200 GENERAL EYOLUTION. 



US in the belief that the use of the angles of the parts in question 

 in these animals would result in a normal exostosis, of a simple 

 kind in the frogs, or as horn cores in the Euminantia.* As to the 

 sheathing of the cores in the Bovidce, and nakedness in the Cer- 

 vidcB, it is in curious relation to their habitat and to their habits. 

 The epidermis and derm would of course share in the effects of 

 friction. In the Bovidae which dwell in treeless plains, or feed 

 on the grasses in great i:)art, the development of these coverings of 

 the horn cores into a horny sheath would naturally meet with 

 no interruption. In the case of the deer, which mostly live in 

 forests or browse on trees, constant contact with the latter would 

 prevent the healthy growth of the dermal covering, and it would 

 be liable to injury or constant excoriation by the animals them- 

 selves on the branches of trees, etc. This we know to be the pres- 

 ent habit of the deer as regards the dermal covering of the horns. f 

 I have elsewhere pointed out the similar connection between the 

 dental structure and habitat among the oxen and the deer. The 

 former, eating the harder grasses, are provided against the conse- 

 quent rapid attrition of the tooth by a prismatic form, which 

 allows of more prolonged growth and more rapid protrusion. 

 The deer, in accordance with their foliage-eating habits, do not 

 wear the crown of the tooth with such rapidity. Long-continned 

 protrusion is not so necessary, hence the teeth are more distinctly 

 rooted and have a prominence or shoulder, distinguishing the 

 body of the crown. 



B. Change in Amount of Groioth- Force. 



1. Absolute increase of Groiutli- Force. — As every type has had 

 its period of greatest development in numbers, size, and complica- 

 tion of structure, the present law indicates as an exjolanation, a 

 culmination of the process of conversion of growth-force from its 

 energetic to its potential state in tissue. The cause is primarily 

 the increased exercise of effort and use, which, while effecting a 

 conversion, increases the capacity of the organs by which further 

 conversion is effected. 



* The now well-known fact that all cranial ossification was primitively dermal, 

 is confirmatory of the idea that its appearance was due to moderate friction of the 

 skin on resistant bodies. (Ed, 1886.) 



f Palaeontological studies go far to show that the origin of the shedding of the 

 deer's horn was due to the loss of their dermal covering, and that this was caused by 

 violent use. (1886.) 



