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 1 ." 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 2 ": 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, Nageli 3 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 Nageli 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 Nageli'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. ni. p. 342. 



3 Nageli, Theorie der Abstammungslehre, Munich, 1884; cf. Chapter in. 



D. 15 



