328 NATURAL CROSS-FERTILIZATION IN INDIA. 



its difficulty, might lead to loss of vigour and it may well be an ad- 

 vantage to sacrifice absolute uniformity to yield. How far a crop 

 in wliicli, say, from 5 to 10 per cent, of natural crossing takes place, 

 can be propagated successfully without loss of vigour from a single 

 fixed plant obtained either by selection or as the result of hybridiz- 

 ation is a matter which, at the present time, calls for long-continued 

 and accurate experiment. Can the method of single plant selection 

 be applied to crops like the Brassica oil seeds or to cotton and 

 natural crossing be avoided without loss of vigour ? If so, the 

 study of pure lines can be applied to such crops. The whole matter 

 turns on the possible ill-elTects of in-and-in breeding in such crops 

 and on the practical difficulties in the prevention of vicinism. 



The extent to which natural crossing takes place obviously 

 affects the production of new varieties by hybridization and the 

 study of the inheritance of characters. Even when spontaneous 

 crossing is rare its possible effects on the various hybrid generations 

 must be expected. In crops where crossing frequently takes place 

 the difficulties are greatly increased if accurate and trustworthy 

 results are to be obtained. Some years must necessarily elapse 

 before pure line cultures can be obtained and the splitting forms 

 eliminated. After the crosses have been made all subsequent 

 work will have to be done with bagged plants so as to exclude the 

 effect of natural crossing. 



2. The Introduction of Exotics. 



Natural crossing has obviously an important bearing on the 

 introduction of exotics. If crossing takes place in a crop, it is likely 

 that any consignment of seed from another country will not be 

 uniform but will contain hybrids. The seed must first be grown, 

 if possible apart from similar local crops, and the constitution of 

 the crop determined. Methods of selection may in consequence 

 have to be applied, and some years may elapse before any of the 

 seed can be distributed to cultivators. The recent failure of the 

 introduction of Egyptian cotton into Texas' appears to have 



1 Cook, Bull, 156, Bureau of Plant Industry, U. 8, Bept. of Ap:, 1909, 



