244 DR R. BROOM ON THE 



vascular plexus and its numerous glands, all point to an affinity with the lower mammals 

 — Monotremes and Marsupials, and the structure of Jacobson's cartilage with its rudi- 

 mentary outer bar gives strong support to this affinity. The mode of opening of the 

 organ is similar to that in Dasypus, and is but an extreme degree of the condition in 

 iEpyprymnus. The well-developed naso-palatine canal, and the process of the outer nasal 

 floor cartilage passing forward with it, reveals a character which seems to remove the 

 rodent from its lower relatives, and suggests an affinity with the higher Eutheria. 

 Taking the various points into consideration, one of two conclusions seems to be 

 possible — (1) Either the rodents are an aberrant group sprung off from the main 

 Eutherian stem somewhat earlier than the development of the common ancestors of the 

 higher Eutheria; or (2) they are a modified and specialised branch of the higher 

 Eutheria. From the primitive characters of the organ and its cartilage found in the 

 rodents and in none of the higher Eutheria, and from the fact that in no known higher 

 Eutherian has a condition similar to that of the rodent arisen by secondary development, 

 the first of the two conclusions, viz., that the rodents are a specialised offshoot from the 

 early ancestors of the higher Eutherians, seems much the more probable. 



Higher Eutheria. 



In a few typical members of the higher Eutheria the organ has been carefully 

 studied, but though much has been done in the way of describing the details of the 

 anatomy practically no attempt has been made to indicate the significance of the various 

 details. Not taking into consideration the Anthropoidea, in which the organ is gene- 

 rally rudimentary or absent, I have studied the organ in the following orders : — 

 Chiroptera, Insectivora, Carnivora, and Ungulata (Artiodactyla and Perissodactyla). 

 Notwithstanding the great differences in the general structure of the members of these 

 different orders, the organs of Jacobson are formed on a common plan, and the differences 

 are very slight. In the bat, when the organ is developed, we have the same type as in 

 the pig, while the organ in the ox scarcely differs in one detail from that in the cat. 



The affinities are such as to lead irresistibly to the conclusion that, in spite of the 

 great outward differences in structure and the differences in habits and dentition, we 

 have the various groups connected by ties of a common ancestry. And, furthermore, 

 not only do the common ties indicate a close relationship, but they distinguish at once 

 the higher Eutheria from the lower mammals. The simplest form of the higher 

 Eutherian type that I have met with is that found in the Chiroptera ; it will, therefore, 

 be convenient to consider first the structure in its simple form as seen in Miniopterus. 



Chiroptera. — Until recently it was believed that there was no organ of Jacobson in 

 the Chiroptera, but in 1895 Messrs Duval and Garnault discovered a moderately 

 developed organ in Vesperugo pipistrellus, and in the same year I found a very well- 

 developed organ in Miniopterus schreibersii. Though the organ is thus seen to he 



