xii PREFACE 



on the staff have given every assistance. Since my 

 transfer to the new Department of Agriculture the work 

 has been continued, the Mendelian laboratory projected by 

 the Society has been established by the Egyptian Govern- 

 ment, and a pure-strain system of seed-supply is in process 

 of adoption. 



Many of the records utilised would not have been 

 obtainable without the steady co-operation after 1909 of 

 my assistant, Mr. Francis S. Hoi ton, to whom I am 

 especially indebted. The members of the Cairo Scientific 

 Society, and of the Botany School at Cambridge, have 

 displayed an encouraging interest in the work. 



To specify individual assistance among so many would 

 be invidious, but on looking back over the train of ideas 

 involved, I find that the most illuminating of these have 

 come from the late Mr. J. R. Gibson, English Com- 

 missioner for the State Domains, from Mr. F. F. Blackman, 

 F.R.S., Reader in Botany in the University of Cambridge, 

 and from Mr. J. A. Todd, Professor of Economics at the 

 Khedivial School of Law, Cairo, while the data on the 

 hydrology of sub-soil water garnered by Mr. H. T. Ferrar, 

 of the local Survey Department, have provided a founda- 

 tion for discussions on this most important topic. 



I should like also to acknowledge the pains which 

 Prof. R. A. Gregory, the Editor of this series, has taken, 

 under all the disadvantages involved by my absence in 

 Egypt. 



The greater part of the work would not have been 

 executed had it not been for the assistance of my wife, 

 who has also helped me in preparing the present volume, 

 and in working up the records on which it is based. 



W. LAWRENCE BALLS. 



GEZIRA HOUSE, 

 CAIRO. 



