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nomena of heredity and variability. The mutual interaction of these two factors gives the development in inorganic nature from the simple to the complicated. During the forty years since Darwin wrote the Origin of Species physiologists have, by experimental work, shown that heredity can be traced to the minute particles of matter.
On the variability side, it has remained for Hugo de Vries to point out in Die Mutationstheorie, Leipzig, 1901-3, just completed that Darwin has not spun out the thread of comparative experimentation. For twenty years de Vries has been making experiments on a large scale, concerning variability in plants. His book turns a new leaf in the history of evolution.
1. The chief result which marks de Vries's work as a step in advance is the conclusion that, while by means of fluctuating variability certain local and improved races may be bred, in nature new species never arise through this agency, but owe their origin exclusively to mutation, to discontinuous variability. Darwin recog- nized two kinds of variability fluctuating and discontinuous. By the Galton curve fluctuating variability has been shown to oscillate within limits. De Vries experimented with the " sports," " saults," the chance, single or discontinuous variations used commercially by nursery gardeners, accepting " mutation " as the name for the phenomenon so described. Wallace, who simultaneously with Darwin discovered natural selection, insists that fluctuating variability is the only source of new species. Thus de Vries and Wallace differ essentially.
2. With Darwin de Vries is less at variance. He quotes Darwin as saying: " The formation of a species I look at as almost wholly due to the selection of chance variations." Darwin also undoubtedly suspected the existence of a certain periodicity. I insist on this because of statements by those who bear a grudge, for other than scientific reasons, against Darwinism. The merit of de Vries is that his extensive experiments have provided a reliable basis concerning a subject about which Darwin had not fully made up his mind.
3. Let us try to picture what de Vries, with his conclusions on mutation, has taught us on the question : How have species originated ? De Vries started with one hundred species of plants. In one, and one only, he actually managed to detect the act of mutation on certain fields of Graveland. This one has continued to do so with perfect distinctness during many years in the Amsterdam Botanical Garden. Thus de Vries has been able actually to witness the very process of the origin of species in nature. For later investigators will be the task to make out how far the laws which de Vries has made out for one genus of plant apply to other plants and animals.
These laws are: (i) New elementary species arise suddenly, without transi- tions. (2) New elementary species are generally perfectly stable from the very first. (3) Most of the new types have all the qualities of elementary species, not of varieties. (4) The elementary species usually appear in a considerable number of individuals simultaneously, or at least within the same period. (5) No important relation whatever exists between individual variability and the new qualities of the elementary species. (6) The mutations which give rise to new elementary species take place in the most various and divergent directions. The modifications con- cern all the organs and are of the most varied descriptions. Part of the new types perish without descendants. Among the others natural selection must wholly decide. (7) The phenomenon of mutability appears periodically. A. A. W HUBRECHT, in Popular Science Monthly, July, 1904. H. E. F.
