150 WORLD-POWER AND EVOLUTION 



the new form. Such mutations seem thus far to have been pro- 

 duced artificially by only two methods. One is by exposing the 

 immature ovules or germ cells to some artificial stimulus. The 

 stimulus may be produced through the injection of a weak chemi- 

 cal solution into the ovary at an early stage of development. In 

 this way MacDougal appears to have obtained a genuine new 

 species of primrose. Other possible stimuli are mechanical shocks, 

 electricity, and light of different colors, but none of these seems 

 to have led to the production of really new forms. The other 

 method of causing mutations is by keeping the developing germ 

 cells under extreme conditions of temperature or humidity during 

 certain critical periods. In the case of plants it is probable that 

 this has occurred again and again although it is not easy to point 

 to specific cases. In the case of animals, with which we are now 

 chiefly concerned, it has occurred in a number of well-authenticated 

 instances, some of which we shall now describe. 



One of the best-known instances of changes in animals through 

 the effect of climatic extremes is the butterfly. Standfuss, 

 Fischer, Merrifield, and others have experimented along this line. 

 To take the wo-rk of only one of these students, Standfuss raised 

 about 42,000 pupa? belonging to about sixty species. His method 

 was to expose the eggs of the butterflies to various temperatures 

 for longer or shorter periods. By exposure to both cold and heat 

 he found that the coloring and other conditions of the butterflies 

 are altered. For instance, a butterfly called Vanessa levana has 

 two distinct generations each year. The summer form of the 

 creature lives during the warm part of the summer, while in the 

 fall there appears a distinctly different type. By subjecting the 

 efggs which would naturally give rise to the summer generation to 

 cold, the fall generation can be secured, or the reverse can be done 

 by warming the eggs of the fall generation. When the eggs of 

 another form called Vanessa urtica are subjected to low tempera- 

 ture they hatch as the variety polaris which lives in Lapland. 

 When the same eggs are subjected to great heat they hatch into 



