442 



SCIENCE. 



[N. S. Vol. XVII. No. 429. 



hypothesis is also the newest. It is that of 

 Professor De Vries, botanist, of Amster- 

 dam, who denies that natural selection is 

 competent to produce species, or that or- 

 ganic ascent is the product of smaU differ- 

 ences gradually enlarging into great ones. 

 According to De Vries 's view, species-char- 

 acters arise suddenly, or all at once, and 

 they are ordinarily stable from the moment 

 they arise. 



I. VARIATION: DE VRIES. 



De Vries conceives that variations, or 

 differences, are of two general categories: 

 (1) Vai'iation proper, or smair fluctuating, 

 unstable differences peculiar to the indi- 

 vidual (only partially transmitted to off- 

 spring), and (2) mutations, or differences 

 that are usually of marted character, ap- 

 pear suddenly and without transition to 

 other forms and are at once the starting- 

 points of new races or species. The vari- 

 ations proper may be due to the imme- 

 diate environment in which the plant lives. 

 The mutations are due to causes yet un- 

 known, although these causes are consid- 

 ered to be physiological. 



Natural selection works on both varia- 

 tions and mutations by eliminating the 

 forms that are least adapted to persist. It 

 is conceived to be a destructive, not a con- 

 structive or augmentative, agency. It 

 merely weeds out. 



"We may first consider selection with 

 reference to variations proper. Among 

 variations, or individual fluctuations, there 

 may be a slight cumulative effect of se- 

 lection, but it is incompetent ever to en- 

 large the differences into stable character- 

 istics; and when natural selection ceases 

 to act, the so-called variety falls back into 

 its original form or splits up into other 

 forms. Varieties of this kind are notably 

 indefinable and unstable. It is impossible 

 to 'fix' them in any true sense; selection 

 only preserves them, and when it is re- 



moved they perish as varieties. They are 

 relatively only temporary and have no ef- 

 fect on phylogeny. Many of the minor 

 adaptations of plants to the particular con- 

 ditions in which they chance for the time 

 being to be placed are of this category. 

 Much of the variation which results in 

 acclimatization belongs here. The fluctu- 

 ating horticultural varieties, and garden- 

 ers' 'strains,' are of this kind. This dis- 

 cussion of the effect of cessation of selec- 

 tion recalls Weismann's panmixia, a name 

 proposed to designate the breaking up of 

 varietal or specific characteristics when 

 natural selection ceases to act. Panmixia 

 is not of itself an original force or an 

 agency ; it is merely a name for the results 

 of all the forces or energies which are 

 allowed to assert themselves when the re- 

 stricting force of natural selection is re- 

 moved. In De Vries 's view, the progress 

 made by selection must be maintained by 

 selection. 



We may next consider selection with 

 reference to mutations. The mutations 

 are practically stable or 'fixed' the moment 

 they arise. Of course there may be indi- 

 vidual fluctuations, or variations proper, 

 amongst plants that have sprung from a 

 mutated individual; but the main charac- 

 teristics of the mutations are heritable. 

 An organism is a complex of organs and 

 attributes. Each attribute is a unit. Prom 

 any unit a new unit may arise by muta- 

 tion. The origination of a new unit con- 

 stitutes at once a full and important 

 character and marks the organism that 

 possesses it as a new physiological species. 

 Not only one unit, but any number of 

 units, may give rise to mutations; and any 

 one of these new mutations may give rise 

 to other mutations. But the point is that 

 these mutations, be they great or small, arise 

 by steps, are full-formed when they arise, 

 and do not grow or enlarge into other mu- 

 tations. The mutations are multifarious 



