444 THE GERM-PLASM 



the characteristics of the part of the body which they represent, 

 in some inexpHcable way. It is not yet possible, for instance, to 

 give an exact account of the changes in the group of determinants 

 for the legs which led to the sudden production of a bow-legged 

 ' otter '-sheep, or of the modifications of certain determinants in 

 the rudiments of the leaves which caused the formation of the 

 notches characteristic of a certain variety of birch. 



b. The Tt'ansjnission of these Variations 



The transmission of 'sports' in plants has hitherto been a 

 very obscure problem. Seed-variations are often propagated by 

 means of seeds, but cannot always be thus reproduced, or at 

 least only in individual cases. Bud-variations can usually only 

 be propagated by cuttings or grafting, but some instances are 

 known in which they have also been reproduced by seeds, 

 though only in a certain percentage of cases. The origin of 

 these irregularities was unknown, and no previous theory of 

 heredity could offer a reason for them ; all that could be said 

 was, that in these instances transmission was very capricious. 

 The theory of the germ-plasm, however, oifers a very simple 

 explanation of them. We will first consider certain facts 

 recorded by Darwin, which are very valuable in respect of this 

 theory. 



Darwin states that ' when a new peculiarity first appears, 

 we can never predict whether it will be inherited.' * If both 

 parents exhibit the variation, ' the probability is strong that it 

 will be transmitted to at least some of their offspring.' Bud- 

 variations are propagated to a much smaller extent than seed- 

 variations, but the power of transmission often appears to be 

 very capricious, inasmuch as one and the same modification 

 on a single plant is transmitted by seed in one case and not in 

 another. Thus vain attempts were for a long time made to prop- 

 agate the weeping ash by seeds. Over twenty thousand seeds 

 developed into trees of the ordinary form, but offspring with 

 hanging boughs were finally raised from the seeds of another 

 specimen of this variety. The same ash-tree, however, did not 

 transmit this character to all its offspring, but only to a certain 



*' The Variation of Animals and Plants under Domestication,' Vol. I., 

 London, 1888, p. 460. 



