January 4, 1894J 



NA TURE 



237 



ever the same in all the mammals, for we know it is very dif- 

 ferent now. After the tee'h were thus divided, some functions 

 became more important than others, and established a mo- 

 nopoly, causing first a marked difference in the relative develop- 

 ment of the series, which we may express in a dental curve, 

 resulting finally in a loss of certain teeth. In the meantime 

 began the special evolution of the form of the back teeth, or 

 molars. Was this ahke in all mammals; was it tritubercular ? 

 It is surprising how many problems of early relationship are at 

 stake in the discussion of these simple processes. 



Primitive Diphyodontisni. 



What does j«^(V5i/c« really consist in? It now appears that 

 Baume was right in denying that the first tooth is the mother 

 of the second ; for the teeth of the lower as well as the upper 

 series spring from the common epithelial dental fold (Schmelz 

 leiste) which dips down from the surface and extends the 

 whole length of the jaw ; at intervals it buds off the dental 

 caps (Schmelzkeim) of the first series ; after these are separated 

 off, the dental fold sinks and buds off the dental caps of 

 the second series, always below and inside the first ; thu^ the 

 fold is the mother and the caps are sisters, twins, or triplets, 

 according to the number of the series. In all young mam nals, 

 including the traditional mono|ihyodont Cetaceans and EHt-n- 

 tates, and excepting only the still unexplored Monotreme 

 embryos, traces of two series of teeth have been found. Both 

 Leche and Rose have detected evidence that the denial fold 

 sometimes buds off parts of a third series, thus explaining the 

 occasional reversion of supernumerary teeth on the inner side 

 of the second series, and Leche has st-en traces of budding 

 preceding the first series — thus giving us vestiges of four suc- 

 cessions ! 



Al! our perplexities as to the relations of the milk and 

 permanent teeth, and the ingenious but mistaken hypotheses of 

 Baume, Flower, Wortman, and Cope have sprung from our want 

 of evidence of the regular and complete diphyodontism of the 

 stem mammals. The solution is in brief that the "milk teeth " 

 and the " true molars " are descended from the fir-t seric-, while 

 the second series is represented by the " permanent incisors, 

 canines, and pre molars" and rudiments of dental caps beneath 

 the true molars. The mammals early began to diverge from 

 this primitive diphyodontism in many ways ; apparently adapt- 

 ing the first and second series, respectively, to their infant and 

 mature feeding habits ; losing parts or all of one series or the 

 other, and in some cases pushing teeth of the second series in 

 among the first ; this intercalation has been a most confusing 

 factor to us. 



In the Marsupials (Kiikenthal) almost the entire first series 

 Ijccame permanent ; thus from the Jurassic period to the 

 present time only a solitary fourth piemolar of the second 

 series has pushed out its elder-sister tooth, and Rose has 

 observed that an outer upper-incisor also pushes up from the 

 second series ; the remainder of the second series still persist 

 as rudimental dental caps beneath the first, even beneath the 

 prst and second molars ! There are wide variations among the 

 |Placentals ; thus in the lowest existing forms, the Insect ivora, 

 ILeche finds that in the Shrew {Sorex) the second series is sup- 

 pressed entirely, while in the Hedgehog (Eriiiaceits) of the 

 twelve permanent teeth in the anterior part of the jaws five 

 pelong to the first series and seven to the second. We thus 

 neet with the paradox, that among the "primitive" Marsupials 

 ind Insectivores the regular reptilian succession was early inter- 

 upted, while in all the "higher" mammals the reptilian sue- 

 ession of two series was retained in the anterior part of the 

 aw. Beneath the posterior highly-specialised molar teeth of 

 )Oth Marsupials and Placentals, the second teeth were early 

 uppressed, although in the Edentates, which also originally 

 lad specialised molars, there is a typical succession of seven 

 eeth behind the canine. These discoveries prove that the 

 irhale teeth, like their paddles, have acquired a secondary 

 daptive resemblance to those of the Icthyosaurs. How did 

 he single and simple teeth of the Edentates and Cetaceans 

 Sevelop ? Clearly by retrogression. As Leche points out in 

 ^e aquatic Carnivora, in which th first series are degenerating, 

 tie single-series condition (monnphyoduntism) advances step 

 y step with retrogressive simplification of the tooth form 

 liomodontism) ; thus in the true seals, the eared seals and the 

 'alruses, as the permanent teeth become simpler, the milk 

 ;eth become smaller. The Edentates, so widely separaied 

 enetically, parallel the seals in tending to suppress the first 



series of teeth and simplify the crowns of the second series at 

 the same time. We might jump to the conclusion that this 

 gives us an explanation of the homodont and apparently mono- 

 phyodont condition of the toothed whales, especially as it has 

 been suppo ed they sprang from aquatic carnivora, but in this 

 Order matters were reversed, for the first series persisted and 

 the second series were suppressed and persist as a rudimental 

 row of tooth caps buried in the jaw. 



