MUTATION THEORY OF PROFESSOR DF VRIES. (>35 



pursued uninterruptedly ever .since. It appears that his first and 

 guiding proposition in connection with his experimental work was 

 that, in the different periods of their chronological life history, all 

 plants vary greatly in the ratio of their mutability; that is, all species 

 and genera, while they are always subject to the full range of fluctuat- 

 ing variability, exist at times in a mutable and at times in an immuta- 

 ble condition, the latter condition being much the more prevalent. 

 Indeed, so prevalent is the immutable condition among plants that only 

 a small proportion of the species embraced in the flora of any given 

 region may be found existing in their mutable period. Furthermore, 

 it was to be expected that some plants, even when in the fullness of 

 their mutable period, would exhibit their mutability more readily than 

 others. The germ of this theory is contained in the authors little 

 book on intracellular pangenesis, 11 written just before he began his 

 experiments, wherein he gives the reasons which led him to believe 

 in mutability. His experimental studies here referred to were under- 

 taken for the purpose of discovering mutating plants and of demon- 

 strating his theory upon them. 



Professor de Vries's first effort was therefore toward the selection 

 of suitable plants for his experimental studies from the flora, both 

 native and introduced, which he found growing in the northern part 

 of Holland. For that purpose he placed under special cultivation in 

 experimental gardens at Amsterdam more than one hundred species of 

 plants, and prosecuted his preliminary experiments upon them. This 

 special cultivation was not for the purpose of producing horticultural 

 variation in those plants, but for the purpose of protecting and aiding 

 such of them as should prove to be in their mutative period. 



At the same time he also made numerous observations upon many 

 other plants in their natural habitat, with the same object in view. 

 The result of these preliminary experiments was that the so-called 

 Evening Primroses were found to respond more readily to mutative 

 influences than would any of the other species that came under his 

 observation. These are American species of plants, of the genus 

 (Enothera, that had been introduced into Holland, where they thrived 

 like native plants, both under cultivation and in a wild state. About 

 the year 1875 one of them, 0. Lamarckiana, began a most vigorous 

 multiplication and dispersion, especially from a center near the town 

 of Hilversum. This unusual exhibition of generative and dispersive 

 force was apparently correlated with the mutative impulse, for there 

 soon appeared among those plants two distinct species of (Enotliera 

 that were before unknown, although the flora of that and other regions 

 had long passed under the severe scrutiny of Professor de Vries and 

 other able botanists. The inference seemed to be legitimate that those 

 plants were then in the full vigor of their mutative period, and that the 



a See Intracellulare Pangenesis; pp. 212. von Hugo de Vries. Jena, 1889. 



