VII.] TRANSMISSION OF ACQUIRED CHARACTERS. 411 



fluence of gravity, continuous through countless generations, 

 had at length conferred upon the root the power of growing in 

 a geotropic direction, how would it be possible to explain the 

 fact that the shoot which has been under precisely the same 

 influence has acquired the power of growing in an exactly 

 opposite direction? The characteristic dilVcrences between 

 root and shoot cannot have appeared until the plant became 

 fixed in the ground, and how can we imagine that the same 

 influence of gravity has since that time directly produced the 

 two antagonistic results of positive and negative geotropism, 

 in two structures, which were originally and essentially similar ? 

 It should also be remembered that it is only the main mot 

 which exhibits true positive geotropism. The lateral roots 

 form angles with the main root, and do not therefore grow 

 towards the earth's centre; and the same is true of the lateral 

 shoots which grow obHquely, and not perpendicularly upwards, 

 like the main shoot. Moreover the angles which the lateral 

 roots make with the main root, and the lateral shoots with the 

 main shoot, are quite different in different species. How is it 

 possible that all these different modes of reaction witnessed in 

 the different parts of plants can be the direct results of one and 

 the same external force? It is quite obvious that these are all 

 cases of adaptation. The main root has not acquired the power 

 of growing perpendicularl}'- downwards under the stimulus of 

 gravity, because this force has acted upon it for numberless 

 generations, but because such a direction for such a part was 

 the most useful to the plant. Hence natural selection has con- 

 ferred upon the root the power of reacting under the stimulus 

 of gravity by growing in a direction parallel to this force. For 

 the main shoot, the opposite reaction was the most useful and 

 has been established by natural selection, while still another 

 reaction has been similarly established for the lateral roots and 

 another for the lateral shoots. 



Each part of a plant has received its special mode of reacting 

 under the stimulus of gravity because it was useful for the 

 whole plant, inasmuch as the position of its different parts 

 relatively to one another and to the soil became thus fixed and 

 regulated. These modes of reaction have become dilVcrent in 

 different species, because the conditions of life peculiar to each 

 require special arrangements. 



