VARIATION AND HEREDITY 



11 



compared witli an organism. Tlicy ''liavc lliis caidinal 

 fact ill coniiiioii, that if oitlicr is disturbed willioiit tiaiis- 

 ,<;res.sing the range of its sta])ility, 

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

 thai is, if tipped to the right or left 

 it will fall hack upon the original 

 side, but if the range is passed, it 

 will toi)plc over into a new position 

 uf stability. Tliis illustrates a mu- 

 tation. There is now a new posi- 

 tion of stability or average condi- 

 tion about wliicli there will be 

 lluctuation. 



In the ])resent state of our knowl- 

 edge of variations we are unable to 

 say dognuitically whether species B 

 have arisen l)y the slow accunni- 

 iated adjustments of fluctuating 

 variation, or by the more rapid 

 process df 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- 1""k:iki': 3- Uia^'nun iiius- 



ji 1 1 i. • li. 1 tratiii'' a Mutation. 



eiitly, 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. 



» Cialtoii, vp. CI/., p. 2S. 



y 



