CHAPTER XIII

EVOLUTION

Darwin’s work has been compared to that of Copernicus and Galileo inasmuch as all these men freed the mind from the incubus of Aristotelian philosophy which, with the efficient co-opera­tion of the church and the predatory system of economics, caused the stagna­tion, squalor, immorality, and misery of the Middle Ages. Copernicus and Galileo were the first to deliver the intellect from the idea of a universe created for the purpose of man; and Darwin rendered a similar service by his insistence that accidental and not purposeful varia­tions gave rise to the variety of organisms. In this struggle for intellectual freedom the names of Huxley and Haeckel must be gratefully remembered, since without them Darwin’s idea would not have conquered humanity.

Darwin assumed that the small fluctuating varia­tions could accumulate to larger varia­tions and thus cause new forms to originate.

It was the merit of de Vries[292] to have pointed out that fluctuating varia­tions are not hereditary and hence could not have played the rôle assigned to them by Darwin, while discontinuous varia­tions as they appear in the so-called “sports” or muta­tions are inherited. This was an important step in the history of the theory of evolu­tion. It did not touch the founda­tion of Darwin’s work, namely the substitu­tion of the idea of an accidental evolu­tion for that of a purposeful crea­tion; it only modified the concep­tion of the possible mechanism of evolu­tion. According to de Vries, there are special species or groups of species which are in a state of muta­tion. He considers the evening primrose on which he made his observa­tions as one of these forms. Morgan and his pupils have observed over 130 muta­tions in a fly Drosophila. From our present limited knowledge we must admit the possibility that the tendency toward the produc­tion of mutants is not equally strong in different forms. Whether this part of de Vries’s idea is or is not correct there can be no doubt that varia­tions occur which consist in the loss and apparently, though in rarer cases, in the gain or a modifica­tion of a Mendelian factor. If we wish to visualize the basis of such a change we may do so by imagining well-defined chemical constituents in one or more of the chromomeres undergoing a chemical change.

This way of looking at the origin of varia­tion has had the effect of putting an end to the vague specula­tions concerning the evolu­tion of one form from another. We demand today the experi­mental test when such a statement is made and as a consequence the amount of mere specula­tion in this field has diminished considerably.

It is possible that any further progress concerning evolu­tion must come by experi­mental attempts to bring about at will definite muta­tions. Such attempts have been reported but they are not all beyond the possibility of error.[293] The most remarkable among them are those by Tower who by a very complicated combina­tion of effects of temperature and moisture claims to have produced definite muta­tions in the potato beetle. The condi­tions for these experi­ments are so expensive and complicated that a repeti­tion by other investigators has not yet been possible.

It is, however, still uncertain whether the mere addi­tion or loss of Mendelian characters can lead to the origin of new species. Species specificity is determined by specific proteins (Chapter III.), while some Mendelian characters at least seem to be determined by hormones or substances which need neither be proteins nor specific for the species.