COMPARATIVE ANATOMY OF THE MAMMALIAN ORGAN OF JACOBSON. 251 



Conclusion. 



From the careful examination of the relations of the organ in the various groups of 

 mammals it will be observed that not only is there a close agreement in the details of 

 the anatomy in allied forms, but that the anatomical details of the structures related to 

 the organ are so little affected by variations in the habits of the animals that, even in 

 allied orders, evidences of affinity are here manifested when lost in most of the other 

 characters. It will also be seen that in the organ and its cartilages we have a steady 

 evolution which has apparently been only but slightly influenced by the great changes 

 in external structure. 



In the Prototheria we have an organ in a highly-developed condition, well supplied 

 with glandular tissue, and having a large vascular plexus along its outer side. In the 

 Marsupialia, though there are numerous little modifications — specialisations and degenera- 

 tions — when these are examined it is found that they all point back to the Prototherian 

 type, and leave little doubt but that in the Marsupial organ we have only a degenerate 

 and slightly specialised variety of the type found in the Monotremes. In the Edentata, 

 so far as known, the organ might be regarded as a more degenerate and slightly aberrant 

 variety of that seen in Marsupials. In Jacobson's organ and its relations we thus have 

 a feature which reveals an affinity between the Monotremes, the Marsupials, and this 

 the lowest order of the Eutheria, notwithstanding the great differences manifested in 

 their modes of development. In the Rodentia we have a well developed organ whose 

 cartilage bears some resemblance to that both in the Marsupials and in the Edentata, 

 with the additional feature which has not been observed in either of these groups — a 

 posterior nasal floor cartilage which is continued forwards as a supporting cartilage to 

 the naso-palatine canal. As this cartilage is found in the higher Eutheria, e.g., Miniop- 

 terus, we see a certain affinity with this higher group. On the whole, however, the 

 agreement is more with the lower than with the higher forms. 



The examination of the organ in the higher Eutheria also reveals some striking 

 relationships. As a rule, the organ itself is more or less rudimentary, the plexus absent, 

 and the glandular tissue much reduced. In the cartilages, however, it has been seen 

 that there is almost invariably a peculiar and characteristic development by which any 

 higher Eutherian in which the organ is developed, and the majority of those even in 

 which it is absent, can be at once distinguished from any of the lower mammals. In the 

 complex development of the nasal floor cartilage we have, apparently, a thoroughly 

 reliable character by which the higher Eutheria can be divided off from the lower into a 

 distinct group by themselves. For this group I would propose the name Ccenorhinata, 

 while for those Eutheria which have the primitive arrangement of the cartilages of the 

 nasal floor the distinguishing name Archceorhinata might be given. In the former- 

 group would be included the following orders : — Primates, Carnivora, Insectivora, 



