Variability 225 
more important factor in the production of new species; as a result 
of this, single individuals are distinguished from one another by 
“slight” differences, first in one then in another character. There 
may also occur, though this is very rare, more marked modifications, 
“variations which seem to us in our ignorance to arise spon- 
taneously.” The selection theory demands the further postulate 
that such changes, “whether extremely slight or strongly marked,” 
are inherited. Darwin was no nearer to an experimental proof of 
this assumption than to the discovery of the actual cause of varia- 
bility. It was not until the later years of his life that Darwin was 
occupied with the “perplexing problem...what causes almost every 
cultivated plant to vary?”: he began to make experiments on the 
influence of the soil, but these were soon given up. 
In the course of the violent controversy which was the outcome of 
Darwin’s work the fundamental principles of his teaching were not 
advanced by any decisive observations. Among the supporters and 
opponents, Nigeli® was one of the few who sought to obtain proofs 
by experimental methods. His extensive cultural experiments with 
alpine Hieracia led him to form the opinion that the changes which 
are induced by an alteration in the food-supply, in climate or in 
habitat, are not inherited and are therefore of no importance from 
the point of view of the production of species. And yet Nigeli did 
attribute an important influence to the external world; he believed 
that adaptations of plants arise as reactions to continuous stimuli, 
which supply a need and are therefore useful. These opinions, which 
recall the teleological aspect of Lamarckism, are entirely unsupported 
by proof. While other far-reaching attempts at an explanation of the 
theory of descent were formulated both in Nigeli’s time and afterwards, 
some in support of, others in opposition to Darwin, the necessity 
of investigating, from different standpoints, the underlying causes, 
variability and heredity, was more and more realised. To this category 
belong the statistical investigations undertaken by Quetelet and 
Galton, the researches into hybridisation, to which an impetus was 
given by the re-discovery of the Mendelian law of segregation, as 
also by the culture experiments on mutating species following the 
work of de Vries, and lastly the consideration of the question how 
far variation and heredity are governed by external influences. 
These latter problems, which are concerned in general with the 
causes of form-production and form-modification, may be treated in 
a short summary which falls under two heads, one having reference 
to the conditions of form-production in single species, the other 
1 Origin of Species (6th edit.), p. 421. 
2 Life and Letters, Vol. m1. p. 342. 
8 Nageli, Theorie der Abstammungslehre, Munich, 1884; ef. Chapter mx. 
Dd 15 
