, &c, for while on the one hand the last or 

 fourth molar may be absent, it may on the contrary be large and 

 may even be succeeded by a fifth molar as an extra tooth. All 

 these conditions were seen in looking over quite a small number 

 of specimens. 



(7) Homceotic Variation in terminal Teeth token a new member 

 is added behind them. 



Upon the remarks made in the last Section the fact here 

 noticed naturally follows. We have seen that there is a fairly 

 constant relation between the size of extra teeth and that of the 

 teeth next to which they stand, so that the new teeth are as it 

 were, from the first, of a size and development suitable to their 

 position. We have now to notice also that the teeth next to which 

 they stand may also undergo a variation in correlation with the 

 presence of a new tooth behind them. 



It may be stated generally that if the tooth which is the last 

 of a normal series is relatively a small tooth, as for example m» or 

 ra^ i n the Dog, then in cases of an addition to the series, by which 

 this terminal tooth becomes the penultimate, it will often (though 

 not always) be found that this penultimate tooth is larger and 

 better developed than the corresponding ultimate tooth of a normal 

 animal of the same size. 



Of this phenomenon two striking examples (q.v.) have been 

 given, Canis azarce No. 249 and Dasyurus maculatus No. 385. 

 Besides these are several others of a less extreme kind e.g. Otocyon 

 megalotis No. 256, Mastiff No. 259, Dog No. 260. The same was 

 also seen in the molars of Bettongia, 



This phenomenon, of the enlargement of the terminal member 

 of a series when it becomes the penultimate, is not by any means 

 confined to teeth ; for the same is true in the case of ribs, digits, 

 &c, and it is perhaps a regular property of the Variation of Meristic 

 Series so graduated that the terminal member is comparatively 

 small. This fact will be found of great importance in any attempt 

 to realize the physical process of the formation of Meristic Series, 

 and it may be remarked that such a fact brings out the truth that 

 the members of the Series are bound together into one common 

 whole, that the addition of a member to the series may be cor- 

 related with a change in the other members so that the general 

 configuration of the whole series may be preserved. In this case 

 the new member of the series seems, as it were, to have been 

 reckoned for in the original constitution of the series. 



(8) Reconstitution of parts of the Series. 



Lastly there are a few cases, rare no doubt in higher forms but 

 not very uncommon for example in the Sharks and Rays (see 



