8] MODIFICATION AND DISTRIBUTIONS OF CROPS 253 



L. H. Bailey et al. Manual of Cultivated Plants, revised edition (Mac- 

 millan, New York, pp. 1-1116, 1949). 



For Details about Forest Products : 

 R. W. ScHERY. Plants for Man (Prentice-Hall, New York, pp. viii -f 564, 



1952)- 

 R. ZoN & W. N. Sparhawk. Forest Resources of the World (McGraw- 

 Hill, New York & London, 2 vols, pp. xiv + 1-493 ^^^ '^ii + 495" 



997. 1923)- 

 S. J. Record & R. W. Hess, Timbers of the New World (Yale University 



Press, New Haven, Conn., pp. xv -\- 640, 1943). 

 S. Haden-Guest, J. K. Wright, & E. M. Teclaff (ed.). A World 



Geography of Forest Resources (Ronald Press, New York, pp. 



xviii + 736, 1956). 



Weeds and their Control : 



L. J. King. World Encyclopaedia of Weeds and their Control (Leonard 



Hill, London, in Press). 

 W. C. MuENSCHER. Weeds, second edition (Macmillan, New York, pp. 



xvi -f 560, 1955) ; mainly northern United States and southern 



Canada. 

 W, W. RoBBiNS, A. S. Crafts, & R. N. Raynor. Weed Control, second 



edition (McGraw-Hill, New York etc., pp. xi + 503, 1952). 



Plant Diseases : 



Sir E. J. Butler & S. G. Jones. Plant Pathology (Macmillan, London, 



pp. xii + 979, 1949). 

 J. C. Walker. Plant Pathology, second edition (McGraw-Hill, New 



York etc., pp. xi + 707, 1957). 

 J. S. BoYCE. Forest Pathology, second edition (McGraw-Hill, New York 



etc., pp. xi + 550, 1948). 



In conclusion it may be interesting to speculate as to the main centres 

 of origin of our crop plants. It has long been thought that the cultivation 

 of plants by Man began independently in each of the three main centres 

 of ancient civilization, viz., the eastern Mediterranean, the Oriental of 

 southeastern Asia, and the American of southwestern North America 

 and at least the northwestern portions of South America. Nowadays 

 there is a tendency to extend or multiply these to include other areas of 

 supposed early cultivation. Vavilov {op. cit.) visualized eight major 

 centres, which he found to be those of greatest diversity of cultivated 

 plants, as follows : (i) Chinese, (ii) Indian (with a suggested separate 

 Indo-Malayan area), (iii) central Asiatic, (iv) near-Eastern, (v) Mediter- 

 ranean, (vi) Abyssinian, (vii) south Mexican and Central American, and 



