28 HEREDITY [CH. 



not necessarily or even usually in the same manner 

 as the body-cells. 



An American zoologist, Tower, describes the pro- 

 duction of mutations by the action of environment in 

 a beetle (Lejrtinotarsa). In nature he found about 

 one such variation among 6000 specimens ; when bred 

 in captivity they were more frequent, but when the 

 full-grown beetles were exposed to extremes of heat, 

 humidity, etc., during the maturation of the eggs, the 

 offspring may include a large proportion (over 80 per 

 cent.) of ' mutations.' These were of several distinct 

 kinds, like those rarely found in nature, and when 

 bred together they are stated to breed true. In this 

 case the abnormal conditions produced no effect on 

 the individuals exposed to them, for they already had 

 their final form, but as their eggs were matured 

 under these conditions the action took effect on the 

 eggs, and mutation resulted among the offspring. 

 When part of the eggs of an individual were matured 

 under abnormal, another part under normal con- 

 ditions, mutation occurred only among the offspring 

 in the first case, all the beetles in the second being 

 normal. It should be noted that as in the experi- 

 ments with butterflies the effect of changed conditions 

 was not specific ; the same conditions may produce 

 more than one kind of mutation in the same batch of 

 eggs, and some eggs were not affected at all. In 

 both cases the abnormal environment seems to upset 



