INTBODUCTION TO MAMMALIAN PALEONTOLOGY 



21 



Brontops phylum, are so closely intergraded and con- 

 nected by "ascending mutations" that the dividing 

 lines between them can be drawn only arbitrarily, 

 according to individual judgment. In the Brontops 

 phylum, for example, the species Brontops hrachyce- 

 pJialus grades imperceptibly into the species Brontops 

 dispar through gradual transitions in a great number 

 of characters, as may be seen in the Hatcher collection 

 in the United States National Museum. There is no 

 evidence of brusque transitions, saltations, or jumps 

 in any structure, such as are presupposed in the 

 mutation theory of De Vries. By contrast, the 

 mutations of Waagen are intergradations between 

 arbitrarily defined species, and through these muta- 

 tions species and genera pass imperceptibly one into 

 another. 



Evolutionary characters of each phylum. — Thus we 

 reach a clear conception of a phylum of the titanotheres 

 in its osteologic and dental characters. A phylum may 

 be further defined as a succession of interbreeding 

 (syngamic, Poulton) individuals of similar (synepi- 

 gamic, Poulton) ancestry, which may or may not 

 occupy a similar range of country (synpatric, Poul- 

 ton), which follow in every structural character a sim- 

 ilar line of evolution (synphyletic, Osborn) and adap- 

 tation (syntelic, Osborn). 



In each horn, in each tooth, in every bone of the 

 skull and skeleton, and by inference in all the hard 

 parts as well as in all the soft parts, each phylum has 

 its distinctive mode and rate of transformation in each 



character, as follows: (1) Distinctive hereditary pro- 

 portion; (2) distinctive tendencies to change of propor- 

 tion; (3) distinctive progressive changes of proportion; 

 (4) distinctive retrogressive changes of proportion; (5) 

 distinctive accelerations and retardations in ontogeny 

 (individual development) ; (6) distinctive rates (veloc- 

 ities) of progression and retrogression in phylogeny in 

 each character. In each phylum are consequently 

 developed distinctive and ever changing proportions 

 and ratios between different single characters and 

 groups of characters, measurable by indices and ratios. 

 Such indices express the degrees of broad-headed, long- 

 headed, broad-footed, short-footed structure and pro- 

 portion, and so on. Each phylum has also its distinc- 

 tive but constantly changing indices and ratios of 

 teeth to skull, of skull to body, of body to limbs, etc., 

 which also are constantly changing as we pass from 

 the lower to the higher geologic levels. 



Old and new meanings of taxonomic terms. — In the 

 following table a comparison is made between the old 

 and the new meanings of the taxonomic terms used 

 by mammalogists. The definitions given in the sec- 

 ond column are those of the old "special creation" 

 system — followed by Linnaeus — which is based on 

 geographic distribution alone; the definitions given in 

 the third column are those of the new phyletic sys- 

 tem — that of Osborn — which is based on both geologic 

 and geographic distribution. The new system was 

 first used for the rhinoceroses (Osborn, 1900.192) and 

 for the titanotheres (Osborn, 1902.208). 



Comparison oj ike Linnaean and the phyletic systems of taxonomic terms 



Desired harmony of mammalian paleontology and 

 zoology. — The methods employed by all zoologists, 

 paleontologists, and anthropologists in their observa- 

 tion and measurement of the hard parts of mammals 

 should be the same. The methods pointed out above, 

 first presented by Osborn (1914.412), are founded 

 on the comparison in time of geologic ascending 

 evolutionary phyla of mammals — such as the rhi- 

 noceroses and the titanotheres — with contemporaneous 



geographic series of species, subspecies, and varieties 

 that may be grouped within a single genus. What 

 applies to the systematic terms used in the classifica- 

 tion and description of animals applies with equal 

 force to those used for single characters, for it is 

 the cumulative sum of evolutionary change in a very 

 large number of single characters which makes up 

 the mutation of Waagen, the species, or the genus, as 

 the case may be. 



