TO a student of biological distribution, Thornthwaite's recent "Climates of North America"' is of particular interest; for the effective humidity, which may be indicated by the relative humidity or by the relation of precipitation and evaporation, is rarely given in either form in Weather Bureau reports. Thornthwaite's formula, giving a means of calculating P /E from easily obtained data, then fills a serious gap. On comparing his map with known types of distribution, there appears to be a close coincidence over the greater part of the country; but on going over the Northeast, which is the part best known to me personally, I noticed two types of areas in which there seemed to be no visible correspondence between known distributions and the districts given on the map. Firstly, the mountain areas were very imperfectly represented if at all, and secondly the humid strip along the coast did not appear.