, 1875 II. pages 426, 427.) But
the building-stones themselves, the variations, have their basis in the
influences which cause variation in those vital units which are handed
on from one generation to another, whether, taken together they form the
WHOLE organism, as in Bacteria and other low forms of life, or only
a germ-substance, as in unicellular and multicellular organisms. (The
Author and Editor are indebted to Professor Poulton for kindly assisting
in the revision of the proof of this Essay.)

IV. VARIATION. By HUGO DE VRIES.

Professor of Botany in the University of Amsterdam.

I. DIFFERENT KINDS OF VARIABILITY.

Before 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 monstrosities. 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 useless. On the
other hand, he strongly combated the belief, that great c