INTBODUGTION TO MAMMALIAN PALEONTOLOGY 



41 



temporaneous phylum may have well-developed horns. 

 In the former the horns may appear much later and 

 may never acquire very great momentum in develop- 

 ment. We can thus note the incipiency of the differ- 

 ences between the short-horned titanotheres and the 

 long-horned titanotheres. 



This principle of unequal phylogenetic velocity in 

 the development of the same characters enables us to 

 distinguish different genera and species. In one genus 

 the development of the internal cusps of the premolar 

 grinding teeth shows high velocity; in another genus 

 it shows low velocity. Apparently these internal 

 cusps are useful adjuncts of the tooth; they make the 

 tooth more effective for grinding up food. Similarly 

 the horns are useful adjuncts of the head in warding 

 off enemies. Yet these characters evolve so slowly 

 in certain phyla that it is unreasonable to believe that 

 utility and natural selection are the prime causes of 

 their evolution. There would seem to be physiological 

 and physical (or chemico-physical) causes of these 

 different velocities. It is the data on the different 

 velocities of the developmen-t of the same characters in 

 related phyla which give the principal biologic value to 

 the long series of detailed measurements and justify 

 the large number of figures that are presented in Chap- 

 ters V and VI. This suggests a summary of the bio- 

 logic aspects of the problems of this monograph and 

 of the features that distinguish this particular field of 

 biologic research. 



SUMMARY OF THE EVOLUTION OF THE TITANOTHEREG 



The known titanotheres were confined to a relatively 

 small area near the fortieth parallel in western North 

 America and to Europe and Asia. The direct lines of 

 descent and the continuous changes in many branches 

 in different or successive life zones were complicated 

 by the occasional incursion of new families from out- 

 side larger regions, probably from northern America 

 and perhaps from northern Asia. (See appendix.) 

 Nevertheless the localities in western North America 

 where the remains of titanotheres have been found 

 were apparently near the main geographic center of 

 the evolution of the family, for the series of known 

 fossils enables us to follow almost every step in the 

 slow transformation of forms that were small and 

 defenseless to forms that were huge and well armed. 



The remains of the titanotheres now collected repre- 

 sent the most complete evolutionary series of mammals 

 thus far discovered except those of the horses. The 

 horses, however, are much less highly differentiated. 

 In the titanotheres we see the growth of a great and 

 vigorous family tree, giving off numerous branches 

 (phyla), which diverge in characters and habits while 

 retaining hereditary resemblances and certain heredi- 

 tary trends and tendencies of transformation. Each of 

 these branches is made up of slowly transforming 

 successive stages (mutations of Waagen), which appear 

 101959^29— VOL 1 5 



to be the more continuous and unbroken by sudden 

 change the more thoroughly we explore the geologic 

 levels where they successively occur. The evolution 

 of the soft parts can only be inferred. The hard parts 

 evolve in a variety of ways, chiefly through increase 

 of size, through changes in proportion, through addi- 

 tion of new parts, and in less measure through loss of 

 parts. Actual addition or loss of parts in the titano- 

 theres is rare; general increase in size is almost uni- 

 versal, though in a few branches the size is diminished 

 or arrested. 



Changes in the proportions (allometrons) of struc- 

 ture were brought about by different velocities of 

 phylogenetic evolution (acceleration and retardation) 

 in the skeletal framework as a whole and in each of 

 its parts. No less important is the definite and 

 successive addition of new characters (rectigra- 

 dations), each developing from infinitesimal begin- 

 nings until it reaches a stage of usefulness and 

 each apparently having its individuality (biocharacter) 

 and its separate history. 



Throughout this wonderful transformation, which 

 is in general adaptive, there were certain manifest 

 germinal (hereditaiy) tendencies and certain unkno\vn 

 interactions between these germinal changes and' the 

 external, habitudrnal, and environmental influences. 

 The more carefully we study the detailed characters in 

 each branch the more evident it becomes that the 

 causes of evolutionary development are neither exclu- 

 sively external nor exclusively internal but are to 

 be sought hypothetically in the interactions between 

 germinal, habitudinal, and environmental forces. The 

 changes in the proportions of the skeletal characters 

 and the new elements added to the teeth and skull, 

 which are the outward expressions of these hypothetic 

 germinal and environmental reactions, become visible 

 more or less contemporaneously but not simultane- 

 ously in all members of the branches and sub- 

 branches of the great family tree — that is, the 

 same characters appear, but at different periods 

 and with different velocities of development. The 

 whole process is an orderly one, which is, however, 

 not predetermined in the germinal constitution of the 

 titanotheres but results from certain innate or germinal 

 potentialities of evolution, which are evoked in response 

 to certain environmental and habitudinal conditions. 



The struggle for existence, or natural selection, is 

 operating continuously and more or less strongly on 

 every single character according as its survival value 

 is greater or less. In each successive geologic level 

 we witness alterations of the family tree — its impover- 

 ishment through the extinction of certain branches or 

 its augmentation through the survival of other 

 branches and the immigration of branches which 

 evolved in other regions. The individual members of 

 all the branches (with two exceptions) become more 

 imposing and more diverse as time goes on. Finally, 



