character takes its origin from a previous one, not from its normal range of variation, but by some sudden change, small though this may be. This „speciesforming variability” will be called hare mutability, a word which was in genera! use before Darwin. The variations of this type, the mutations, emerge from processes, the nature of which is stil! very unclear.”

Takingthisquotation from de Vries’ work as our starting point, it appears that he means by mutation a sudden change in characters of an organism, which character again is raised from units. And when we set ourselves the task to define ~mutation in the sharpest way possible, we find first of all the necessity to tracé the causes, by which characters of organisms can change.

Since the publication of de Vries’ classical work thirty-five years have elapsed now, and in these years the building of genetics has been constructed on the foundations laid by Mendel and his rediscoverers, more or less skyscraperlike. And from the top of this skyscraper we are now able to observe the various causes by which differences in characters are produced. And we may state now that not so much the characters as the units are the pith of the matter.

Firstly there are modifications, changes in characters which are produced by externa! conditions without any change in the genotypica! constitutions and which usuaüy do not surpass the ümits drawn by the duration of the individua! üfe.

But not always: there is the possibility that such modifications, stamped upon one individual, pass on to its offspring, not only within vegetative clones, but also after sexual reproduction. It seems self-evident that such „continuous modifications (by which the suitable German term ~Dauermodijikationen” may be translated) occur chiefly with unicellular organisms, but it should be mentioned that, among higher animals and plants, analogous cases have been found. In his studies on the influences of high temperature on the development of the fruit-fly (Drosophila) Jollos has observed a clear case of such a continuous modification. In a culture of wild type animals with normal wings, by applying high temperatures on larvae of 5 or 6 days old, he obtained some individuals with a remarkable change in their wing structure, called the ..aeroplane” type. This abnormality seemed to be subject to matemal inheritance: normal females crossed with abnormal males brought normal