Nature 170: 680 (Oct 25, 1952)
Factors of Evolution: The Theory of Stabilizing Selection.
By I. I. Schmalhausen. Translated by Isadore Dordick. Edited by Theodosius Dobzhansky.
Pp. xiv+327. (Philadelphia: The Blakiston Company, 1949.) 6 dollars.

ALTHOUGH I. I. Schmalhausen's book appeared in Russian in 1947, and in an American translation as long ago as 1949, it has received rather little attention in Great Britain, and it still seems worth while to review it. It is a ponderous work, written in a style which, one suspects, was oven before translation heavy and repetitious, and the translator has not succeeded in making it easy to read. But the labour brings rewards. The book contains accounts of most of the recent experimental and theoretical work on the genetical mechanisms of evolution; but it is not in this that its main value lies: indeed, better summaries of this material will be found in the well-known monographs of Dobzhansky, Huxley and others. Schmalhausen provides rather fuller references than most such publications to recent Russian work, which is not so accessible, and this already gives his book certain distinctive usefulness.

The main value of the work, however, lies in the particular point of view which Schmalhausen has derived from his studies as an embryologist, and which differentiates him from most of the other recent writers on evolution. He makes a consistent attempt to envisage the evolving animal, not as a mere adult form, nor as an assemblage of genes, but as a developing being characterized by a certain norm of reaction to its surroundings. This leads him to emphasize the conservative role of selection. He lays stress on the distinction between 'dynamic selection', which brings about changes in the animals submitted to it by favouring certain new types, and 'stabilizing selection', which perpetuates the norm by eliminating deviants. He points out that stabilizing selection does not merely conserve the status quo; it will actually entrench the prevalent and well-adapted norm, rendering its development more stable and less responsive to disturbing influences. It would have made Schmalhausen's thought clearer if he had distinguished these two aspects, both of which are included in his use of the term 'stabilizing selection' as Simpson has pointed out in a review in the Journal of Heredity, it is also used in still other different senses.

Schmalhausen directs attention to the importance of stabilizing selection as n mechanism for the evolution of adaptations. He suggests that an originally variable but adaptive response to an environmental stimulus may become stabilized and will then eventually arise autonomously-that is, by heredity. Owing to the lack of precision of the general phrase 'stabilizing selection', it is not easy to make quite certain how Schmalhausen envisaged the passage from the 'acquired' to the 'inherited' status of the character. Sometimes he seems to be thinking along the lines of the theory of 'organic selection', suggested some fifty years ago by Baldwin, according to which an adaptive environmental response acts by enabling a species to occupy an otherwise unsuitable niche until such time as a mutation turns up which endows it with a hereditary adaptation. In other places he seems to imply, but perhaps never quite clearly states, what is surely the more fruitful suggestion, that if a response to an abnormal environment becomes stabilized in the sense of being rendered less susceptible to disturbing influences, this means that it will be less easily abolished when the abnormal environmental influence is removed, and will in this way pass from being dependent on the environment to being relatively independent of it. Similar ideas were put forward in a short article by me, using rather different terminology, at about the same time as Schmalhausen's book first appeared; but in this latter work they have received for the first time a full-length discussion against the background of the relevant embryological, genetical and evolutionary facts.

Waddington Bibliography