GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



221 



The duck-mole is entirely confined to Aus- 

 tralia and Tasmania; the porcupine ant-eaters, 

 on the other hand, as recent discoveries have 

 shown, extend to New Guinea. 



In contrast to this the origin of these crea- 

 tures is very obscure. The palaeontological 

 data bearing on the question are limited to a 

 few remains of an echidna which have been 

 found in the Quaternary strata of Australia. 

 This poverty of fossil remains combined with 

 the low organization of the creatures has 

 called forth the most singular speculations, to 

 which we must devote a few words. 



First of all we cannot fail to be struck by 

 the insufficiency of the dental system. The 

 echidna, like the manis, has no teeth at all, 

 and it is noteworthy that it approaches the 

 latter animal also in the structure of the jaws, 

 tongue, and nails, besides resembling it in the 

 nature of its food. The duck-mole possesses 

 a few horny plates which in respect of their 

 structure may perhaps be compared with the 

 teeth of certain fishes. This reduction of the 

 dentition must certainly be pronounced an 

 acquired character due to a long process of 

 modification, for all old mammals possess a 

 very complete dentition and also in general 

 more teeth than their descendants. 



Now the skeleton exhibits the closest af- 

 finity not only to that of reptiles and birds 

 but also to that of amphibians, in addition to 

 which it has certain marsupial characters. 



We cannot here enter into the details of 

 these relationships, the exposition of which 

 would lead us beyond the scope of this work. 

 It is enough to mention the fact that all 

 investigators are agreed upon these points, 

 and recognize the characters which these 

 creatures possess in common with the lower 

 vertebrates on the one hand, and the marsu- 

 pials on the other hand. 



What conclusion shall we draw then from 

 this singular commingling of characters? 

 May we from the facts just mentioned deduce 

 the inference which has recently been put for- 

 ward and forced into publicity by some natu- 



ralists of mark, that the monotremes are an 

 older form than the marsupials, and that the 

 latter are direct descendants of the former? 



There are two facts which stand in opposi- 

 tion to this view: first, the rudimentary den- 

 tition; and second, the present geographical 

 distribution of the monotremes. 



With respect to the first point the difficulty 

 has sometimes been got over by a wholly 

 groundless assertion. According to those 

 who get over the difficulty in this way "the 

 now extinct monotremes, which comprised all 

 the forms of the then existing mammalian 

 fauna, had certainly a more highly developed 

 dentition, which had been transmitted to them 

 by the fishes," and Microlestes and Droma- 

 therium of the Jurassic Period "are probably 

 descended from these primitive mammals." 

 This hypothesis is in fact altogether without 

 foundation. Wherever we have discovered 

 the inner side of the lower jaw of those pri- 

 mitive mammals, there we have been able to 

 observe the characteristic inflexed angle of the 

 marsupials, nothing of which is seen in the 

 monotremes. Yet that is of no consequence! 

 The descendants may have acquired or lost 

 this character, may also have gained or lost 

 any number of teeth. 



Further, the present geographical distribu- 

 tion does not give the slightest indication of 

 any great antiquity belonging to this type. 

 Wherever we meet with old groups, which 

 have reproduced themselves with ceaseless 

 modifications through the geological epochs, 

 while retaining their general characters, like 

 the Insectivora, Ungulata, and so forth, we 

 find them spread over wide areas, taking 

 possession of almost entire continents; and 

 when we have to do with old types, which 

 are near their point of extinction, we find the 

 few survivors in the most widely different 

 geographical regions. The tapirs afford a 

 striking example of geographical discontinuity 

 of this kind in types which are undoubtedly 

 very old, but which are gradually disappear- 

 ing. We must, therefore, look with con- 



