A COMMENTARY ON LAMARCKISM 



failed to spread to non-industrial areas because their advantage 

 in toughness is more than outweighed by their greater con- 

 spicuousness. 



(iv) Sladdeii's experiments on the inherita?ice of altered food 

 habits in stick-insects. These are perhaps the best of the experi- 

 ments that purport to demonstrate Lamarckian inheritance; 

 all sorts of genetical complications are avoided by the fact that 

 reproduction in the subject species is parthenogenetic. Sladden 

 (1934, 1935; Sladden and Hewer, 1938) studied the inheritance 

 of the acquired ability of stick-insects of the species Dixippus 

 morosus to subsist upon ivy instead of their normal diet, privet. 

 The life cycle in this species is 9-10 months long, and somewhat 

 more than 500 eggs are produced by each individual. 



The insects feed at night, and must feed every night. The 

 alternative foods were offered for consumption in such a way 

 as to provide a reliable measure of their degree of accept- 

 ability. In the ''presentation test\ ivy and privet were offered 

 on alternate nights, the privet being necessary to keep the 

 insects alive if they failed to eat sufficient ivy. Acceptability 

 was measured by the number of trials necessary before the 

 final acceptance of ivy. In the 'preference tesf ivy and privet 

 were thrice offered simultaneously: the result was scored as 

 'ivy preference** if ivy was chosen on all three occasions, and so 

 for privet; otherwise the result was held to be unindicative. 



After six generations there was a clear-cut increase in the 

 } acceptability of ivy, but it is noteworthy that a high proportion 

 of this increase occurred in the first generation after the first 

 presentation of ivy. There is also an echo of the difficulties that 

 bedevil the interpretation of McDougalPs work, in that the 

 control insects, reared upon privet throughout, also showed a 

 distinct increase in preference for ivy. Sladden''s own inter- 

 pretation of this finding, which turns upon seasonal changes of 

 food preference, is unconvincing. 



These are good experiments: the facts are well set out and 



G 97 



