HOMOLOGIES OF TEETH. 381 



the ascent of p 3 into its place (ib. E). Afterwards m 3 is ac- 

 quired ; and in the Macropus giyas, p 3, simultaneously pushed 

 out (ib. F). 



Thus, four individuals of this species may be found to have 

 the same number of molars, i. e. -J:-J ; two of these individuals 

 may seem, on a cursory comparison, to have them of the same 

 shape, e. g., as in c and E, or as in D and F, fig. 296. In fact, to 

 determine the identity or difference in such instances, it requires 

 that the substance of the jaws be examined, to see if the germs 

 of successional teeth are present, as at p 3, C and D, or at m 3, E. 

 The result of such examination may be to show that not one of 

 the four Kangaroos with the m -f:-J had the same or homologous 

 teeth. 



The four grinders, e. g. may be d 2, d 3, d A, m i ; as in c ; or 

 d 3, d 4, m i, m 2 ; as in D ; or p 3, d 4, m i, m 2 ; as in E ; or d 4, 

 m i, m 2, and m 3 ; as in F. 



The changes, however, do not end here. As age advances, 

 d 4 is shed, and the molar series is reduced numerically to the 

 condition of B ; but, instead of d 2, d 3, and d 4, it consists of 

 m i, m 2, m 3. 



Finally, m i is shed, and the dentition is reduced to the same 

 numerical state as at A ; the teeth, however, being m 2 and m 3. 



The symbols used, it is hoped, are so plain and simple as to 

 have formed no obstacle to the full and easy comprehension of 

 the facts explained by means of them. If these facts, in the 

 manifold diversities of Mammalian dentition, were to be de- 

 scribed in the ordinary way, by verbal definitions, e. g., ' the 

 second deciduous molar representing the third in the typical 

 dentition,' instead of d 3, and so on, the description of dental 

 development would continue to occupy much unnecessary space, 

 and would levy such a tax upon the attention and memory as 

 must tend to enfeeble the judgment and impair the power of 

 seizing and appreciating the results of the comparison. 



Each year's experience has strengthened the writer's convic- 

 tion that the rapid and successful progress of the knowledge of 

 animal structures, and of the generalisations deducible therefrom, 

 will be mainly influenced by the determination of the homology 

 of parts and organs, and by the concomitant power of condensing 

 the propositions relating to them, and of attaching to them signs 

 or symbols equivalent to their single substantive names. In the 

 writer's Works, CXL, CXLI, CXLIY, he has denoted most of the 

 bones by simple numerals. The symbols of the teeth are fewer 



