^;^8 THE GERM-PLASM 



from Darwin's • Animals and Plants under Domestication " * will 

 best show how much our insight has since then advanced : — 

 • In purelv-bred races, occasional reversion to long lost characters 

 of the ancestors often occurs without our being able to assign 

 any proximate cause." 



6. Reversion in Asexual Reproduction 

 (a) Reversion in the Process of Gemmation 



As alreadv stated, reversion does not entirely depend on am- 

 phimixis, and ma\' in fact, also occur apart from the crossing 

 of two individuals. The bud-variations of plants form a well 

 known instance of this kind. 



For many years I possessed in my garden a maple (Acer 

 negundo^ with variegated leaves which were almost entirely 

 white, and one branch of this tree bore ordinar\- green leaves, 

 flowers, and seeds. Owing to the greater amount of chlorophyll, 

 this branch grew and bore flowers and seeds far more luxuriantly 

 than the main branch from which it arose. If we look upon the 

 offshoots of a tree as persons, this would be an instance of a 

 person of the plant-stock produced asexually. which reverted 

 to the ancestral form. 



We must go back to the origin of variegated species in 

 order to lind a theoretical explanation of this phenomenon. 

 Like most similar varieties, indeed, of our trees and shrubs, 

 this form must have arisen by bud-variation ; in other words, 

 a normal maple, from certain unknown causes, gave rise to a 

 branch bearing variegated leaves. The cause of this modification, 

 traced to the idioplasm, must have been due to the determinants 

 of the leaves and other green parts of the shoot becoming 

 modified in such a way as to result in the production of organs 

 deficient in chlorophyll. If. however, only the majority, and not 

 all the ids in the apical cells of the first variegated shoot became 

 modified in this manner, a reversion of the variegated variety 

 to the green ancestral species would become possible. 



Another assumption, however, which cannot vet be proved to 

 be tnie. is required in order to account for the appearance of a 

 green branch upon a variegated tree : we must suppose that 

 even in ordinary cell and nuclear divisions, the division of the 

 idioplasm may take place in an irregular manner, so that all the 



* Vol. IL. p. 25. 



