OTHER THEORIES OF SPECIES-FORMING. 361 



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 condi- 

 tions, to revert to the earlier type. The model further illustrates 

 the fact that sometimes a sport may occur of such marked pecu- 

 liarity 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 I, 

 on the edge A B, corresponding to the shorter diameter of the oval, 

 it stands in its most stable position, and in one from which it is 

 equally difficult to dislodge it by a tilt either forwards or back- 

 wards. So long as it is merely tilted it will fall back on being 

 left alone, and its position when merely tilted corresponds to a simple 

 deviation. But when it is pushed with sufficient force, it will tumble 

 on to the next edge, B C, into a new position of stability. It will 

 rest there, but less securely than in its first position ; moreover, its 

 range of stability will no longer be disposed symmetrically. A com- 

 paratively slight push from the front will suffice to make it tumble 

 back, a comparatively heavy push from behind is needed to make it 

 tumble forward. If it be tumbled over into a third position (not 

 shown in the figure), the process just described may recur with 

 exaggerated effect, and similarly for many subsequent ones. If,, 

 however, the slab is at length brought to rest on the edge C D, most 

 nearly corresponding to its longest diameter, the next onward push, 

 which may be very slight, will suffice to topple it over into an 

 entirely new system of stability ; in other words, a 'sport' comes 

 suddenly into existence. Or the figure might have been drawn with 

 its longest diameter passing into a projecting spur, so that a push 

 of extreme strength would be required to topple it entirely over. 



"If the first position, A B, is taken to represent a type, the other 

 portions will represent sub-types. All the stable positions on the 

 same side of the longer diameter are subordinate to the first position. 

 On whichever 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 condi- 

 tions may co-exist: (i) variability within narrow limits without 

 prejudice to the purity of the breed; (2) partly stable sub-types; 



