ORGANIC EVOLUTION—-MACNAMARA. 373 
occurred a new species is already in existence, and will remain in 
existence unless all the progeny of the mutatien are destroyed.” 
According to De Vries, therefore, species originate by mutation 
instead of by continuous selection. He adds: ‘‘Natural selection 
may explain the survival of the fittest, but it can not explain the 
arrival of the fittest.’’! 
Tt is clear that De Vries’s hypothesis of evolution of species by 
mutation, if established, would mean a profound change in the 
ideas received from Darwin. The survival of the fittest among a 
crowd of fresh elementary species or subspecies ready-made by 
mutation is a different conception from that of the progressive 
building up of the fittest types, by the improvement through selec- 
tion of existing characters and qualities, and the gradual addition of 
a unit here and a unit there to a complex structure of species.2 
Darwin, as far back as the year 1859, stated that it was the fine- 
ness of gradation in the shells of substages of the chalk formations, 
which led him to maintain the gradual, as against the sudden evolu- 
tion of species. 
The evidence upon which De Vries founds his hypothesis as to 
the sudden production of new species from existing types—that is, 
by mutations—is largely derived from his own observation of changes 
which took place in specimens of plants of the evening primrose (Zno- 
thera lamarckiana) he found growing in the sandy soil of a field at 
Hilversum. De Vries took seeds from two species of these wild 
plants and sowed them in a well-manured garden in Amsterdam. 
Seeds collected from these cultivated plants produced, according to 
De Vries, seven constant elementary species of the evening primrose; 
but these species differed so slightly from one another and from the 
parent stock, that we should rather refer them to varieties than as 
constituting distinct species. Varieties of this kind might be ac- 
counted for by a change of environment, the plants having origi- 
nally grown in a sandy soil and been transferred to a well-manured 
garden. De Vries, however, attributes the changes observed in 
these plants to the latent qualities possessed by the parent stock, 
He assumes that the characters of organisms are made up of ele- 
ments that are sharply separated from each other, and that at 
certain periods these elements become impressed by an impulsive 
mutability. It was at one of these periods De Vries supposes he 
chanced to secure his evening primrose, hence the changeful state 
of the plant and its production of seven elementary species within a 
short time. 
1 Heredity, by J. Arthur Thomson, M. A., regius professor of natural history in the University of Aber- 
deen, pp. 90, 98. 
2 The Quarterly Review, July, 1909, p. 26. E. B. Poulton, Darwin and his Modern Critics. 
