GENETIC CHANGE AND EVOLUTIONARY CHANGE 425 



mals could be born with them. Eventually genetic changes may occur 

 which mimic the adaptive characteristics each generation has been acquir- 

 ing for itself. Since it will be desirable for the organism to be relieved of 

 the necessity of acquiring these characteristics in every generation, natural 

 se.'ection may be expected to favor individuals which are born with them. 

 So in time hereditary characteristics may replace the individually acquired 

 ones. "Characters individually acquired by members of a group of organ- 

 isms may eventually, under the influence of selection, be reenforced or re- 

 placed by similar hereditary characters" (Simpson, 1953b). 



As commonly conceived the theory is that the organism lives in the en- 

 vironment to which it can accommodate and "waits" for appropriate chance 

 mutations to occur. An experimental instance of the Baldwin effect operat- 

 ing with chance mutations seems to have been afforded by Waddington's 

 experiment on the bithorax phenotype (1956). The experimenter "re- 

 quired" the strain to have this phenotype. He enforced the requirement 

 by subjecting the eggs to ether vapor. Eventually, apparently in two in- 

 stances as noted above, what seem to have been dominant mutations ap- 

 peared giving rise to the required phenotype. 



But in addition it is conceivable that the genetic change involved in the 

 Baldwin effect may be the genetic assimilation of an acquired character 

 we have been discussing. If so, we note that the environment is thought to 

 be concerned with the instigation of the genetic change (through canaliza- 

 tion of development) as well as with the final fate of the change. Genetic 

 assimilation of acquired characters forms a possible additional means (sup- 

 plementing the mutation process) of supplying grist to the mill of natural 

 selection. 



References and Suggested Readings 



Bonner. D. M. "Genes as determiners of cellular biochemistry," Science, 108 

 (1948), 735-739. 



Colbert, E. H. "Siwalik mammals in the American Museum of Natural His- 

 tory," Transactions of the American Pliilosopliical Society, new series, 26 

 (1935), 1-401. 



Dobzhansky. Th. Genetics and tJie Origin of Species, 3rd ed. New York: Colum- 

 bia University Press, 1951. 



Goldschmidt, R. The Material Basis of Evolution. New Haven: Yale University 

 Press, 1940. 



Huxley, J. S. Problems of Relative Growth. New York: Dial Press, 1932. 



Jepsen, G. L. "Selection, 'orthogenesis," and the fossil record," Proceedings, 

 American Philosophical Society . 93 ( 1949), 479-500. 



Mayr, E. "Speciation and systematics." In G. L. Jepsen, E. Mayr, and G. G. 



