CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



825 



1. No addition oj hiocharacters in Palaeosyops. — In 

 Palaeosyops no distinctively new elements or organs 

 are added to the skull through progressive brachy- 

 cephaly and no characters are lost, but allometrons 

 are established by the abbreviation of some parts 

 and the expansion of conjoining parts. For example, 

 the postglenoid and post-tympanic processes tend to 

 close in the auditory meatus. 



2. Correlation oj hiocharacters. — Age and sex both 

 exert a positive divergent influence on all the 

 quantitative characters of the skull, such as 

 massiveness and robustness. Old male skulls 

 are more progressive in form, more prophetic 

 of higher successive stages than young male 

 skulls, and some female skulls are more con- 

 servative in form than male skulls. In brachy- 

 cephalic phyla the females preserve the more 

 mesaticephalic indices of ancestral forms 

 Similar conclusions are summarized by Dr- 

 C. Hart Merriam (1895.1), from his studies 

 on the pocket gophers (Geomyidae), as follows: 



The female generally has the brain case broader and 

 flatter, the zygomata narrower and less angular, the 

 jugal narrower anteriorl.y, the rostrum and nasals 

 shorter, and the skull as a whole smoother. In other 

 words, the cranium of the female is much less special- 

 ized than that of the male and often points sugges- 

 tively to the stock from which the species was derived. 

 It thus happens in the case of series of species in which 

 the suocsssive forms in the development of a partic- 

 ular type are still extant (as in the texensis-bursarius 

 series) that the female resembles the male of the species j 

 next below in the line of descent more than the male of 

 her own species. 



Concerning the influence of age and mus- 

 culature the same author observes (op. cit., p. 32): 



Fundamental characters are based on structures and rela- 

 tions that enter into the ground plan of the skull and are of 

 high morphologic weight; superficial characters are the result 

 of special adaptations and particular miuscular strains and are 

 of little value except as affording recognition marks for species 

 and in some instances for genera also. The fundamental struc- 

 tures are mostly hidden, comprising the floor of the brain case, 

 the craniofacial axis, and the turbinated bones. 



Thus young jaws of the species of Palaeosyops and 

 Limnohyops are more similar to each other than old 

 jaws. 



3. Remoteness of zoologic affinity (Jamily, supra- 

 generic, and generic). — Such affinity apparently exerts 

 little or no controlling influence on the brachycephalic 

 evolution of Palaeosyops, because we observe that 

 other related generic forms {Limnohyops, Telmathe- 

 rium) are evolving differently along their own lines. 



4. Closeness of zoologic affinity {generic, specific, 

 varietal). — These relationships exert a positive con- 

 trolling influence because of the kinship of (1) a 



Figure 742. — Outlines of skulls of Palaeosyops, Dolichorhinus, and 

 Eotitanops, illustrating the evolution of brachycephalic and dolicho- 

 cephalic types 

 A, Brachyceptialic .skull of P. leidyl (shaded), superposed upon B, dolichocephalic skull of 

 D. Injognaihus (heavy outline); C, hypothetic outline of the skull of E. gregoryi, drawn 

 to the same scale as A and B. This illustration shows the contrasts produced in the 

 evolution of brachycephaly, dolichocephaly, and cyptocephaly from the primitive mesa- 

 ticephaly of Eotitanops. 



common progressive brachycephalic tendency, (2) the 

 kinship of similar habit and habitat, and (3) the kin- 

 ship of a similar incidence of competition and selection 



DIFFERENTIAL ALLOMETRONS OF SPECIFIC VALUE 



Whfle the skull as a whole is increasingly brachy- 

 cephalic the differentia] rate of increase in its separate 

 parts furnishes an important share of the material 

 which paleontologists have seized upon in the sys- 

 tematic description of "species." This is shown in 

 the careful analysis of the measurements given in the 

 following table: 



