PALAEONTOLOGY AS A DISCIPLINE. 57 



spout-shaped odontoid process of the axis has independently 

 developed in the horses, the tapirs, and in three artiodactyl 

 series. The true ruminants (Pecora) of the present day are, 

 among other characteristics, distinguished from the remaining 

 artiodactyls by the hollow tympanic bullae, which in the pigs, 

 tragulines, and camels are filled with cancelli, or spongy bone. 

 In Oligocene times only the camels had acquired the cancelli ; 

 the other groups, though already differentiated as such, still 

 had hollow and inflated tympanics. Lists of such parallelisms 

 in single characters might be multiplied almost indefinitely, 

 but they also occur in whole groups of structures. The 

 camels have in teeth, skull, vertebrae, and limbs many points 

 of resemblance to the true ruminants, which demonstrably are 

 not due to inheritance from a common ancestor. The two 

 great series of ungulates, the artiodactyls and perissodactyls, 

 which are usually grouped together as the Ungulata par excel- 

 lence, are examples of parallel development on a grand scale, 

 their many resemblances being, for the most part, independ- 

 ently acquired. The flesh-eaters known as Carnivora include 

 at least two, and probably three lines, which have been sep- 

 arately given off from the primitive flesh-eaters, or creodonts. 



Such a mode of development greatly increases the difficulty 

 of determining phylogenies, which would be very much easier 

 could every notable resemblance at once be accepted as proof 

 of relationship. It often renders impossible the proper classi- 

 fication of some isolated genus which seems to have several 

 incompatible affinities. It emphasizes the necessity of found- 

 ing schemes of classification upon the totality of structure, and 

 of determining the value of characteristics, whether they are 

 primitive or acquired, divergent, parallel, or convergent, before 

 attempting to assign them their proper taxonomic value. 



We may find a practical identity in teeth, skull, or feet as 

 the outcome of these processes, but as yet no case is known 

 where all these structures have become alike through the opera- 

 tion of either parallel or convergent development. Among the 

 invertebrates the case is different. Hyatt has shown that the 

 degenerate, straight-shelled, ammonoid genus Baculites is a 

 polyphyletic group, and derived from several distinct stocks, 



