EXPERIMENTAL EVOLUTION I47 



as the starting point of a phylum. There is at present no clue whatever as 

 to what calls forth this essential difference in behavior. This is not surpris- 

 ing in view of the fact that there have been no comparative experimental 

 studies of stable and plastic species. Until these have been made, it is im- 

 possible to do more than to formulate a working hypothesis as to the effect 

 of stability, and an explanation of the forces which cause or control it is 

 altogether out of the question. 



186. Variation and mutation. New forms of plants are known to arise 

 by three methods, viz., variation, mutation, adaptation. The evidence in 

 support of these is almost wholly observational, and consequently more or 

 less inexact, but for each there exist a few accurate experiments which 

 are conclusive. Origin by variation and subsequent selection is the essence 

 of the Darwinian theory of the origin of species. According to this 

 the appearance of a new form is due to the accumulation, and selection, 

 through a long period, of minute differences which prove advantageous to 

 the plant in its competition with others in nature, or are desirable under 

 cultivation. Slight variations appear indiscriminately in every species. 

 Their cause is not known, but since they are found even in the most uniform 

 habitats, it is impossible to find any direct connection between them and the 

 physical factors; In the case of origin by mutation, the new form appears 

 suddenly, with definite characteristics fully developed. Selection, in the 

 usual sense of the term, does not enter into mutation at all, though the 

 persistence of the new form is still to be determined by competition. Muta- 

 tions are known at present for only a few species, and their actual appearance 

 has been studied in a very few cases. Like variations, they are indiscrimi- 

 nate in character. The chief difference between them is apparently one of 

 degree. Indeed, mutation lends itself readily to the hypothesis that it is 

 simply the sudden appearance of latent variations which have accumulated 

 within the plant. DeVries regards constancy as an essential feature of 

 mutation, but the evidence from the mutants of Onagra is not convincing. 

 Indeed, while there can be no question of the occurrence of mutation in 

 plants, a fact known for many years, the facts so far brought forward in 

 support of the "mutation theory" fall far short of proving "the lack of 

 significance of individual variability, and the high value of mutability for 

 the origin of species."^ Mutations do not show any direct connection with 

 the habitat, but their sudden appearance suggests that they may be latent 

 or delayed responses to the ordinary stimuli. Origin by adaptation is the 

 immediate consequence of the stimuli exerted by the physical factors of a 



^DeVries, H. Die Mutationstheorie, 1:6. 1901. 



