No. 580 J 



ORIGIN OF SINGLE CHARACTERS 



213 



which is a matter of phylogeny, is quite different from 

 comparison of the relative movement of a number of 

 different characters in single lines of descent, which is 

 the basis of Hyatt '& law. 



To illustrate the distinction between ontogenetic and 

 phyletic movement: a rudiment of a horn may appear 

 upon the skull in one phylum of titanotheres during the 

 period of deposit of the base of the Bridger beds (Fig. 5), 

 which are 1,500 feet in thickness, and in another phylum 

 (Fig. 5) at the summit of these beds, many thousands 

 of years later; this is its relative phyletic ■niorrmfiit. 

 Second, after the same horn-character has appeared long 

 subsequent to the birth of the individuals, in both phyla 

 it begins to be thrust forward in the ontogeny of individ- 

 uals, so that in Lower Oligocene time it begins to appear 

 long before birth; this is its acceleration or ontoyenol'u- 

 movement. 



Paleontology lias also revealed the marked distinction 

 in the mode of origin of the two kinds of characters ob- 

 served in zoology, namely, between the almost universal 

 changes of proportion and the comparatively rare new 

 "numerical characters. 



To the former I have applied the term alio met rons, 1 * 

 which signifies that differences of measurement express 



and ratios mav ho calculated. Such differences arising in 

 the head and in the feet are indicated in the familiar 

 terms dolichocephaly, brachycephaly, dolichopody, brachy- 

 pody, and many other convenient combinations of Greek 

 terms. That these changes of proportion become distinct 

 hereditary "characters" is proved in certain hybrids of 

 mammals where they appear to be partly or completely 

 separable. Thus the cross of human broad-heads with 

 long-heads does not produce a blend between the two but 



