PREFACE vn 



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groups. The former is physiologically the same in all 

 proteins. There probably are chemical differences in the 

 primary groups of varied proteins, and it is possible that 

 fine physiological differences may be detected by more 

 careful study, but the primary group is the ring about 

 which all proteins are built, or at least, it contains this 

 ring; and just as innumerable compounds may be built 

 with the benzol ring as a nucleus, so all proteins are con- 

 structed about a common centre. The secondary groups 

 are not identical in any two kinds of proteins. There may 

 be one or more common to the two, but in some respects 

 there are differences. 



The cell is not the unit of life; life is molecular. The cell 

 is not only made up of protein molecules, but its form and 

 function are determined by the chemical structure of its 

 constituent molecules. The lines along which the spore, 

 seed, or ovum develops are determined by the chemical 

 structure of its proteins. Growth in other directions is 

 impossible, and this accounts for stability in reproduction. 

 However, gradual changes in the chemical structure of 

 living proteins occur, and in these lies the basis of organic 

 evolution. 



The basic points of our theory, as stated above, will be 

 in evidence throughout this volume. The experimental 

 work devoted to the development of this theory could not 

 have been done without the aid of able assistants who 

 have devoted much time to it, and all without adequate 

 reward. Besides those associated w r ith me in the prepa- 

 ration of this volume, special mention is due Drs. Sybil 

 May Wheeler and Mary Leach. The former gave eight 

 years and the latter two years of most devoted and skilful 

 service to the elaboration of the problems discussed here. 



