88 
THE PLANT WORLD. 
firmed his results and added new and valuable contributions to this 
important phase of the experimental study of evolution. 
The introductory chapter reviews the various theories of evolution and 
methods of investigation. The theory of descent and of natural selection 
are discussed, and the true function of natural selection emphasized by 
the use of the striking figure of the sieve. Natural selection “ is the 
sifting out of all organisms of minor worth through the struggle for 
life. It is only a sieve, and not a force of nature, no direct cause of 
improvement, as many of Darwin’s adversaries, and unfortunately many 
of his followers also, have so often asserted. It is only a sieve, which 
decides which is to-live, and what is to die. But evolutionary lines are 
of great length, and the evolution of a flower or of an insectivorous plant 
is a way with many side paths. It is the sieve that keeps evolution on 
the main line, killing all, or nearly all, that try to go in other directions. 
By this means natural selection is the one directing cause of the broad 
lines of evolution. 
“ Of course with the single steps of evolution it has nothing to do. 
Only after the step has been taken, the sieve acts, eliminating the unfit. 
The problem as to how the individual steps are brought about, is quite 
another side of the question.” 
As to this last point two methods are conceivable: (i) fluctuations, and 
(2) mutations. Of these Darwin recognized both, Wallace only the first. 
At present there are two camps of evolutionists, the Neo-Lamarckians, 
who hold that organisms may be changed by the direct influence of 
environment, and the Darwinians, who “ assume fluctuating variations 
in all directions and leave the choice between them to the sieve of natural 
selection.” 
DeVries elaborates his theory first “by a critical survey of the facts of 
agricultural and horticultural breeding,” “ combined with some correspond¬ 
ing experiments concerning the real nature of species in the real state ”; 
second, by his own observations and experiments at Amsterdam. 
After describing the methods of scientific pedigree culture, the dis¬ 
tinction between species and “ elementary species ” is carefully drawn. 
Species “ are the practical units of the systematists and florists, and . . . 
are not really existing entities.” They may be broken up into varieties 
and sub-species. The real units are the elementary species; “. . . any 
form which remains constant and distinct from its allies in the garden is 
to be considered as an elementary species” (p. 12). Elementary species 
are exactly what Linnaeus designated as varieties. 
Elementary species must not be confused with retrograde varieties. The 
latter result from the loss of some special character, while the former 
have originated in a progressive way and attained something entirely new. 
Thus orchids have originated by progressive development, hundreds of 
steps being required. 
Retrogression is as much evolution as is progression. Monocotyledons 
have evolved from primitive dicotyles by the loss of characters. The 
investigator in experimental evolution deals with the single steps, and 
