DARVINISMO. — In D.’s work, one must distinguish the original scientific investigations from the speculative views that bear his name. D. was certainly one of the most productive and lucid naturalists of the nineteenth century, even though some of his hypotheses and interpretations, such as that concerning the formation of coral reefs or the circumnutation of climbing plants, have been abandoned. His investigations into fertilization in the plant kingdom (1858–77), which established the law of dichogamy, and into the importance of worms in the formation of humus, would suffice to preserve his fame. But concerning the problem of the “species,” which nevertheless occupied him constantly for 40 years, he accomplished nothing comparable to the work of Mendel, Naudin, or Jordan. In the history of biological philosophy and the social sciences, on the other hand, “Darvinismo” is important; by this one must properly understand the theory of the causes of the alleged transformation of living forms. D. began from the facts offered by the domestication of the breeds of animals that man has made domestic: how did the numerous breeds of pigeons, horses, cattle, and dogs arise? They were brought forth by the breeder’s “selection,” whether conscious or unconscious. Now, in nature there are species that vary, and the variability and heredity of the characters appearing in the varieties of species are perhaps effective in discovering the factor corresponding to the breeder’s artificial selection. D. postulated natural selection. The breeder’s selection draws from species the most diverse and distant varieties; natural selection, having a much longer period at its disposal, succeeds in making forms cross the boundaries of the species, that is, in producing new species. In the struggle for existence, individuals do not perish by chance; the less fit perish, while the fittest survive; natural selection is the consequence of the struggle for existence among continuously varying forms. Nature selects individuals that display and accentuate useful variations, and these are slowly modified until they produce new species. Darwinism is therefore based essentially on the analogy between natural (hypothetical) and artificial selection; natural selection accounts for the usefulness of organs and for adaptation. D. felt the paternity of this conception so strongly that, after 1859, he only with difficulty consented to “diminish the glory of selection” (letter to Hooker of 24 November 1862), acknowledging the importance of the “direct action of physical conditions” (on living organisms) and subsequently appealing to “sexual selection.” But that pure Darwinism, that is, the efficacy of natural selection, was merely an interpretive hypothesis, D. himself admitted: “Belief in natural selection must today be founded entirely upon general considerations. When we descend to particulars, we can prove that no species has changed—that is, we cannot prove that even a single species has changed, nor can we prove that the supposed changes are useful, which is the foundation of the theory” (letter to Bentham of 22 May 1863). In the last century, at least one conclusion was clear: artificial selection cannot produce a new species. Therefore, from the methodological point of view, Huxley was compelled to make the reservation: “I accept D.’s hypothesis with the reservation that evidence be produced of the possibility of producing physiological species as a result of elective mating.” The same experimental method dictated Spencer’s rigorous criticism, which he ultimately summarized as follows: “There is no inductive evidence of natural selection. The facts attesting the effects of artificial selection are very numerous, but none appears to attest the effects of natural selection. Until the production of a single species by natural selection has been demonstrated, there is not even the beginning of inductive proof.”
Then came the work of the geneticists, with Johansen at their head: the “pure race,” isolated through artificial selection, is fixed; for certain characters it fluctuates only within certain limits, beyond which it is impossible to proceed by means of small, continuous, hereditary variations.
Then came other observations, suggested to evolutionists themselves by the development of certain series of fossil forms, which they were obliged to call “orthogenetic”—that is, such as to suggest the idea of an “evolution” directed from the outset in a determinate direction, without that multitude of failed attempts, inefficient, useless, and unselected variations that the theory predicts and requires. According to Osborn, paleontology demonstrates that “in evolution law prevails over chance.”
It is therefore understandable that some naturalists, evolutionists by principle and philosophical preconception, should declare themselves anti-Darwinists and write that “Darwinism may very well have succeeded in making biologists themselves forget the obsessive problem of life” (Brien); V. EVOLUZIONE.