Periodic Mutations 691 



agency of external circumstances. Should this 

 be granted for the evening-primrose, it would 

 have to be predicated for other species found in 

 a mutable state. Then, of course, it would be 

 useless to investigate the causes of mutability 

 at large, and we should have to limit ourselves 

 to the testing of large numbers of plants in 

 order to ascertain which are mutable and 

 which not. 



If, on the other hand, mutability is not a per- 

 manent feature, it must once have had a begin- 

 ning, and this beginning itself must have had an 

 external cause. The amount of mutability and its 

 possible directions may be assumed to be due to 

 internal causes. The determination of the mo- 

 ment at which they will become active can never 

 be the result of internal causes. It must be as- 

 signed to some external factor, and as soon as 

 this is discovered the way for experimental in- 

 vestigation is open. 



In the second place we must consider the past. 

 On the supposition of permanency all the ances- 

 tors of the evening-primrose must have been 

 mutable. By the alternative view mutability 

 must have been a periodic phenomenon, produc- 

 ing at times new qualities, and at other times 

 leaving the plants unchanged during long suc- 

 cessions of generations. The present mutable 

 state must then have been preceded by an im- 



