March 20, 1903.] 



SCIENCE. 



443 



{all-seitig), occurring apparently at ran- 

 dom and in diverse directions, and without 

 regard to fitness. They may be either 

 quantitative or qualitative. Variations 

 proper arise mostly in a definite line. 

 Now, natural selection may weed out mu- 

 tated individuals as it does mere variant 

 individuals; and thus breaks may arise in 

 the chain, and we have left what we know 

 as taxonomic species. 



Natural selection, with survival of the 

 fittest, is, therefore, of two distinct cate- 

 gories,— that which operates within the 

 species and results in the formation of 

 local minor races, and that which operates 

 between species and results in the forma- 

 tion of a line of ascent. 



Everywhere and always plants are vari- 

 able. Now and then and relatively rarely, 

 plants are mutative. Any man who sees 

 two plants, sees variation ; there are no two 

 plants alike. Only he who studies and ob- 

 serves critically, sees mutation. One must 

 examine a hundred or a thousand or ten 

 thousand individuals. In De Vries's ex- 

 tended experiments with CEnothera, only 

 1.5 per cent, of the plants were mutative, 

 and mutation is undoubtedly more common 

 in cultivation than in the wild, and the 

 mutated individuals are more likely to per- 

 sist. The investigator should employ only 

 statistical methods of comparison. He 

 should contrast unit-characters, rather than 

 individuals as a whole. Moreover, not 

 only are the numbers of mutating indi- 

 viduals relatively uncommon, but the spe- 

 cies may not now be in a- mutative epoch. 



In other words, there are epochs in the 

 history of the plant when mutations occur. 

 These are the 'mutation-periods' of De 

 Vries. There are epochs of non-mutations, 

 when no progress seems to be making. It 

 may be conceived that some force is then 

 withholding or restraining the mutative 

 impulse. This force is what we are in the 

 habit of calling heredity. "When heredity 



is overcome, there arises a ' premutation- 

 period,' in which the mutations are begin- 

 ning to express themselves ; and eventually 

 the full mutation-period may appear. 

 Heredity and non-heredity, these are the 

 ever-opposing and ever-contrasting forces 

 of organic life, the one resulting in the 

 survival of the like, the other resulting 

 in the survival of the unlike. One is 

 heredity; the other is variation. One 

 makes for continuity; the other for evolu- 

 tion. No hypothesis of the energy of evo- 

 lution is perfect that does not account for 

 both. A theory of heredity, or continuity, 

 must also account for the opposite of itself. 

 It is not easy to construct an hypothesis or 

 a metaphor that will accomplish this. 



The phenomena of continuity and dis- 

 continuity are well contrasted by Kor- 

 schinsky. These phenomena, he conceives, 

 are the results of two antagonistic tend- 

 encies. Under normal or usual conditions 

 heredity is the stronger force. The 

 tendency to vary is always present, be- 

 ing predisposed by environment but not 

 caused by it ; when it gathers the necessary 

 energy it overbreaks the power of inherit- 

 ance and sudden variations or sports arise, 

 and these sports are the starting-points of 

 evoliition. This sudden appearing of new 

 forms is called by him heterogenesis. 



The conceptions of per saltum variations 

 of Korschinsky and De Vries seem to be 

 practically identical. De Vries has carried 

 his work further, into the realm of actual 

 experimental investigation. He stiidied 

 many species of plants in the hope of find- 

 ing one or more that might be in its muta- 

 tion-period. Finally, he chose the common 

 evening primrose, CEnothera Lamarchiana, 

 and by continual sowing of seeds and rais- 

 ing of great numbers of plants he discov- 

 ered several truly mutative forms. These 

 forms reproduce themselves by means of 

 seeds as acciirately as accepted species do. 

 He has given them specific names. The full 



