A COMMENTARY ON LAMARCKISM 



describes a biological history of the origin of certain inherited 

 character differences. The '"strong"' form of Lamarckism is the 

 weak form strengthened (in the sense of being made more 

 particular) by the categorical statement that the origin of 

 acquired character differences is accompanied by the origin of 

 adaptive genetical differences in the individuals in which they 

 are induced. By an ''adaptive'' genetical change is only meant 

 such a change as will reproduce the character difference 

 originally elicited by the environment: the enlargement of a 

 particular muscle by habitual use must be accompanied by 

 such a genetical change as will entail the enlargement of that 

 muscle. The qualification ''adaptive'' is therefore of central 

 importance. That differences of environment or of ""treatment"* 

 may bring about genetical transformations has not been in 

 dispute since Muller"'s demonstration, now a quarter of a 

 centur}^ old, of the mutagenic action of X-rays, and the number 

 of physical and chemical treatments known to increase muta- 

 tion rate is being steadily added to. 



Lamarckists do not suppose that adaptive genetical changes 

 are completed within a single generation; the ''strong"' form of 

 Lamarckism may therefore be expressed in such a way as to 

 take this qualification into account: 



The repeated induction of character-differences within the 

 lifetimes of individuals of successive generations is accompanied 

 by a genetic change in each individual, the change being such as 

 eventually to reproduce the character-difference elicited by en- 

 vironmental stimuli even when those stimuli are withheld. 



It will be clear that the only acceptable evidence for Lam- 

 arckian inheritance in the strong sense will be that in which 

 the possibility of selection is scrupulously eliminated. This 

 section will begin by a consideration of four examples of 

 supposedly Lamarckian inheritance in higher animals, choosing 

 the experiments on the grounds that they have been conducted 

 with care and reported in sufficient detail to make an appraisal 



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