CHAPTER XI 



CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



In section 1 of this chapter two chief processes of 

 evolution are traced in detail in the long history of 

 the titanotheres — the evolution of new proportions 

 (allometrons) and the rise and evolution of new char- 

 acters (rectigradations). Both processes proceed along 

 direct, continuous, orthogenetic lines, and both are of 

 great biologic and sytematic significance. 



In section 2 the causes of the extinction of the 

 titanotheres and other quadrupeds are considered in 

 the light of Darwin's theory of natural selection. 



Biocharacters {single characters). — Throughout the 

 research made for this monograph far more attention 

 has been devoted to a study of the modes of evolution 

 of "single characters," which we may term "biochar- 

 acters," than to artificial groupings into "mutations," 

 "species," and "genera." The titanotheres exhibit 

 with extraordinary clearness the origin, evolution, 

 survival, and elimination of biocharacters. Detailed 

 observations extending over nineteen years are not 

 without reward. The long history disclosed by hun- 



Figure 725. — Restorations of Eoiitanops borealis (right) and Broniotherium plalyceras (left): the earliest and 

 the latest stage in the evolution of the titanotheres 



SECTION 1. MODES AND CAUSES OF THE ORIGIN 

 AND EVOLUTION OF NEW ADAPTIVE CHARAC- 

 TERS (RECTIGRADATIONS) AND NEW PROPOR- 

 TIONS (ALLOMETRONS) IN OLD CHARACTERS 



DISTINCTIONS BETWEEN MODES AND CAUSES 

 Evolution of the heredity germ. — A host of new char- 

 acters and of new proportions in old characters separate 

 Brontotherium from Eoiitanops, and all these characters 

 are germinal, hereditary. The problem here consid- 

 ered is. How and why did the heredity germ of Eoiitanops 

 evolve into that of Brontotherium ? "Ho w ' ' is a question 

 of the modes of evolution; this question we have in 

 large part answered. "Why" is a question of the causes 

 of evolution; this question we have answered only in 

 small part. Let us examine four points of distinction 

 between "modes" and "causes." 



dreds, even thousands, of titanothere biocharacters in 

 the teeth and skeleton throws light upon three of the 

 still unsolved great problems of the modes of evolution, 

 namely : 



Modes of origin of new adaptive biocharacters = 

 rectigradations. 



Modes of origin of new proportions in biocharac- 

 ters = allometrons. 



Modes of survival and elimination of biocharacters 

 = selection. 



Distinctions to he noted. — We must keep constantly 

 in mind the distinction between the comparatively 

 well understood modes of evolution, which follow cer- 

 tain well-established principles that apply to all living 

 beings, and the still mysterious causes of evolution, 

 which follow principles yet to be discovered. The 



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