28 NATURAL INHERITANCE. [chap. 



intended to illustrate the meaning of primary and sub- 

 ordinate stability in organic structures, although the 

 conditions of these must be far more complex than 

 anything we have wits to imagine. The model and the 

 organic structure have the cardinal fact in common, that 

 if either is disturbed without transgressing the range of 

 its stability, it will tend to re-establish itself, but if the 

 range is overpassed it will topple over into a new 

 position ; also that both of them are more likely to 

 topple over towards the position of primary stability, 

 than away from it. 



The ultimate point to be illustrated is this. Though a 

 long established race habitually breeds true to its kind, 

 subject to small unstable deviations, yet every now and 

 then the offspring of these deviations do not tend to 

 revert, but possess some small stability of their own. 

 They therefore have the character of sub-types, always, 

 however, with a reserved tendency under strained con- 

 ditions, to revert to the earlier type. ■ The model further 

 illustrates the fact that sometimes a sport may occur of 

 such marked peculiarity and stability as to rank as a 

 new type, capable of becoming the origin of a new race 

 with very little assistance on the part of natural selection. 

 Also, that a new type may be reached without any large 

 single stride, but through a fortunate and rapid succession 

 of many small ones. 



The model is a polygonal slab, the polygon being one 

 that might have been described within an oval, and it is 

 so shaped as to stand on any one of its edges. When the 

 slab rests as in Fig. 1, on the edge a b, corresponding to 



