xb PREFACE 



and the possibility of the control of development by appropri- 

 ately altering those conditions. Gi-owth phenomena show them- 

 selves, indeed, particularly susceptible to this control, and are 

 consequently especially valuable for experimental study. 



In the preparation of the Second Part, I have been put again 

 under heavy obligations to my friend and colleague, Dr. G. H. 

 Parker, who has read most of the manuscript and made 

 important suggestions. I am also indebted to Dr. H. E. Saw- 

 yer for reading Chapter XI in the manuscript, and to my wife 

 for much painstaking work on the manuscript and proofs and 

 for compiling the index. 



C. B. D. 



Cambridge, Mass., Dec. 11, 1898. 



