IV 
VARIATION 
By Huco DE VRIEs, 
Professor of Botany in the University of Amsterdam. 
I. 
Different kinds of variabihty. 
BeroreE Darwin, little was known concerning the phenomena of 
variability. The fact, that hardly two leaves on a tree were exactly 
the same, could not escape observation: small deviations of the same 
kind were met with everywhere, among individuals as well as among 
the organs of the same plant. Larger aberrations, spoken of as 
monstrosities, were for a long time regarded as lying outside the 
range of ordinary phenomena. A special branch of inquiry, that of 
Teratology, was devoted to them, but it constituted a science by 
itself, sometimes connected with morphology, but having scarcely 
any bearing on the processes of evolution and heredity. 
Darwin was the first to take a broad survey of the whole range 
of variations in the animal and vegetable kingdoms. His theory of 
Natural Selection is based on the fact of variability. In order 
that this foundation should be as strong as possible he collected all 
the facts, scattered in the literature of his time, and tried to arrange 
them in a scientific way. He succeeded in showing that variations 
may be grouped along a line of almost continuous gradations, 
beginning with simple differences in size and ending with monstro- 
sities. He was struck by the fact that, as a rule, the smaller the- 
deviations, the more frequently they appear, very abrupt breaks in 
characters being of rare occurrence. 
Among these numerous degrees of variability Darwin was always 
on the look out for those which might, with the greatest probability, 
be considered as affording material for natural selection to act upon 
in the development of new species. Neither of the extremes complied 
with his conceptions. He often pointed out, that there are a good 
many small fluctuations, which in this respect must be absolutely 
