BRITISH ASSOCIATION : ZOOLOGICAL ADDRESS. 355 



changes in the adult individuals reared from them. To prove that 

 the deviations were not inherent in the germ-plasm of the parents, 

 the latter were kept under normal conditions during the periods of 

 development and fertilization of the last two batches of eggs ; the 

 larvae and pupae reared from these eggs were similarly subjected to 

 normal conditions, and gave rise to sixty-one unmodified decem- 

 lineata,- which, when bred together, came true to type for three 

 generations. The decemlineata forms produced under experimental 

 conditions also came true to type when bred together. Of the 

 pallida forms produced by experimental conditions all but two males 

 were killed by a bacterial disease. These two were crossed with 

 normal decemlineata females, and the result was a typical Mendelian 

 segregation, as shown by the following table : — 



2 $ pallida x 2 $ decemlineata 



5 <? x 3 ? 



pallida 



Hybrids, decemlineata characters dominant 



decemlineata and hybrids 

 16 $. 14 ? 



pallida pallida decemlineata and hybrids 



18 <?, 23 ? 8 <?,7 ? 26 <?, 28 ? 



This is a much more detailed experiment than those of Standfuss, 

 Merrifield, and Fisher, and it shows that the changes produced by the 

 action of altered conditions on the maturing germ-cells were definite 

 and discontinuous, and therefore of the nature of mutations in 

 De Vries' sense. 



In another experiment Tower reared three generations of decem- 

 lineata to test the purity of his stock. He found that they showed 

 no tendency to produce extreme variations under normal conditions. 

 From this pure stock seven males and seven females were chosen and 

 subjected during the maturation periods of the first two batches 

 of ova to hot and dry conditions. Four hundred and nine eggs 

 were laid, from which sixty-nine adults were reared, constituted as 

 follows : — 



Twenty (12 $ , 8 ? ) . 

 Twenty-three (10 J\ 13 ?) 

 Five (2 c? , 3 2 ) 

 Sixteen (9 $ , 7 ? ) . 



apparently normal decemlineata. 



pallida. 



immaculothorax. 



albida. 



These constituted lot A. 



The same seven pairs of parents subjected during the second half 

 of the reproductive period to normal conditions gave eight hundred 

 and forty eggs, from which were reared one hundred and twenty- 



