MODIFICATION OF SPECIES 



355 



should not be called mutations, but the name has been applied to 

 them. 



]\Iodifications that are due to changes of single genes — and hence are 

 true mutations — have, however, been abundantly witnessed in other 

 organisms. Over a thousand alterations have occurred in pedigreed 

 strains of the vinegar fly Drosophila melanogaster, and many of these 



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Fig. 302. — Mutation in Oenothera involving the rosettes, or young plants. Below 

 (8 and 9), Oenothera pratincola; above (3 and 4), Oenothera pratincola mutation nummularia, 

 a mutant of the preceding form. {Photograph by Prof. H. H. Bartlett.) 



are presumabl}^ changes in single genes. The first of these mutations to 

 be discovered was a change from red eye to white in one fly in the labora- 

 tory of Prof. T. H. Morgan in the year 1910. Since then there has been 

 almost a continuous procession of mutations, affecting eyes, wings, body 

 color, bristles, legs, antennae, and physiological properties (Fig. 303). 

 Most of these mutants have been bred so that the mode of inheritance of 

 their new characters was ascertained, and most of them turned out to be 



