30 NATURAL INHERITANCE. [CHAP. 



of them the polygon may stand, its principal tendency 

 on being seriously disturbed will be to fall back towards 

 the first position ; yet each position is stable within 

 certain limits. 



Consequently the model illustrates how the following 

 conditions may co-exist : (1) Variability within narrow 

 limits without prejudice to the purity of the breed. 

 (2) Partly stable sub-types. (3) Tendency, when much 

 disturbed, to revert from a sub-type to an earlier form. 

 (4) Occasional sports which may give rise to new types. 



Stability of Sports. Experience does not show that 

 those wide varieties which are called " sports " are 

 unstable. On the contrary, they are often transmitted 

 to successive generations with curious persistence. 

 Neither is there any reason for expecting otherwise. 

 While we can well understand that a strained modi- 

 fication of a type would not be so stable as one that 

 approximates more nearly to the typical centre, the 

 variety may be so wide that it falls into different condi- 

 tions of stability, and ceases to be a strained modification 

 of the original type. 



The hansom cab was originally a marvellous novelty. 

 In the language of breeders it was a sudden and re- 

 markable " sport," yet the suddenness of its appearance 

 has been no bar to its unchanging hold on popular 

 favour. It is not a monstrous anomaly of incongruous 

 parts, and therefore unstable, but quite the contrary. 

 Many other instances of very novel and yet stable 

 inventions could be quoted. One of the earliest 