Each dental series has an adaptive evolution of its own, in 

 Erinaceus the first series has an ancient and the second a 

 modern form ; in Ericulus both series are alike ; in the Bats 

 the first series is homodont, the second is heteroHont (Leche) ; 

 in the Edentates the first series is ancient and heterodont, the 

 second is modern and homodont (Thomas, Rheinhardt), so 

 among the Cetacea and Ungulata. 



What deep and ancient clefts the different laws of succession 

 mark between the Marsupials and these three Placental groups ! 



Primitive Heterodontism and Formula. 



Now that all mammals are led back to a distant diphyodont 

 stem, it is also true that the further we go back both in palin- 

 genesis and embr>ogenesis, the more widespread heterodontism 

 is — all modern homodontism proving to be secondary. The 

 simple conic teeth of the porpoise, for example, bear a mislead- 

 ing resemblance to those of a reptile. Flower, Weber, Julin, 

 and Kiikenthal agree that the ancestral whales and edentates 

 were heterodont and had a smaller number of teeth than the 

 existing forms. 



Heterodontism is the second problem. Did the differenti- 

 ation of the teeth into incisors, premolars, and molars 

 occur before or after the Monotremes, Marsupials, and Pla- 

 centals separated ^ It i> well settled that the canine was the 

 first maxillary tooth, and developed from the most anterior 

 bi-fanged premolar ; also, from the discovery of complete 

 succession, we must now define the first molar as the most 

 anterior specialised or triconid tooth, not as the most anterior 

 permanent tooth. It seems to me we now find strong evidence 

 that the stem mammals had a uniform number of each kind 

 of teeth ; in other words, a uniform dental formula. The 

 Monotremes are most doubtful as the existing forms point only 

 to primitive heterodontism. It will be a great step forward 

 when we learn whether or not the Multituberculates are Mono- 

 tremes — the resemblance of their molars to those of the duck- 

 bill is very superficial, for the duckbill upper molars lack the 

 intermediate row of tubercles universally seen in the Multitu- 

 berculates, and look to me rather like degenerate Trituberculate 

 teeth. Cope has recently found in the cretaceous rocks a re- 

 markable Trituberculate, which he names ThLcodoti ; the jaw 

 of this animal is neither Placental nor Marsupial ; it is like that 

 of the Multituberculates — and both resemble remotely the de- 

 generate modern Monotreme jaw. All we can say, therefore, 

 is that the Multituberculates are an archaic group, highly 

 specialised even in the Trias, that they were probably Mono- 

 tremes, and neither structurally nor functionally akin to the 

 Diprotodont Marsupials (Owen), nor to the Microbiotheridas 

 (Ameghino). With a dental mechanism and a condyle exactly 

 like that of the rodents, they show no trace of canines, and the 

 mode of evolution of their peculiar molars was probably 

 paralleled later in the rodents. They present vestiges of a 

 primitive dental formula like this : 73. C?. P^. JI/4 + . Thlceodon 

 shows Ci. P^. M^. .Thus, so far as this doubtful palaeonto- 

 logical evidence goes, the Monotremes had a typical formula. 



Our next step is to unify the typical 5. i. 3. 4 of recent Mar- 

 supials with the 3. I. 4. 3 of higher Placentals. Thomas has 

 shown in his studies of recent Marsupials that they have pro- 

 bably lost one of the four typical premolars (pm. 2) ; this obser- 

 vation, fortunately, is partly confirmed by Rose's finding an em- 

 bryonic germ of this tooth. Ignoring the incisors of the Jurassic 

 Marsupi lis, Thomas raised the number of ancestral incisors to 

 five, the highest number known among recent Marsupials ; 

 Rose therefore made another step towards uniformity when he 

 showed that the Marsupial 2.5 is probably a member of the 

 second series of incisors, and should not be reckoned with the 

 first. Nov\, if we suppose that the Placentals have lost one 

 incisor, and one molar, abundant evidence of which is found in 

 Otocyon, Cenietes and Homo, we derive as the ancestral 

 formula of both orders : I^. C &^ P^- ^M- 



The aberrant placental Cetacea point in the same direction, as 

 we read in the conclusion of Weber's fine memoir : " All the 



NO. 1262, VOL. 49] 



