In his first publications in the beginning of the twentieth century, Thomas Hunt Morgan claimed that the evolutionary process occurred by jumps and denied the principle of natural selection. Around 1915, taking into account his own work on the genetics of Drosophila, he started to accept that the evolutionary process could be gradual and that new genes offering slight advantages would gradually spread in the population. Only in 1932 he finally admitted that the evolutionary process was gradual and accepted natural selection as the main mechanism of evolution. In his early works Morgan argued that natural selection could not explain regeneration. However, in his publications after 1910 he omitted this criticism, although some objections and problems he had pointed out before had not been answered by himself or other researchers. This attitude is similar to the one he adopted concerning the chromosome theory in the first decade of the twentieth century: he devoted his efforts to a successful line of research, despite its foundational problems.