My colleague Lam, in whose honour this volume is composed, has made it very easy for contributors to find a subject in a field in which he has worked himself. His versatile interest nearly covered the whole of the taxonomy and phylogeny of vascular plants, subjects in theoretical biology, plant morphology, plant geography, and plant ecology. In the latter section his “Fragmenta papuana” contains an inspiring picture of tropical vegetation in correlation with environment. I have pleasure on this occasion in offering some considerations in the field of plant ecology. The subject which I have chosen deals with the way of reasoning when interpreting a correlation found to exist between vegetation and environment. I have not infrequently traced a deficiency in such interpretation and I feel a need of discussing this point which is, in my opinion, a matter of vital importance.