VARIATION AND HEREDITY 



11 



compared with an organism. They "have this cardinal 

 fact in common, that if either is disturbed without trans- 

 gressing the range of its stability, 

 it will tend to re-establish itself, ' ' 9 

 that is, if tipped to the right or left 

 it will fall back upon the original 

 side, but if the range is passed, it 

 will topple over into a new position 

 of stability. This illustrates a mu- 

 tation. There is now a new posi- 

 tion of stability or average condi- 

 tion about which there will be 

 fluctuation. 



In the present state of our knowl- 

 edge of variations we are unable to 

 say dogmatically whether species B 

 have arisen by the slow accumu- 

 lated adjustments of fluctuating 

 variation or by the more rapid 

 process of mutation. In support of 

 the first theory there are numerous 

 cases where species are connected 

 by intermediate grades. There is 

 much experimental evidence to sup- 

 port the second theory. 



In 1900, when De Vries in Hol- 

 land, Correns in Germany, and 

 Tschermak in Austria independ- FIGURE 3 

 ently, and almost simultaneously, 

 reached results from the experimental study of heredity 

 which have modified our views of the origin of species, 

 the whole subject of heredity took on added interest. 



Galton, op. cit., p. 28. 



Diagram illus- 

 trating a Mutation. 



