CHARACTER EVOLUTION 



149 



character complexes of similar ancestry in which each char- 

 acter evolves differently and is in a high degree heteromorphic 

 (diversely formative), as, for example, in the grinding teeth of 

 mammals (Fig. 38). 



This individuality and separability inherent in character 

 form is equally observed in character velocity and is the basis 

 of the shifting of characters from adult to youthful stages, 

 or vice versa, as well as of all the pro- 

 portionate and quantitative changes 

 which make up four-fifths of verte- 

 brate evolution. Increasing character 

 velocity is a process of acceleration; 

 decreasing character velocity is a proc- 

 ess of retardation. For example, in 

 the evolution of any group of ani- 

 mals, as in plants (p. 108), two char- 

 acter forms side by side, like the 

 fingers of the hand or toes of the 

 foot, may evolve with equal velocity 

 and maintain a perfect symmetry, or 

 one may be accelerated into a very 

 rapid momentum^ while another may be held in a state of 

 absolute inertia or equilibrium, and a third may be retarded. 

 These are the extremes of character velocity which result in 

 the anatomical or visible conditions respectively known as de- 

 velopment, balance, and degeneration. 



^ In physics momentum equals mass X velocity. In biology momentum and inertia 

 refer to the relative rate of character change, both in individual development (ontogeny) 

 and in evolution (phylogeny). Character parallax would express the differing velocities 

 of two characters. Thus the character parallax of the right and left horns in the Bron- 

 totheriinae (titanotheres) is very small, i. c, they evolve at nearly or quite the same 

 rate; on the other hand, the character parallax between the first and second premolar 

 teeth in these animals is very great. The character-parallax idea has innumerable ap- 

 plications and can be expressed quantitatively. W. K. Gregory. 



Fig. 38. Dissimilarly 

 Formed Characters of 

 Similar Origin. 



Surface of the upper grinding 

 teeth of two ancient Eocene 

 mammals. Type B is 

 known to be related to 

 type A. In Euprologonia 

 {A) all the cusps are of a 

 somewhat similar rounded 

 form. In Mcniscotherium 

 (B) each cusp has its own 

 peculiar form. 



