MUTATION 179 



vegetation sprang up on the lake bottom from 

 seeds lying dormant there. One Winter a ditch 

 was dug through a salt marsh, where the only- 

 higher plant was a species of marsh grass — 

 Spartina. The black peat cut from the ditch 

 was piled in a ridge by its side so high that it was 

 no longer covered by the tide. In the Spring 

 various roadside weeds sprang up along the 

 ridges, forming striking lines of vegetation run- 

 ning athwart the marsh. In these cases the 

 germs were present, but failed to germinate until 

 conditions suitable to their organization inter- 

 vened. So, in general, there are abundant means 

 of dissemination, and for almost every character 

 there is a situation for which it is best suited. In 

 that situation the new character will prove itself 

 adapted to its environment. 



THE MUTATION THEORY A KEY TO 

 DIFFICULTIES 



The notion of mutation, when fuUy grasped, 

 solves two difficulties which formerly confronted 

 evolutionists. The first difficulty is the swamp- 

 ing effect of intercrossing. If the usual result 

 of crossing a new character with its absence were 

 a blend of the two conditions, then the difficulty 

 would be a real one. But even in wild species 

 any unit character typically fails to blend when 

 crossed with its absence.^ The unit characters of 

 violets, shepherd's purse, and spots of beetles are 

 experimentally tested instances. The characters 



