354 



PRINCIPLES OF ANIMAL BIOLOGY 



Mendel's law (page 18), in the evening primrose Oenothera lamarckiana 

 before the year 1890. Since that time, individuals of this species and 

 others of the same genus have continued to produce offspring unlike 

 themselves in some permanent way. Not a year passes without the 

 production of one or more new forms. Some of them represent changes 



Fig. 301. — Mutation in Oenothera involving the length of the seed capsule. The two 

 specimens at the left arc Oenothera reynoldsii mutation dehilis, a foiin which gives lise by 

 mutation to the form represented by the two figures at the right, Oenothera reynoldsii 

 mutation bilonga. {Photograph by Prof. H. H . Bartlett.) 



in the seed capsules (Fig. 301), others the whole habit of growth. Some 

 mutations are detectible only in the adult plant, others in the young stage 

 known as the rosette (Fig. 302). The alterations arising in Oenothera 

 are not the simplest examples of evolutionary change, for it has been 

 found that most of them are due not to simple changes of genes but to 

 rearrangement of large fragments of the chromosomes and ]-egrouping of 

 whole chromosomes that adhere to one another. Probably such changes 



