EVOLUTION 



345 



were practically wanting. The Ijody color, normally a brownish graj'', has 

 changed to yellow and black. Additional bristles on the body and bristles 

 forked at the end, are other new features. Besides these, there are a 

 number of physiological changes not detectible by morphological means, 

 but easily demonstrated by breeding experiments. More than two hun- 

 dred mutations of all kinds have been detected. All these new features 

 are inherited. All of them therefore exemplify the process of evolution. 

 Some of these mutations closely parallel changes that have evidently 

 occurred in wild flies, showing natural evolution to be like that which oc- 

 curs in captivity; and some of the modifications (in bristle characters, for 

 example) concern features used in the classification of flies in nature. 

 Furthermore, new characters arising in nature have not always been 



Fig. 236. — Mutations in the fruitfly Drosophila melanog aster. A, normal wing; B, 

 beaded wing; C, notch wing; D, vestigial wing; E, miniature wing; F, club wing. G, rudiment- 

 ary wing; H, truncate wing; 1, normal red eye; J, bar eye; K, eyeless; L, white eye. (C from 

 Morgan; D and L original; the rest from Morgan, Sturtevant, Muller and Bridges, courtesy 

 of Henry Holt and Co.) 



advantageous. The question whether the individuals possessing the new 

 characters would have survived if they had occurred wild instead of in a 

 laboratory is of no importance in this connection. Survival would be of 

 importance in deciding what kind of animals would be left after evolu- 

 tion had been taking place for a time. Survival must be taken into account 

 in explaining the species alive on the earth today, for without doubt mil- 

 lions of evolutionary changes have taken place without leaving a trace 

 either in Hving animals or in the fossils of past animals. But a change 

 which is temporary, merely because the only individuals which possess 

 it perish, is not for that reason without the pale of evolution. All that 

 is required is that the new character be one which will be, in case of its 

 possessor's survival, transmitted to its progeny. 



Other organisms furnish equally good examples of mutations. The 

 evening primrose, Oenothera, is every year producing new forms. Seeds 

 of plants that have been self-fertilized, and whose ancestors have been 

 self-fertilized for generations, and which should therefore be genetically 

 nearly pure, give rise to plants strikingly unlike their parents in certain 



