After the great achievements in the field of molecular foundations of genetics and protein synthesis, molecular biology undertook the successful deciphering of a number of other important biological problems. By this time ecology in its various branches was far enough advanced to tackle the problems arising at the level of molecular biology. The monograph of Professor Alexandrov, which takes as an example the adaptation of organisms to habitat temperatures, presents a vivid picture of this major ecological problem as viewed at the cellular and molecular levels. As main theme of the book the author advances a hypothesis on a correlation between the level of conformational flexibility of protein molecules and the temperature ecology of a species, as a result of which the protein molecules are maintained in a semilabile state. This principle may also be applied to other factors of the environment which affect the level of flexibility of protein macro­ molecules. The principle of semistability is shown to be applicable also to the nucleic and fatty acids.