234 DR R, BROOM ON THE 



of the space enclosed by the cartilage, it receives the ducts of glands throughout 

 its whole leutrtli. 



O 



While it will thus be noticed that there may be a very considerable difference in the 

 degree of development of the organ in even closely allied forms, I have been led to a 

 much more importaut conclusion, viz., that though the degree of development may vary 

 greatly, there is throughout large groups a very marked constancy of the type, followed 

 by the organ in its general relations and connections. For example, in all the Marsupials 

 that have been examined, with one exception, which will be referred to later, the organ 

 opens into the upper end of the naso-palatine canal, near the point where the canal 

 opens out into the nasal cavity, while there is never more than a rudimentary 

 cartilaginous support given off to support the naso-palatine canal either from Jacobson's 

 cartilage or from the nasal floor cartilage. While this holds good for practically all 

 Marsupials, of no Eutherian that has yet been examined can the same be asserted. The 

 two known Monotremes have in their organs a number of features in common, but which 

 differ from those of any other animal. And the same can be said of the different 

 Rodents which have been examined. Again, in all the higher Eutheria in which 

 Jacobson's organ is well developed, there is a very complex development of the nasal 

 floor cartilage, giving rise to a cartilaginous support for the naso-palatine canal as well 

 as to a posterior nasal floor cartilage, while Jacobson's cartilage is produced downwards 

 and forwards to form a support to Jacobson's duct ; and so unvarying is this type, that 

 even in forms in which the organ of Jacobson is completely aborted, as in Pteropus, the 

 cartilages still retain the same general arrangement. From the small tendency to 

 variation in the organ and its cartilages, we have in them a factor of considerable value 

 in the classification of the Eutherian orders, probably of more value than either dentition 

 or placentation. 



In the following pages the relations of the organ in the principal orders will be 

 considered, and the affinities and significance of the various arrangements dwelt upon. 



MONOTREMATA. 



The differences of the organ in the two Monotremes are such that it will be more 

 convenient to consider the two separately. 



OrnitJtorhynchus. — The presence of the organ in the Platypus was recognised by Sir 

 W. Turner in 1885, but was first described in detail by Symington in 1891. Since 

 then further details have been supplied by Wilson, Wilson and Martin, W. N. Parker, 

 Elliot Smith, and myself. 



It is unfortunate that in Ornithorhynchus — the most primitive mammal at present 

 existing — there is a most remarkable degree of specialisation of the structures in the 

 anterior part of the snout, and that this specialisation is in a direction entirely dissimilar 

 to that found in any other known vertebrate. This peculiar development in the snout 

 to some extent affects the organ of Jacobson and its cartilages, and renders it difficult to 



