382 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



however, it may be assumed that stimulus is stress 

 due to a want of harmony between an organism and 

 its environment, and that kinetogenesis is the result of 

 the effort of the organism to adjust itself. So soon as 

 equilibrium is attained, the stress of stimulus ceases 

 or is much reduced, and evolution in this direction 

 ceases. Such equilibrium is attained when the mechan- 

 ism of an animal is sufficient for the satisfaction of its 

 needs. When this is the case severe exertion is no 

 longer necessary, and a period of easy use follows 

 .which is sufficient to maintain the mechanism in work- 

 ing condition. In the case where circumstances should 

 become so favorable for the easy satisfaction of the 

 necessities of life, as to call for little or no exertion, 

 degeneracy of the organism is sure to follow. The 

 well-known phenomena of degeneracy from disuse 

 show that a large part, and in some cases all, of the 

 stimulus of use, is only sufficient for the maintenance 

 of the organism in working condition, and that there is 

 no surplus to be expended in progressive evolution. 



It is however true that some organs are stimulated 

 to excessive growth by active use. Such are some of 

 the teeth, which, if not worn at the crown by the op- 

 position of those of the opposite jaw, soon grow to an 

 inconvenient length. This occurs in the hypsodont 

 molars of horses and artiodactyles, and in the pris- 

 matic molars and incisors of Glires. Hypsodonty in 

 general is an illustration of continuous growth induced 

 by long-continued stimulus in those orders of Mam- 

 malia, and in the Toxodontia and Proboscidia. The 

 excessive growth of the canines in the South American 

 saber-tooth tiger, and of the incisors of the mammoth, 

 are cases where the energy of growth has not subsided 

 in time to prevent excess. 



