DARWIN AND PALEONTOLOGY 235 



ible form, such as the beginning of a cusp or of 

 a horn. The origin of horns has always been a 

 famous problem, because horns are eminently in 

 the nature of new characters. In the great quad- 

 rupeds known as titanotheres rudiments of horns 

 first arise independently at certain definite parts 

 of the skull ; they arise at first alike in both sexes, 

 or asexually; then they become sexual, or chiefly 

 characteristic of males; then they rapidly evolve 

 in the males while being arrested in development 

 in the females; finally they become in some of 

 these animals dominant characters to which all 

 others bend. 



The form, the proportion, the modeling, both 

 of the cusps and of the horns, accord with the 

 proportions of the teeth or of the skulls in which 

 they appear; they are thus correlated in form 

 with other organs. The cusps of the grinding 

 teeth of mammals form a peculiarly advanta- 

 geous field of observation because they do not 

 depend either upon ontogeny or environment for 

 their perfection ; they are born complete and per- 

 fect, use and environment destroy rather than 

 perfect them. They thus contrast with the 

 bones, muscles, and many other tissues of the 

 body which depend upon ontogeny for their per- 

 fection. 



Failure of attempted explanation of adaptive 

 origins hy transmission of acquired characters. 

 In seeking explanation of this definiteness or 

 adaptiveness of direction in the origin of certain 



