238 BOTANY; PRINCIPLES AND PROBLEMS 



never been able to produce by artificial selection a group of 

 individuals which could clearly be regarded as a new species? 



These and other objections have been answered in whole or 

 in part by Darwinians, but they are still of sufficient weight to 

 convince most biologists that the theory just as Darwin left it 

 cannot well be maintained today. No doubt natural selection 

 eliminates vast numbers of obviously unfit individuals, but that 

 it has been the most important factor in producing new forms, 

 and thus in directing evolutionary progress, is now rather 

 generally doubted. 



De Vries's Theory. — Another attempt to explain the cause and 

 method of evolution has been made in recent years by the Dutch 

 botanist de Vries, who believes that the small and almost imper- 

 ceptible variations, regarded as most important by Darwin, are 

 merely "fluctuations" around the normal type which have been 

 produced by the environment and are therefore not inheritable. 

 The real variations which lead to evolutionary change, according 

 to de Vries, are the mutations. These are permanent and 

 inheritable, and are often large and conspicuous. The founder 

 of the Mutation Theory thus looks upon organic nature as advanc- 

 ing by distinct and usually rather long steps rather than by an 

 almost infinite number of small ones. 



Although de Vries recognizes the importance of natural 

 selection in evolution, his theory has certain advantages over 

 that put forward by Darwin. If complex new characters and 

 even new varieties and species can arise by one or even a few 

 steps, the problem of the preservation of the early stages in the 

 development of a useful structure is partly solved, and the exis- 

 tence side by side of distinct but very similar species is explained. 

 The length of time necessary for evolution is also reduced. Many 

 of the objections which have been urged against Darwin's theory, 

 however, apply with equal force to that of de Vries, and although 

 the latter has taught us the necessity of distinguishing sharply 

 between inheritable and non-inheritable characters in evolution, 

 it has not been accepted as a complete solution of the problem. 



Orthogenesis. — These various theories lack a convincing 

 explanation of the progressive appearance of new characters and 

 their harmonious incorporation into the organism. The environ- 

 ment evidently cannot produce them, and it seems unlikely 

 that mere random variations, whether large or small, would be 

 any more successful. In view of all this, some biologists have 



