834 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Thus the proportions of the horse skull acquired 

 through a long process of allometric evolution prove 

 to be separable biocharacters in the germ. This also 

 proves (Osborn, 1912.372) that proportional charac- 

 ters, although evolving continuously, may exhibit the 

 same germinal separability as the suddenly appearing 

 saltations and mutations of De Vries. Out of 28 

 characters that were carefully compared in the skull 

 and the teeth of the horse, the ass, and the mule 18 

 found in the mule are obviously derived either from 

 one parent or the other and show very slight tendency 

 to blend, whereas 10 characters show a distinct tend- 

 ency to blend. 



Yet, so far as our observation shows, rectigradations 

 are more decidedly and distinctly separable in the 

 germ than allometrons. 



CORRELATION, COORDINATION; COMPENSATION OF 

 RECTIGRADATIONS AND ALLOMETRONS 



In the evolution of rectigradations and allometrons 

 all the biocharacters are harmoniously adjusted to 



Figure 748. — Contrasts between the hypotheses of Lamarck (A), 

 Weismann (B), and Osborn (C) regarding the causes of evolution 



such as the development of the horns at the expense of 

 the nasal bones. Such compensation correlation is 

 comparatively rare in the titanotheres in contrast with 

 other quadrupeds. 



4. Sex correlation or linkage with male or female sex 

 of proportion biocharacters and to a less extent of 

 rectigradation biocharacters, as in the size of the 

 canine teeth, the size of the horns, the proportions of 

 the skull, the varying velocities of ontogenetic and 

 phylogenetic changes in the dentition and all parts of 

 the skeleton, the brachycephaly of males and mesa- 

 ticephaly of females of the same species. 



5. Germinal correlation, by which titanotheres of 

 common ancestry exhibit similar evolutionary tend- 

 encies, as in progressive brachycephaly and doli- 

 chocephaly, acceleration and retardation. 



6. Psychic correlation, as of the horns, with psychic 

 predisposition to use the horns, as explained above in 

 a consideration of ontogenesis of the horns in cattle. 



The accompanying diagram (fig. 748) represents 



crudely the extreme contrasts between the Lamarckian, 



the Darwinian, and the tetrakinetic points of 



f,ypot/,es,s view. 



THEORETIC CAUSES OF THE EVOLUTION OF NEW 

 CHARACTERS AND NEW PROPORTIONS 



THEORIES ADVANCED TO EXPLAIN THE ORIGIN 

 OF RECTIGRADATIONS AND ALLOMETRONS 



Having compared some of the modes of the 

 evolution of the titanotheres with those of the 

 evolution of other mammals, we may now con- 

 sider theories that have been advanced to explain 



The Lamarckian conception of causes is chiefly external, centripetal; the Darwin- Weismann the tWO kinds of evolution upon which OUr atten- 



conception of causes is chiefly internal, centrifugal; the tetrakinetic hypothesis is chiefly tion is here Centered bv rCCtiffradationS and bv 



internal-external, or both centripetal and centrifugal. The balance of existing evidence is in ,. o • * j • 'Z! ii 



favor of B, the centrifugal hypothesis, since the tetrakinetic theory rests only upon inference ailomctrons. oetting aside aS unsCientlllC all 

 from the observed modes of evolution rather than upon actual observation of the external, 

 centripetal origin of any single somatic character. 



serve the organism as a whole. The adjustments are 

 of six different kinds, as follows: 



1. Mechanical correlation of the rectigradations and 

 allometrons in the upper and lower grinding teeth. 

 The reason for this continuous and perfect adjustment 

 is perhaps the most difficult to comprehend in the 

 whole range of titanothere evolution. 



2. Mechanical correlation of the developing horns, 

 head, shoulders, and entire locomotor skeleton, with 

 the use of the horns as offensive and defensive struc- 

 tures, with the use of the lips and grinding teeth in 

 the prehension and comminution of food, with the 

 development of the limbs, partly for offense and de- 

 fense, partly for locomotion. In this general correla- 

 tion the evolution of various allometrons and recti- 

 gradations is continuously and perfectly adjusted. 



3. Proportion compensation, a mode of correlation 

 and coordination, by which the development of one 

 character is effected through the sacrifice of another. 



ideas of "vitalism" and "internal perfecting 

 tendencies," we may group all modern theories 

 of the evolution of form and function under three 

 heads, as follows: 



1. External initiation and causation (Lamarckism) ; 

 modifications of the environment and of the soma. 

 Modifications introduced by change of environment 

 and change of habit are in some manner impressed 

 upon the germ, so that they reappear more or less 

 fully developed in offspring. This idea is inherent in 

 all theories of "direct action of environment" and 

 "inheritance of acquired characters," as developed by 

 Buffon, Lamarck, Spencer, Cope, and others. This 

 general interpretation may be refeiTcd to as the 

 Lamarckian. 



Evidence: According to this theory germinal varia- 

 tions should be found to originate, to follow, and to 

 correspond closely with individual somatic modifica- 

 tions. They should be orthogenetic and should 

 invariably be followed by similar changes inherited 

 in the bodies of descendants. 



