The Occurrence of Amino Acids in Nature 229 



tides [64, 65] incorporating a pentapeptide sequence which also occurs, repeated 

 twice, in the cychc decapeptide gramicidin S [66] which is produced by a 

 different strain of Bacillus brevis. In all these cases there is variation in chemical 

 structure without much change in biological activity. Thus study of the inter- 

 play of form and function in living organisms, which led up to Darwin's formu- 

 lation of evolution, is now being extended, as chemical study, to the ultramicro- 

 scopic aspects of living organisms. It seems Hkely that this study, which is bound 

 to be extremely laborious, will throw light on many of the problems of systematics 

 which are still obscure, and particularly on the systematics of micro-organisms. 

 It is likely also that, as we begin to understand something about mechanism as 

 well as structure, we shall be able to deduce much that is relevant concerning the 

 problem of the origin of life. 



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