change. Many of the forests have not yet been studied in detail while field work in others is still in progress. Current ideas concerning them are, therefore, in a state of flux but it might, nevertheless, be pointed out that few items have been discovered over the past several years (the first version of this report was written in 1948) that have not proved susceptible of ready explanation in terms of the working hypothesis. No facts have been discovered which would serve in any way to disprove the hypothesis.