THE UNIQUENESS OF THE INDIVIDUAL 



their truth is not in question. Thorpe (1938, 1939) has however 

 suggested an alternative and rather unexpected interpretation 

 based upon the fact that insects are susceptible of a high degree 

 of olfactory conditioning, in the sense that odours normally 

 distasteful to adults may be acceptable if larvae are exposed to 

 them early enough. For example: the ichneumon fly Nemeritis 

 canescens normally lays its eggs in the Mediterranean flour 

 moth Ephestia kuhniella^ and is strongly attracted by the smell 

 of its normal host. It does not normally lay eggs in, and is not 

 normally attracted by, the smell of the related wax moth 

 Meliphora. But if the ichneumon flies have been deliberately 

 reared in Meliphora, or have been exposed to its larvae shortly 

 after emergence from the cocoon, then they do show a strong 

 attraction to Meliphora. This transformation of host preference 

 was complete in one generation; ten successive generations of 

 rearing on Meliphora did not increase it. 



With this and other evidence of similar import in mind, 

 Thorpe therefore suggests that, in Sladden''s experiments, some 

 olfactory emanation from ivy caused a conditioning which 

 increased its acceptability to stick-insects. Enough might arise 

 from the egg to condition the newly hatched nymphs, particu- 

 larly if their first food is egg-shell. This does not account for 

 the progressive increase in the acceptability of ivy over six 

 generations, but as the greater part of this increase occurred 

 after the first generation, and as there was some increase in 

 tolerance by the controls, it is difficult to regard this as a grave 

 shortcoming of Thorpe''s explanation. 



These four examples inspire one with no confidence in the 

 applicability of the Lamarckian scheme of inheritance to higher 

 animals. Two are susceptible of clear-cut alternative explana- 

 tions; a third, McDougalPs, is open to question on the grounds 

 of empirical fact; and the fourth, that of the inheritance of 

 induced eye defects, is urgently in need of reinvestigation. I am 

 not aware of any experiments that have a greater claim upon 



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