DIV. II 



PHYSIOLOGY 



325 



descendants. If the material to begin with is really pure, selection 

 has no effect. 



COMBINATIONS. When a plant originates not from self-fertilisation 

 but from a cross, this may be termed a hybrid even if its parents 

 belonged to very nearly related races. In this sense in every cross 

 between two individuals heterozygotes must appear. The descend- 

 ants of a hybrid will have the characters of the one parent or of 

 the other or of both, and will thus appear diverse. This form of 

 variation is superficially not to be distinguished from modification, 

 for it can also show the curve of chance. It is, however, essentially 

 different since it is inheritable. The descendants vary according to 

 the Mendelian rules. This form of variation is termed combination. 



FIG. 272. Habit of 1, Chelidonium majus; 2, Chelidonium majus laciniatum. (After LEHMANN.) 



MUTATIONS ( S6 ) are variations that are distinguished from com- 

 binations in not having arisen by hybridisation, but resemble them in 

 being inherited. Mutations can only be recognised with certainty 

 under experimental conditions, when in the descendants of a pure line 

 individuals appear which possess a new character or are wanting in 

 a character of the parent organism, the departure being maintained 

 in their offspring. The appearance of such mutations has been 

 observed in experiments both with seedlings and with buds. It is also 

 highly probable that many variations met with in nature should be 

 regarded as mutations. Thus, for example, Chelidonium laciniatum, a 

 mutation of Chelidonium majus with incised leaves, was found at 

 Heidelberg in 1590 (Fig. 272). Fragraria monophylla, which was first 

 noticed in 1761, differs from the ancestral form of the Strawberry 

 in having simple instead of trifoliate leaves. The remarkable 



