PANGENESIS. 173 



Variability. — Changed conditions may lead to 

 irregularity in the number of gemmules derived from 

 various parts of the body; deficiency in number might 

 cause variation in any part by leaving some of the 

 cells free to unite with allied gemmules. 



The direct action of surroundings, or the effect of 

 use or disuse on a part, may cause corresponding 

 modifications of the gemmules, and through these of 

 the part in the succeeding generation. " A more per- 

 plexing problem can hardly be proposed," and yet it 

 receives an explanation on this hypothesis. Such 

 causes must, as a rule, act during many generations 

 before the modification reappears in the offspring. . 

 This may be due to the unaltered gemmules derived 

 from earlier generations, and their gradual replace- 

 ment by the increasing number of altered gemmules. 

 Variation in plants is much more frequent in 

 sexually produced than it is in asexually produced 

 individuals. This may be due to the absence in the 

 latter of that great cause of variability, changes in 

 the reproductive organs under altered conditions. 

 Furthermore, the former alone pass through the 

 earlier phases of development, when structure is most 

 plastic and yields most readily to the causes in- 

 ducing variability. 



The stability of hybrids and of many varieties 

 when propagated by buds, as compared with their 

 reversion to the parent form when propagated by 

 seed, remains inexplicable. 



Hence variability is explained as due (1) to the 



