814 



TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA. 



Contrasts between rectigradations and allometrons — Continued 



Features of rectigradations 



11. Rectigradations appear to indicate a germinal predisposition 



or predetermination to vary in the same direction, these 

 predispositions being most apparent in the more closely- 

 related phyla and less apparent in remotely related phyla. 



12. The rise of similar rectigradations may confidently be 



predicted in geologic descendants of similar phyla — for 

 example, similar and convergent rectigradations succes- 

 sively develop in the premolar teeth of the entire order 

 Perissodactyla. 



Features of allometrons 



11. Similar or entirely dissimilar allometrons may arise in 



closely related branches of the same stock; dolichocephaly 

 and brachycephaly, for example, may arise in closely 

 affiliated branches of Homo sapiens. 



12. Occurrence of similar allometrons can never be predicted; 



even within the family Brontotheriidae, even within the 

 genus Telmaiherium, dissimilar and divergent allometrons 

 may arise during the same geologic stage. 



Theoretic conclusions concerning predisposition to vary (Oshorn) 



Causes of rectigradations 



Causes of allometrons 



13. Rectigradations appear to be due in part to germinal potenti- 



alities as to definite or determinate variation which are 

 found in the common ancestors of several phyla. The 

 apparent potentiality to definite variation may be analo- 

 gous to the similar mutations (De Vries) arising from the 

 same stocks. (See section on causes of evolution, p. 834.) 



14. Similar rectigradations arise in connection with changes of 



environment and of habit (or ontogeny), but it is not 

 known whether there is any real causal relation between 

 changes of environment and of ontogeny and the origin 

 of definite rectigradations like dental cusps and horns. 



13. Allometrons exhibit hereditary predispositions and definite 



variations only within descendants of the same phylum — 

 for example, a certain brachy cephalic or dolichocephalic 

 tendency once displayed within a phylum seems to be 

 cumulative in successive generations. 



14. Allometrons are also directly observable as the result of 



changes of environment and of habit, but it is not known 

 whether these are merely somatic, ontogenetic, or are 

 actually germinal. 



PRINCIPLES OF EECTIGRADATION 



HORNS ARISE AS TYPICAL RECTIGRADATIONS 



ORTHOGENETIC, CONTINUOUS 



Phylogenesis of horns in the titanotheres. — Horns are 

 biocharacters having very complex germinal and 

 somatic relations. That they are separable in heredity 

 is shown by the sudden and complete disappearance 



1. Horns arise from excessively rudimentary begin- 

 nings — inconspicuous bony swellings which in their 

 initial stages can hardly be detected on the surface 

 of the skull. 



2. These horn rudiments arise independently in the 

 same region of the skull in different phyla of the 

 titanotheres at earlier or later geologic periods, 

 respectively. 



fALAeOSYOPS 



OAOH/PPUS 



Figure 732. — Rectigradations in the teeth of Eocene ungulates 



A, Orohippus sp., a primitive equine: B, Falaeosyops palvdosus, a primitive titanothere. From specimens in the 

 American Museum of Natural History. Internal view of the lower teeth. The circles mark the cusps, which 

 appear independently in the phyla of the horse and the titanothere. They are at first barely visible but increase 

 in size in successive geologic levels. 



of horns in certain domestic breeds of cattle and by the 

 sudden appearance of rudiments of horns as major 

 saltations in many orders of mammals that are 

 naturally hornless. Horns are typical rectigradations 

 in the titanotheres, as distinguished from allometrons, 

 for the following seven reasons: 



3. The horn rudiments evolve continuously and 

 very gradually change in form and proportion 

 (allometrons) . 



4. In the earliest geologic stages in which horns 

 have been observed they are found only in adult 

 individuals, but through ontogenetic acceleration 



