806 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



so-called "laws of evolution" thus far discovered relate 

 chiefly to modes and in but small degree to causes. 

 Nevertheless the observation of the modes of evolu- 

 tion exemplified in the titanotheres and other animals 

 bears adversely or favorably upon one or another of 

 the various theories that have been advanced as to 

 the causes of evolution. 



The study of evolution may be divided into a 

 study of facts more or less fully ascertained by com- 

 parative studies of the invertebrates and vertebrates 

 through the sciences of zoology, comparative anatomy, 

 experimental zoology, genetics, invertebrate and verte- 

 brate paleontology, and a consideration of hypotheses 

 as to the causes of evolution, such as those presented 

 by Buffon, Lamarck, Spencer, Darwin, Wallace, Cope, 

 Weismann, De Vries, Osborn, Loeb, Morgan, and 

 other naturalists, experimentalists, and biochemists. 



£otitanops yre^oryi 



^rontotheriUTrt platyceras 



Lower £x>cene 



Figure 726. 



ZSO.ooo generations 



soo.ooo years of 



^eoloqic 'time 



/.ower Olifocene 



Relations of the heredity germ of Eotitanops gregoryi to that of 

 Brontotherium platyceras 

 Animals are one-fiftietb natural size. 



SPECTJIATION AS TO THE CAUSES OF EVOLUTION 



Many vertebrate paleontologists have contributed 

 to our knowledge of the modes of evolution of the 

 extinct vertebrates, notably Cuvier, Owen, Huxley, 

 Gaudry, Leidy, Cope, Marsh, Kowalevsky, Williston, 

 Scott, Osborn, Deperet, Smith Woodward, Andrews, 

 Dollo, Abel, Merriam, Matthew, and Gregory. Only 

 a few — among them Buffon, Lamarck, St. Hilaire, 

 Spencer, Cope, and Osborn — have attempted even to 

 speculate upon the causes of evolution of the verte- 

 brates. Lamarck, Spencer, and Cope are the principal 

 contributors to the Lamarckian hypothesis of inherit- 

 ance by the germ of the mechanical modifications of 

 the body, and to the related hypotheses of kineto- 

 genesis, of growth force, and of "bathmism, " all 

 dependent upon the supposed inheritance by offspring 

 of the influences of bodily characters acquired through 



the habits of the parents. Scott has pointed out the 

 very important distinction observable in vertebrates 

 between the principle underlying the "mutations" of 

 Waagen and the "variations" and "variability" of 

 Darwin. Osborn has developed a hypothesis for the 

 principle of rectigradation as of potential homology, 

 and has proposed a theory of tetraplasy in growth 

 and development and of tetrakinesis in evolution, the 

 latter based upon conceptions of energy. 



DISTINCTION BETWEEN INVISIBLE GERM EVOLUTION 

 AND VISIBLE BODILY EVOLUTION IN THE TITANO- 

 THERES 



In observing the modes of evolution, and still more 

 in speculating upon the causes of evolution, it is 

 necessary to keep in mind always the sharp distinction 

 between the invisible evolution of the germ and the 

 visible evolution of the body. The germ 

 contains the heredity units of all the predis- 

 positions and potentialities of body form and 

 function, units figuratively known as 

 "factors," "determiners," or "genes," 

 which may control the development into 

 normal visible body form only under the 

 favorable influence of normal habit and en- 

 vironment. The body of the titanothere, on 

 the other hand, is the component of the four- 

 fold (tetraplastic) influence of the animal's 

 germ cells, of its habits, of its physical 

 environment, and of its life environment. 



Germ evolution represents incessant 

 changes in heredity; bodily development 

 represents four factors — heredity, physical 

 environment, life environment, and habit 

 (ontogeny). With this distinction clearly 

 in mind, if we contrast the beginning and 

 the end of titanothere bodily evolution we 

 observe that the heredity germ of Bron- 

 totherium platyceras was very different 

 from that of its remote ancestor Eotitanops 

 lorealis. Perhaps 500,000 years of geologic time 

 separated Eotitanops from BrontotJierium. Allowing 

 one generation on the average for every two years, 

 two hundred and fifty thousand generations of titano- 

 theres separated Eotitanops horealis from Brontotherium 

 platyceras. 



Since the adult Brontotherium differs very widely 

 from the adult Eotitanops in all its visible biocharac- 

 ters it certainly reflects a great number of new or 

 transformed germinal predispositions. For example, 

 say the germ of Eotitanops contains 300,000 predis- 

 positions, that of Brontotherium contains perhaps 

 500,000. How did the predispositions of these 

 200,000 new or transformed biocharacters enter the 

 heredity germ? Whatever the number, certainly 

 the heredity germ of Brontotherium was in its predis- 

 positions and potentialities vastly different, more 



