374 



Evolution by Jumps 



not feel either its wobbling or its movement. Yet we have 

 good reason to believe that it is moving at a rate that staggers 

 the imagination, and that its flight through space is not 

 absolutely smooth. The germ plasm is also stable, compared 

 to the developing protoplasm of an individual plant or ani- 

 mal. Yet this stability is compatible with the kind of change 

 which the idea of evolution presupposes. 



Francis Galton developed this thought by using a simile 

 from geometry. A material body may be in such stable 

 equilibrium that although it yields considerably to pressure 

 from various directions by rocking on its base it always re- 



B 



B 



Fig. ^6. Galton*s Principle of Primary and Secondary 



Stability 



A polygonal block that is not symmetrical can be made to stand on any 

 one of its faces, on a level surface. It is more stable in some positions than in 

 others, however. Similarly a natural species may fluctuate considerably about 

 a mean and yet remain fixed; or it may transcend the normal range and jump 

 into a new, more or less stable, position. See text. After Galton. 



turns to its normal position (on A-B in Fig. 96) . A push 

 above the ordinary, however, would shove it into a new posi- 

 tion. This corresponds to a sub-species; and here it could 

 maintain itself, subject to minor disturbances (on C-B in 

 the figure) . A second violent disturbance might push the 

 block into its original position, or into a third position cor- 

 responding to a new type (on D-C in Fig. ^6) . In this way 

 a species may be conceived to endure a considerable range of 

 fluctuating variation and yet remain always essentially the 

 same species. A change in hereditary constitution would 

 make the fluctuations transcend those of the original condi- 

 tion, and would shift the whole of succeeding generations into 



