56 VARIATION. 



we will see later, experiments with selection in pure clones have 

 always given negative results, and the apparent exceptions are 

 founded upon a play of words. The negative results of selection 

 in clones show plainly that at least one of the three main 

 theories of evolution, namely that of Lamarck, is untenable. 



We will later show, why populations of habitually autoga- 

 mous plants tend to consist of mixtures of families, which each 

 consist only of individuals of identical genotype. Within each 

 of these families selection has proved to be wholly ineffectual. 

 We think it is not superfluous to repeat, that this does not im- 

 ply that in such families the variability isnecessar ly continu- 

 ous. As we saw in the case of de Vries' teazel, the variability in- 

 duced by the environment may be distinctly discontinuous. 



Inasfar as in this book we are concerned with the process of 

 evolution, we can simplify the question as to the causes of var- 

 iation, by leaving "modification" out of account, and asking: 

 How do individuals originate which have new combinations of 

 genes, a new genotype? 



If we put the question in this way, it becomes clear, that the 

 answer cannot be selection. Selection is of no effect in a popu- 

 lation without geno- variability, and therefore it can obviously 

 not produce geno- variation. The idea, that by selection small 

 fluctuating variations can be accumulated into inheritable var- 

 iations, has been proved untrue. Even these small, fluctu- 

 ating, continuous variations have been shown to be of two dis- 

 tinct kinds, the result of environmental action and the result of 

 geno- variation. 



Two possible causes of geno- variation remain, spontaneous 

 geno- variation and re combination of genes after crossing, We 

 know that crossing is a cause of geno- variation. It is possible 

 that outside crossing in the widest sense, geno variation can 

 take place spontaneously? And if so, in how far can we discover 

 the causes of such spontaneous geno- variation ? 



