588 THE HISTORY OF BIOLOGY 



and succumb to natural selection. All fresh characters have thus been formed 

 as a result of mutations; between the mutations a species survives with its 

 characters unchanged; the slight variations that occur daily in the life of 

 the species have no effect on evolution, because they are not hereditary, and 

 the recombinations of characters that arise through the crossing of different 

 forms have no new significance. "As many steps as an organism has made 

 from the beginning, so many mutation periods must have occurred." These 

 mutation periods, de Vries believes, must have arisen at a far more rapid 

 pace during previous geological periods than they do nowadays, and he is 

 thus able to explain on the basis of this theory the origin of the forms of 

 life without the assumption of those infinite spaces in time that the old 

 Darwinism required at its disposal. 



When it first appeared, de Vries's theory naturally met with violent 

 opposition on the part of loyal Darwinists. It was certainly admitted that 

 in all essentials he really accepted the point of view of the old Darwinism, 

 in that he maintained the theory of natural selection as the principle govern- 

 ing life, but the fact that he denied the heredity of the slight variations 

 and their importance for selection and maintained the immutability of species 

 in the normal existence between mutations was far too much at variance 

 with old traditional ideas to be acceptable. Every possible effort was made 

 to get away from the facts that he had adduced and the conclusions that 

 he drew therefrom. As a matter of fact, these certainly have been open to 

 objection. His cultural experiments with CEnothera have failed to withstand 

 the criticism of later years. Johannsen has objected that the material with 

 which the experiment was carried out was casually selected and was not 

 kept as pure as it should have been, and finally a Swedish naturalist, Heri- 

 BERT-NiLssoN, Carried out the entire experiment over again and came to the 

 conclusion that the new generations of Oenothera lamarckiana only show fresh 

 combinations of characters that already existed in the main species. De Vries, 

 who was one of those who rediscovered Mendel's law of cleavage, has, in 

 fact, denied the validity of that law as regards mutations, such as those of 

 CEnothera, but this has been found to be a mistake. Consequently his theory 

 of the formation of new species of that plant collapsed. His service to sci- 

 ence, on the other hand, lies in the fact that he revealed the phenomenon 

 of mutation, for that this phenomenon exists has since been proved over 

 and over again. Moreover, on the basis of his "pangen" theory he insisted 

 upon the necessity of analysing with regard to their elementary units the 

 hereditary qualities that characterize the species. "It is not a question," he 

 says, "of the origin of species, but of the development of the species-char- 

 acters." Through these assertions he became a pioneer of modern heredity- 

 research. "His mutation theory," Johannsen declares, "has represented the 

 principal milestone in the transition from the old ideas to the modern 



