His idea of gemmules is not. Mutations Prominent among the detractors of Darwin was Hugo de Vries in Holland who, around the turn of the century, suggested larger evolutionary changes called mutations. He considered these to be the significant evolutionary process, in contrast to Darwin's smaller changes. While the interpretation of his experiments turned out to be largely erroneous, real mutations were discovered later by F. H. Morgan. Unfortunately for the evolutionary viewpoint, these changes turned out to be overwhelmingly detrimental.
Some evolutionists still stake their hopes on the potential of a few beneficial mutations.
Early this century, R. A. Fisher in England and Sewall Wright in the United States developed sophisticated mathematical models of evolution that helped shift the emphasis of an evolutionary mechanism from individual organisms to populations. Fisher emphasized small changes in large populations. Wright wanted smaller populations to facilitate the manifestations of new mutations, but not so small as to engender the deleterious effects of inbreeding. The question of proper population sizes for progressive evolution is still debated. The modern synthesis is a vague combination of the mutation concept and Darwin's idea of survival of the fittest.