MUTATION 325 



question. Another fact which makes one hesitate to 

 accept mutation as a general explanation is the rarity 

 of the cases observed. It is true that since De Vries' 

 book was published many new observations have been 

 made and new cases reported. De Vries himself 

 confesses, however, that those cases are yet too few. 

 His explanation of the fact is rational enough but 

 would fit any other theory as well. When a mutation 

 takes place in the wild state, he says, naturalists regard 

 it as an old variety and take no account of it. Thou- 

 sands of mutations may thus appear and disappear 

 and never be noticed, at least, as mutations. On the 

 other hand, the struggle for life wipes out easily all 

 the individuals which diverge too far from the normal 

 and are incapable of developing under certain con- 

 ditions. This is true of mutations as well as of fluctu- 

 ating variations, and as useful mutations are rare, 

 many useless mutations must be wiped out in the 

 struggle for life. If De Vries was able to observe a 

 certain number of mutations, it is because he took all 

 the precautions known to cultivation in order to pre- 

 serve them. 



The practical import of De Vries' observations is 

 the concrete evidence they give that new species can 

 appear and that their appearance may be due to other 

 factors besides slow and gradual variations. 



