8o AVERS. [Vol. VI. 



after the forces of the organism have been expended in building 

 it up to a state of physiological perfection, while its place is 

 taken by an organ very similar to it in the earlier stages of its 

 development, but which, it must be admitted, develops along a 

 different line from the Sauropsid organ. Of the greatest impor- 

 tance in tracing the descent of the mammalian ear from the 

 Sauropsid condition is the recognition of the fact that at least 

 two species of lower mammals retain the papilla acustica lagenae 

 in a condition of development entirely identical with that found 

 in Birds and Hydrosaurians. The existence of the crista acu- 

 stica abortiva has not been determined in the Monotremes. 



An explanation of the origin and significance of the pillars of 

 Corti at once in harmony with their origin and with their func- 

 tion, as I conceive it, and as it seems clearly indicated by the 

 new light we have received as to the nature of the whole 

 organ, is that, oiving to the mcreasing length of the hairs of 

 the hair cells, there arose need of greater isolation and firmer 

 support. 



The inner hair cells, being the oldest row, are first ajfected and 

 first stipplied. 



There are indications of a progression of this process out- 

 wards, between the rows of the so-called outer hair cells, and 

 there is evidence (see PI. X, Fig. 4) that this process has already 

 begun in the organ of Corti of the Alligator : it is here seen 

 between the innermost and the second rows of hair cells. 



The pillars of Corti are then isolating and specially and pecu- 

 liarly constituted supporting structures. As the ear differen- 

 tiates we may expect similar pillars to form between the second 

 and third, and third and fourth rows, as well as between the 

 latter and the fifth or outermost row of hair cells. 



There is evidence that the number of rows of hair cells in the 

 mammalian cochlea is on the increase. All the facts point to 

 the conclusion that the number of rows is increasing at the 

 same time that they are becoming more highly modified, i.e. 

 specialized and capable of executing the functions of the organ 

 in ever-increasing degree of perfection. 



The Alligator's organ of Corti shows a larger number of rows 

 of hair cells than the mammalian organ, and all mammal forms 

 which have been studied so far show unmistakable indications 

 of the increasinof differentiation of the orsran of Corti. In the 



