CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



821 



Allometrons differential {disharmonic) in origin. — 

 At first glance it would seem that if the bones of the 

 skull were slretched longitudinally as a whole the 

 stretching would give rise to dolichocephaly, or if 

 they were stretched transversely as a whole the 

 stretching would give rise to brachycephaly, and 

 approximately this mode of evolution is seen in skulls 

 where all parts are harmonically lengthened or broad- 

 ened in nearly equal percentages; but the actual mode 

 of evolution in the titanotheres involves disharmonic 

 and differential lengthening or broadening of each 

 part. On analyzing closely by comparative measure- 

 ments the progressive changes in titanothere skulls 

 we find that each of the adjacent bony elements acts 

 differently and more or less independently; the skull 

 is not broadened or lengthened as a whole, as if com- 

 posed of and stretched like India rubber, but each 

 part of it acts as a distinct biocharacter and has its 

 separate velocity or rate of change. This differential 

 principle is illustrated on the accompanying diagram 

 (fig. 738) showing some of the contrasts in the parts 

 evolving in typical broad-headed {Palaeosyops) and 

 long-headed {DolicJiorJiinus) skulls. 



Allometrons continuous in origin. — Do changes of 

 proportion arise by saltation or continuously? Observa- 

 tions made by the author on titanotheres, rhinoceroses, 

 and horses compared with those made by numerous 

 anthropologists on man demonstrate beyond doubt that 

 all adaptive changes in proportion are continuous in 

 origin; also that, like rectigradations, they take certain 

 directions (from causes which will be considered in the 

 next section) and are cumulative or progressive in 

 successive generations. This kind of proportional 

 change, as demonstrated below, may be suspended or 

 even reversed in direction. This continuity in the 

 evolution of proportions is demonstrated whenever a 

 series that is nearly complete, or unbroken, is meas- 

 ured, as they are in the measurement tables of Menodus, 

 Brontotherium, and Megacerops in the sections treating 

 of these genera. (See Chap. VI.) 



Exactly similar results have been obtained by Osborn 

 in the comparative measurement of closely successive 

 series of Oligocene horses. The same is indicated 

 among the Oligocene rhinoceroses but is not yet posi- 

 tively demonstrated. According to the unanimous tes- 

 timony of anthropologists (Ripley, Races of Europe, 

 1899.1, p. 624), the form of the human head is the 

 result of very gradual change, either by elongation 

 (toward dolichocephaly) or by broadening (toward 

 brachycephaly) . In the native Indian races of America 

 also, which are believed to be of the same remote racial 

 origin, considerable diversity in the proportions of the 

 head has gradually evolved under geographic isolation. 

 Similarly Keith (1911.1) observes that in course of their 

 evolution the tendency of one Negro tribe has been 

 toward the accentuation of one set of skeletal propor- 

 tions, of another tribe toward another set. The 



Dinka acquire high stature and narrow heads, the 

 typical Nigerians low stature and narrow heads, the 

 Basoko wide, short heads and low stature, the Bunms 

 broad heads and high stature. 



Causes of head allometrons unhnown. — It is probable, 

 but not yet demonstrated, that whatever the causes 



FiGTJRE 737. — Harmonic and disharmonic natural brachj'cepli- 

 aly and dolichocephaly and artificial broadening or lengthen- 

 ing of the outline of a skull 



A. Palaeosyops robustus, a natural brachycephalic skull; Ai, outlines of the same 

 skull artificially stretched into dolichocephalic form (harmonic); B, Mesatirhinus 

 megaThinus, a natural mesaticephalic skull; Bi, outlines of the same skull arti- 

 ficially stretched on India rubber into dolichocephalic form (harmonic); C, 

 DoUchOTliinus hijognailius, a natural dolichocephalic skull, viewed from above; Ci, 

 DolkhorMnus hyognathus, a natural dolichocephalic skull (disharmonic) . The con- 

 trast between) Ai artificial) and C (natural) and between Bi (artificial) and Ci 

 (natural) is clearly displayed. 



of proportional change may be they are similar in 

 the evolution of "bi'oad heads," "long heads," "long 

 limbs," "short limbs," etc., among the titanotheres, 

 the horses, man, and all other mammals. In man 

 also there is considerable evidence that the evolution 

 of proportion biocharacters is differential, namely, 

 that while there is often harmonic evolution of head 



