THE origin of species, the way in which evolution has taken place and is still progressing, seems, even now, to remain an open question. So much so, that three absolutely diverse theo­ ries of evolution still find their adherents. These three main theories are so different, that it would seem as if the real na­ ture of the process of species-formation must for ever remain a subject for speculation only, and as if good facts are wholly wanting. A minority of Biologists adhere to Lamarck's theory of the inheritance of adaptative changes induced by the environ­ ment; some incline to the view of Darwin and Weismann, that natural selection on· small, individual variations gradually changes species, and still others believe with de Vries, that new species spring into being spontaneously, by mutation, saltation. Judging from this diversity of opinion, it would certainly seem as if no important headway had been made since Darwin. This is very surprising, when we see how in the last fifteen years Genetics has become established as an actively develop­ ing branch of science, and counts by hundreds the Biolo­ gists who are engaged in genetical experimental investigations. All the data thus acquired, should have their significance for an insight into the process of evolution, the origin of species.