This paper investigates the reception and discussion of Jakob von Uexküll’s biological theory by two German thinkers of his time, Helmuth Plessner and Kurt Goldstein. It demonstrates how their bio-philosophical perspectives are on the one hand indebted to Uexküll’s theory and, on the other, critical of its tendency to excessively harmonize the relationship between living beings and their environment. This original critical reading of the _Umweltlehre_ is rooted in ambiguities within Uexküll’s own thought - between a dynamic conception of the (...) organism-environment relationship and the idea of "conformity to a plan" -,, which is here examined in the second section. In the third and fourth sections we will then focus on Plessner and Goldstein respectively, demonstrating how for these two authors the harmony between organism and environment is not an original state, but only reveals itself against the background of a tension; as such, it can only be partial, unstable and always changing. The two thinkers avoid the rigid alternative between Darwin’s concept of adaptation (_Anpassung_) and Uexküll’s “fitting into” (_Einpassung_) by theorizing the ideal state of the relationship between organism and environment in terms of “adequacy” (_Adäquatheit_) and “adaptability” (_Adaptiertheit_). Between organism and environment there is neither absolute separation nor perfect harmony, but rather a gap which can never be definitively fixed. (shrink)