22 



TITANOTHEEKS OF ANCIENT AVYOMING, DAKOTA, AND NEBEASKA 



SUMMARY OF DIFFERENCES BETWEEN OLD AND NEW 

 SYSTEMS 



To sum up: (1) The Linnaean genus or species is 

 defined (statically) by the presence of certain propor- 

 tions and by the presence or the absence of certain 

 characters, whereas the phyletic genus or species is 

 defined (dynamically) by the progressive evolution of 

 certain proportions and by the gradual gain or loss of 

 certain characters; (2) the Linnaean genus or species 

 was clearly distinguished from a related genus or 

 species, whereas the phyletic genus or species may 

 gradually fade into its ancestor or successor, and the 

 point where we make the dividing line is largely arbi- 

 trary; (3) consequently the phyletic genus actually 

 has a new meaning, but to avoid innovation in nomen- 

 clature we apply the phyletic term genus to a number 

 of species having a wide range in time and space, in 

 the same manner that Linnaeus applied the term 

 genus to a number of species having a wide range in 

 space only. 



STUDY OF THE EVOLUTION OF SINGLE CHARACTERS 



In the hard parts of living as of extinct animals 

 only three kinds of changes are observable — (1) 

 changes of proportion, which the author terms "allo- 

 metrons"; (2) the appearance of absolutely new char- 

 acters, which the author terms "rectigradations"; (3) 

 the disappearance or retrogression of characters. 



Changes of proportion. — Changes of proportion 

 (allometrons) make up by far the larger part of the 

 evolution of the titanotheres, as of that of all other 

 mammals. At least 95 per cent of the differences 

 between the skeletons of Eotitanops horealis and 

 Brontotherium plafyceras are due to changes of pro- 

 portion, and not more than 5 per cent to additions of 

 absolutely new characters, such as horns. Conse- 

 quently a very careful study has been made of allo- 

 metry — that is, of the methods of calculating, measur- 

 ing, recording, and describing changes of proportion — 

 and the result has been the discovery of a number of 

 general principles that apply to all mammals, extinct 

 and living, including man. Probably also the un- 

 discovered causes of changes in proportions are the 

 same in all mammals, but their discovery constitutes 

 a very difficult problem. (See Chap. XL) In this 

 difficult work the paleontologists may be greatly aided 

 by the zoologists, especially by very precise field 

 observers, such as Allen, Merriam, Miller, Osgood, and 

 Sumner. 



Although the mammalogists have demonstrated that 

 there is an apparently causal relation, direct or in- 

 direct, between certain types of coloration and of size 

 (harmonic increase or decrease) and the geographic 

 environment, the relation between change of environ- 



ment and changes in proportion (disharmonic) is very 

 obscure. It is known that a harmonic increase or 

 decrease in size of the entire mammal is correlated 

 with certain differences in habitat, often for the 

 obvious reason that a favorable environment favors 

 development of larger races, whereas an unfavorable 

 environment dwarfs growth. It remains to be 

 determined, however, whether certain environments 

 induce uniformly similar disharmonic changes of pro- 

 portion. Anthropologists, for example, have failed 

 to establish a definite causal relation between environ- 

 ment and the broad-headed (brachycephalic) or the 

 long-headed (dolichocephalic) form of the human head. 



The chief contribution that the paleontologist has 

 made to this obscure matter is to show that when a 

 proportionate change of head form is once established 

 in a certain direction there is a tendency to go to 

 extremes, so that, for example, extremely long heads 

 or extremely broad heads tend to evolve longer or 

 broader heads. These evolutionary tendencies are 

 illustrated in the titanotheres. 



Adaptive new characters. — The second mode of mam- 

 malian evolution — by the appearance of absolutely 

 new characters — lies in a field where the paleontologist 

 has a great advantage over the zoologist, because in 

 a series of fossils a new character (rectigradation) can 

 be traced back to its incipient, rudimentary stage, in 

 which it is so inconspicuous that it would not attract 

 the attention of the zoologist. Many characters that 

 eventually may exert a most profound influence on 

 the evolution of a race — that may, in fact, dominate 

 aU other characters — arise, so far as observed, from 

 excessively minute beginnings. These origins of new 

 characters are pointed out with great precision in 

 Chapters V and VI, in which the evolution of the 

 skuU and teeth is described in detail as observed in the 

 Eocene and lower Oligocene titanotheres. This very 

 precise study of the origin and evolution of similar 

 characters in many different lines of descent has led 

 to the important discovery that phyla differ less 

 through the possession of this or that new character 

 than through the different rates of evolution at which 

 the same character arises and evolves. In one 

 phylum a new character like the horns will arise in 

 an early geologic stage and evolve very rapidly, 

 whereas in a related phylum it will arise relatively late 

 in geologic time and will evolve very slowly. Thus a 

 phyletic genus is defined not only by the characters 

 which it exhibits but by the rate of the evolution of 

 these characters. This principle, again, is observable 

 only thi-ough paleontology. 



The origin of new characters, as manifested in dif- 

 ferent ways in the members of twelve subfamilies of the 

 titanotheres and as indicated by comparison with the 



