FERTILIZATION TX PIMA COTTON. 61 



given on preceding pages that in cotton most of the ovules normally 

 are self-fertilized. This would be expected to result in a degree 

 of " immunity " to the supposedly injurious effects of continued in- 

 breeding, comparable in some degree to that of wheat and other 

 plants in which cross-fertilization rarely takes place under natural 

 conditions. 



The hypothesis of a direct physiological effect of inbreeding is re- 

 jected, however, by recent investigators (#, lo s 23, 45), who attribute 

 the often injurious results to segregation of deleterious recessive fac- 

 tors or to elimination of favorable dominant factors upon which de- 

 pends the vigor of heterosis. To quote Jones (23, p. 95), " whether 

 good or bad results from inbreeding depends solely on the constitu- 

 tion of the organisms before inbreeding is commenced." As stated 

 by East and Jones (15, pp. 139, 140) : "The only injury proceeding 

 from inbreeding comes from the inheritance received. The con- 

 stitution of the individuals resulting from a process of inbreeding 

 depends upon the chance allotment of characters preexisting in the 

 stock before inbreeding was commenced. If undesirable characters 

 are shown after inbreeding, it is only because they already existed 

 in the stock and were able to persist for generations under the pro- 

 tection of more favorable characters which dominated them and kept 

 them from sight/' Substantially the same idea is expressed by 

 Stout (45, p. 124) as follows : " The accumulation of evidence that 

 inbreeding is not necessarily injurious has led to the view that de- 

 creased vegetative and reproductive vigor in inbred stock is due to 

 an inherently weak constitution existing before inbreeding was 

 begun." 



The assumption seems justifiable that in Pima cotton factors con- 

 tributing to low fertility had been eliminated in large part in the 

 ancestiw of the plant which gave rise to the variety or by the 

 subsequent selection and isolation which resulted in development 

 of the present highly uniform commercial stock. The fertility of 

 the latter maj' be regarded, therefore, as due to segregation rather 

 than to heterosis. 



SUMMARY. 



The three principal types of cotton grown in the United States, 

 upland, sea island, and Egyptian, although very different in their 

 botanical characters, intercross readily, giving rise to hybrids which 

 are extremely fertile in the first generation. 



When any two of these types or any two varieties of the same type 

 of which the hybrid offspring is easily distinguishable are grown in 

 close proximity in a locality where pollinating insects are abundant, 

 the proportion of vicinists (natural hybrids) in populations grown 

 from the resulting seed seldom exceeds 20 per cent and is often much 

 lower. Upland cotton usually produces more vicinists than Egyptian 

 cotton when each type is equally exposed to cross-pollination by the 

 other. 



From the point of view of maintaining varietal purity, a very 

 small proportion of vicinism must be regarded as a menace. A few 

 accidental hybrids, unless they are promptly eliminated, will eventu- 

 ally contaminate the stock. 



The percentage of recognizable vicinists does not indicate the total 

 extent of cross-fertilization occurring under natural conditions, many 



