Evans: Hepaticae of Puerto Rico 289 



It will be seen from the foregoing discussion that our knowl- 

 edge of the species in this difficult genus is still far from complete. 

 At the present time it is often quite impossible to determine 

 whether differences are specific in value or merely due to varia- 

 tion. Apparently the plants will have to be studied in the field as 

 well as in the laboratory, to learn if possible something about the 

 causes which induce variation in them, before we can hope for 

 definite conclusions on certain disputed points. 



Yale University. 



