Heredity and Natural Selection. 297 



was great enough to overpass the limit of the facet on 

 which it has been resting. 



In this case it would tumble over into a new position 

 of stability, which it will retain until the pressure again 

 becomes great enough to dislodge it and roll it another 

 step onwards. He says, " The various positions of stable 

 equilibrium may be looked upon as so many typical at- 

 titudes of the stone, the type being more durable as the 

 limits of its stability are wider. We also see clearly that 

 there is no violation of the law of continuity in the move- 

 ments of the stone, though it can only repose in certain 

 widely separated positions." 



Mivart, who has discussed this subject at some length, 

 has given many reasons for believing, in opposition to 

 Darwin, that such sudden jumps do occur, and that 

 evolution is not always by minute changes. 



It is clear to every one that any theory of the cause of 

 variation, which recognized the possibility of sudden and 

 extensive modification, would very greatly diminish the 

 time which is demanded for the origin of species by nat- 

 ural selection, and would thus greatly simplify our con- 

 ception of the working of this law. 



We have just seen that as our theory of heredity ex- 

 plains how a variation in one part causes related parts 

 to vary, it removes one great objection to the theory of 

 natural selection, and I wish now to call attention to 

 the fact that, since a change in any part will disturb the 

 harmony of related parts, thus causing their cells to 

 throw off gemmules, a slight change in one genera- 

 tion may become, in following generations, a very con- 

 siderable modification. There is therefore no reason 

 why natural selection should not often be presented with 

 great and extended variations the saltations which Mi- 

 vart believes in and the evolution of organisms may 



