28 



KNOWLEDGE 



[Febru.\ey 1, 1894. 



of Monte Hermoso. Now, in the nesodons, the structure 

 of the molar teeth clearl}' approximates to that character- 

 izing the odd-toeci ungulates, although belonging to what 

 naturalists term a more specialized type. It is further note- 

 worthy that in these nesodons, although the molar teeth 

 grow for a considerable portion of life, yet they eventually 

 form roots in the ordinary manner ; the same being true 

 of the incisors, with the exception of a single pair, which 

 grow permanently. We see, therefore, that the per- 

 manently growing teeth of the toxodon are a specialized 

 feature, which do not justify the reference of the creature 

 to a separate subordinal group, while the older genus 

 shows that these animals are clearly allied to the odd-toed 

 ungulates, although sharply distinguished by the structure 

 of their feet. Their feet being of a less specialized type 

 than those of the latter (as is especially shown by the 

 almost flat huckle-bone), while their teeth are more 

 specialized, it is further evident that neither group can be 

 ancestral to the other. Hence, the toxodon and its allies 

 must be regarded as forming a separate subordinal group 

 of equal value with the other sub-divisions of the great 

 ungulate order. Where theseremarkablecreaturesbranched 

 off from the primitive ancestral types of the latter, and 

 how they first obtained an entrance into South America, 

 where they gradually increased in size and specialization 

 till the period of the Pampean, when they finally dis- 

 appeared, is still an unsolved problem. 



The interest of the toxodons does not, however, by any 

 means end here. We have seen that the toxodon itself 

 shows certain resemblances to rodents in the structure of 

 its teeth ; while it will be perfectly evident that such 

 resemblances indicate no genetic affinity between the two 

 groups, seeing it is certain that rodents are neither the 

 ancestors nor the descendants of the toxodons. In a 

 much smaller animal known as the typotherium, of which 

 the skull is shown in the accompanying figure (2), these 



Fig. 2. — Skull of tlie Typothciv. Half nntiu'al size. 



rodent resemblances are, however, still more pronounced, 

 as is especially shown by the incisor teeth, which are 

 essentially those of a rodent. Moreover, in the hind feet 

 the toes had lost the hoofs characterizing the more typical 

 ungulates, and were probably protected by small nails. A 

 still further step is exhibited by a much smaller Argentine 

 mammal, of the approximate size of a hare, and rejoicing 

 in the euphonious name of Paihi/nicus. If it were not 

 for the intermediate links, this creature would almost 



certainly be put down as a rodent, with which group it 

 agrees in the structure of its teeth and toes, as well as in 

 many other parts of the skeleton. Nevertheless, it is 

 clearly a near ally of the typotherium, and therefore a 

 member of the toxodon group. Here, then, we have one of 

 the most remarkable known instances of the phenomenon 

 of parallelism — a feature to which especial attention has 

 been directed in previous articles. We have, in fact, 

 displayed before us the origin of what we may call a 

 rodent-ungulate ; that is to say, an animal which, while 

 certainly an ungulate by descent, has acquired such a 

 marked resemblance to a rodent that, if we had not the 

 intermediate links, it would certainly be regarded as a 

 member of the same order. This wonderful instance gives 

 us some insight into the intricacies of evolution , and serves 

 to show the amount of value to be attached to the majority 

 of phylogenies of the animal kingdom. 



In addition to the slightly grooved huckle-bone, the 

 toxodon group is characterized by at least one of the upper 

 incisor teeth growing throughout life, and by the molar 

 teeth being either rootless or not forming roots till a very 

 late period. There is, however, a second group of allied 

 extinct ungulates peculiar to the Argentine, in which all 

 the molars are rooted at the usual period, while the huckle- 

 bone is as flat as in the elephants, although of somewhat 

 different form. This group is represented solely by two 

 genera, both of which are confined to the Patagonian 

 deposits, where they are represented by animals rivalling 

 rhinoceroses in size, and furnished with molar teeth some- 

 what resembling those of the latter. One of these creatures, 

 on which the somewhat cumbersome name of Homahidon- 

 tiitlu'rium has been conferred, presents the rare peculiarity 

 of having the teeth arranged in a regular even series without 

 gap or interval, and with their crowns of equal height. 

 Very different in dental characters were the members of the 

 allied genius,, i-v/^rt^/'it/fcciM/c, in which each jaw was furnished 

 with a huge pair of tusks, those of the lower jaw curving 

 outwards and upwards after the manner of those of a wild 

 boar, while both were kept sharp and keen by their points 

 wearing against one another. In the presence of these 

 euormoits upper tusks, the astrapotheres resembled the 

 extinct uintatheres of North America, alluded to in our 

 article on " Tusks " ; but they differed in the possession of 

 tusks in the lower jaw, while it is probable that those 

 of the upper jaw were incisors instead of canines. The 

 most curious feature connected with these animals is, 

 however, the close resemblance of their upper molar 

 teeth to those of rhinoceroses ; the similarity being 

 so marked that if we were acquainted with the South 

 American animal only by these teeth, it would probably be 

 classed with the rhinoceroses. From the structure of the 

 bones of the ankle it is, however, quite certain that these 

 two groups of ungulates have no direct connection with 

 one another, and that their common ancestor must have 

 had molars of a much simpler type of structure. It 

 follows, therefore, that the form of molar teeth charac- 

 teiizing both the astrapotheres and the rhinoceroses must 

 have been evolved independently in the two groups ; and 

 that we have consequently here another very interesting 

 case of parallelism. Although this type of tooth (which it 

 must be remembered is one of considerable complexity) 

 is admirably adapted for crushing vegetable substances, it 

 is by no means the only one which could have been evolved 

 from what we may probably regard as the primitive type ; 

 and it is, therefore, hard indeed to see how it can have 

 been produced by evolution unaccompanied by design. 



Strange and unique as are the foregoing creatures, they 

 are exceeded in the former respect by the long-necked and 

 long-limbed animal rejoicing in the name of Mucniuclirni'i, 



