152 WORLD-POWER AND EVOLUTION 



remembered that in a former chapter we saw that such extremes 

 appear to have been the most notable feature of the climate of 

 the Glacial Period. In earlier times during the Permian glacia- 

 tion they appear to have given rise to the habit of metamorphosis 

 which the nymphae of the butterflies illustrate. Moreover, such 

 extremes, though less severe than in the Glacial Period, are one 

 of the most marked characteristics of the climate of the United 

 States. Cold waves alternating with hot waves occur in this 

 country not only in winter but in summer when the eggs of crea- 

 tures like the butterfly are hatching. 



Another set of experiments on insects has been conducted by 

 Tower, chiefly in Tucson, Arizona, at the Desert Laboratory of 

 the Carnegie Institution of Washington. To that dry region he 

 brought beetles from Mexico, Illinois, and other less arid regions. 

 In this way he obtained many mutants, some of which bred true, 

 and thus perpetuated their variations. In many cases the new 

 environment selected certain types for preservation and destroyed 

 others, so that the inherent character of the insects was changed. 

 These changes were evident not only in the color and pattern of 

 the insects but in other respects also: For instance, one type of 

 beetle was brought from Chicago to the Desert Laboratory at 

 Tucson. During the first winter many insects died because they 

 were not able to stand the drying effect of the arid climate. 

 Those that survived were bred in Tucson for some generation's 

 and then a part were taken back to Chicago. There they proved 

 quite unable to stand the cold winters because they could not 

 give up their moisture fast enough. In Arizona they had been 

 forced to hold much moisture, but in Chicago this caused them 

 to freeze so that all perished. In still other cases Tower's beetles 

 showed marked mutations which were passed on to later genera- 

 tions. In all cases he found that whatever changes take place 

 occur quickly. A given species is subjected to climatic extremes 

 to which it is not accustomed. In the next generation mutants 

 arise; these start new lines; those thajb are not adapted to the 



