Variability in Cereals 77 



The external causes of this curious period of mutability are as yet 

 wholly unknown and can hardly be guessed at, since the origin of 

 the Oenothera Lamarckiana is veiled in mystery. The seeds, intro- 

 duced into England about 1860, were said to have come from Texas, 

 but whether from wild or from cultivated plants we do not know. 

 Nor has the species been recorded as having been observed in the 

 wild condition. This, however, is nothing peculiar. The European 

 types of Oenothera biennis and O. muricata are in the same condition. 

 The first is said to have been introduced from Virginia, and the 

 second from Canada, but both probably from plants cultivated in the 

 gardens of these countries. Whether the same elementary species 

 are still growing on those spots is unknown, mainly because the 

 different sub-species of the species mentioned have not been system- 

 atically studied and distinguished. 



The origin of new species, which is in part the effect of mutability, 

 is, however, due mainly to natural selection. Mutability provides the 

 new characters and new elementary species. Natural selection, on 

 the other hand, decides what is to live and what to die. Mutability 

 seems to be free, and not restricted to previously determined lines. 

 Selection, however, may take place along the same main lines in 

 the course of long geological epochs, thus directing the development 

 of large branches of the animal and vegetable kingdom. In natural 

 selection it is evident that nutrition and environment are the main 

 factors. But it is probable that, while nutrition may be one of the 

 main causes of mutability, environment may play the chief part in 

 the decisions ascribed to natural selection. Relations to neighbour- 

 ing plants and to injurious or useful animals, have been considered 

 the most important determining factors ever since the time when 

 Darwin pointed out their prevailing influence. 



From this discussion of the main causes of variability we may 

 derive the proposition that the study of every phenomenon in the 

 field of heredity, of variability, and of the origin of new species will 

 have to be considered from two standpoints ; on one hand we have 

 the internal causes, on the other the external ones. Sometimes the 

 first are more easily detected, in other cases the latter are more 

 accessible to investigation. But the complete elucidation of any 

 phenomenon of life must always combine the study of the influence 

 of internal with that of external causes. 



III. 



Polymorphic variability in cereals. 



One of the propositions of Darwin's theory of the struggle for life 

 maintains that the largest amount of life can be supported on any 



