THE UNIQUENESS OF THE INDIVIDUAL 



interpretation has no bearing whatsoever. Flexure Hnes are 

 mysterious, but not mysterious in any way that is particularly 

 discreditable to Darwinism. 



This section may well conclude with a description of an 

 important experiment in which Waddington (1952) has demon- 

 strated the genetical pre-emption of a change originally brought 

 about by environmental means. If fruit flies are subjected to a 

 mild temperature shock shortly after pupation, a certain pro- 

 portion develop without the cross veins that bridge the prin- 

 cipal veins of the wings. Flies of this susceptible fraction were 

 bred from, and their ofl'spring again shocked; the susceptible 

 fraction again bred from, and so on. The proportion of suscept- 

 ible flies steadily increased, as was to be expected; but from 

 the twelfth generation onwards, the cross-veinless condition 

 began to appear in flies which had received no temperature 

 shock at all. Selection has thus, in eff'ect, converted an acquired 

 into an inherited character diff'erence.* 



The gist of the foregoing argument is as follows. Darwinism 

 and Lamarckism may be thought of as competing interpreta- 

 tions of the origin of inherited character difi'erences in metazoan 

 individuals. An examination of these character diff'erences 

 shows that only a certain category, described as Class B, is 

 open to a Lamarckian interpretation at all. But there is, on 

 the one hand, no evidence to suggest that the Lamarckian 

 interpretation is the correct one; and, on the other hand, 

 Darwinism is no less competent to explain the origin of Class B 

 adaptations than the origin of any other. 



3. THE ^STRONG' FORM OF LAMARCKISM 



The weak form of Lamarckism, which we have seen to be 

 unobjectionable, is purely descriptive in intent; it merely 



■^ [Some of Waddington's more recent experiments are reported in 

 Evolution, 10, p. 1, 1956.] 



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