Periodic Mutations 699 



Species of algae belonging to the well-pre- 

 served group of the diatoms, are said to have 

 remained unchanged from the Carboniferous 

 period up to the present time. 



Summing up the results of this very hasty 

 survey, we may assert that species remain un- 

 changed for indefinite periods, while at times 

 they are in the alternative condition. Then at 

 once they produce new forms often in large 

 numbers, giving rise to swarms of subspecies. 

 All facts point to the conclusion that these pe- 

 riods of stability and mutability alternate more 

 or less regularly with one another. Of course 

 a direct proof of this view cannot, as yet, be 

 given, but this conclusion is forced upon us by 

 a consideration. of known facts bearing on the 

 principle of constancy and evolution. 



If we are right in this general conception, we 

 may ask further, what is to be the exact place 

 of our group of new evening-primroses in this 

 theory? In order to give an adequate answer, 

 we must consider the whole range of the obser- 

 vations from a broader point of view. First of 

 all it is evident that the real mutating period 

 must be assumed to be much longer than the 

 time covered by my observations. Neither the 

 beginning nor the end have been seen. It is 

 quite obvious that Oenothe^^a lamarchiana 

 was in a mutating condition when I first 



