RECAPITULATION 233 



T. H. Morgan l states that a mutation of gametic 

 origin may affect any stage in the development of 

 the individual. This may be true when there are 

 already distinct stages in the life history. The 

 more important question is whether distinct stages 

 can be caused by mutation. It is true that in 

 heterozygous individuals characters may develop 

 more fully in the adult stage than in the young. But 

 when we find different stages evidently adapted to 

 different modes of life, it is impossible to explain them 

 by mutations affecting different stages of life. In 

 such cases as the larval stages of Insects we find that 

 the larvae have become adapted to new habits while 

 the adults have remained unchanged, or have evolved 

 quite independent adaptations. For example, the 

 adults in the chief orders of Insects have the typical 

 three pairs of legs, while the maggots or grubs of 

 the Diptera or Hymenoptera have no legs at all, 

 the caterpillars of Lepidoptera have evolved pseudo- 

 legs on the abdomen, and the larvae of Coleoptera 

 have the ordinary legs and no more. This is the 

 reverse of recapitulation : in the case of legless 

 maggots, and caterpillars with pro-legs, the adult is 

 more similar to the ancestor than the larva. But 

 the same principle holds, that where functions and 

 habits are different, there organs are different. No 

 mutationist has yet produced by breeding experi- 

 ments a caterpillar without the three pairs of thoracic 

 legs and yet developing into a moth that had the 

 normal three pairs. Morgan, with all his mutations 

 of the adult Drosophila, says nothing of larval 

 mutants possessing legs. The only rational con- 

 clusion is that legless larvae have lost the legs through 



1 A Critique of the Theory of Evolution, p. 18. 



