26 HEREDITY [CH. 



that most cases which at first sight seem to support 

 the theory of the inheritance of acquired characters 

 are equally explicable in the view that both parent 

 and offspring are susceptible to the action of the ex- 

 ternal factor ; what is inherited is not the character 

 acquired, but the innate power of acquiring it. 



But it is always possible that some forms of 

 external conditions may act on both the body-cells 

 and germ-cells concurrently, and produce similar 

 effects in each. For example, it may happen that 

 extremes of temperature produce striking colour- 

 variations in certain butterflies. Weismann has 

 pointed out that, according to his theory, in a 

 developing butterfly the determinants for producing 

 colour not only exist in the germ-cells which will 

 transmit the character to the offspring, but also in 

 the embryonic cells of the body which go to produce 

 the coloured parts of the perfect insect. If extremes 

 of heat or cold cause changes in the colour-deter- 

 minants in the developing wings, so that abnormal 

 colours result, it is possible that the determinants 

 in the germ-cells which transmit the colour-pattern 

 to the next generation will be similarly modified, so 

 that the offspring will show similar abnormalities. 

 This would not be the transmission of an 'acquired 

 character ' in the strict sense of the expression, but 

 the simultaneous modification of body and germ-cells 

 in the same manner. 



