266 



EVOLUTION AND GENETICS 



resist the same disease, to digest the same food, or to respond to 

 the same stimulant we have common examples of physiological 



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Fig. 154. — Radiograph showing Polydactyly in a child's hand. (From 

 Guyer's Being Well Born, copyright 1916-1927, after Dr. W. B. Helm; 

 used by special permission of the publishers, The Bobbs-Merrill Company.) 



variation. Everyone is familiar with the difference in individual 

 response to caffeine. Some people cannot drink a demi-tasse in 

 the evening without enduring a sleepless night, while others drink 



