In his early publications in the beginning of the 20th century, Thomas Hunt Morgan (1866-1945) considered that the evolutionary process took place by jumps and criticized several features of Darwin's evolutionary proposal. He thought that regeneration could not have been acquired through natural selection, for instance. At this time he was also a strong opponent of both Mendelian and chromosome theories. This happened until 1910-1911. In the books A critique of the theory of evolution (1916) and its revised edition Evolution and genetics (1925) in which he intended “to review the evidence on which the old theory rested its case, in the light of the newer evidence of the recent years”, Morgan did not discuss regeneration nor mentioned the objections he had raised before against the action of natural selection in this process. He mainly presented the evidence got from the experimental breeding of Drosophila, claiming that the slight mutations that were inherited, according to “Mendel's law” furnished the material in which natural selection would act. Only in 1932 he finally admitted that the evolutionary process was gradual and accepted natural selection as the main mechanism of evolution. The aim of this communication is to discuss the epistemic and non epistemic factors that could have contributed to Morgan's change of view concerning the subject, during the first decades of the 20th century, trying to elucidate at to extent the studies developed by Drosophila group could have contributed to it.