8 4 MOSTLY MAMMALS 



smaller Argentine mammal, of the approximate size of a 



hare, named Pachyrucus. 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 instances 



of the phenomenon of parallelism in development. 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 might be regarded as a member 



of the same order. This instance gives us some insight 



into the intricacies of evolution, and serves to show the 



amount of value attaching to many phylogenies of the 



animal kingdom. 



In addition to the slightly grooved huckle-bone, the 

 toxodon group is characterised by at least one of the upper 

 incisor-teeth growing throughout life, and by the cheek- 

 teeth being either rootless or not forming roots till very 

 late. 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 somewhat resembling 

 those of the latter. One of these creatures, on which the 

 name of Homalodontotheriam has been conferred, presents 

 the rare peculiarity of having the teeth arranged in a 



