TRANSMISSION OF ACQUIRED CHARACTERS. 399 



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 1 perpendicularly 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 

 conferred 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 different in different species, because 

 the conditions of life peculiar to each require special arrangements. 



The same argument also holds with regard to heliotropism. 

 The power of growing towards the light possessed by green shoots 

 cannot be a primitive character of the plant : it must have arisen 

 secondarily. If it were an essential and original character it could 

 not be reversed in certain parts of the plant ; but the roots are 

 negatively heliotropic, for they grow away from the light. There 

 are also shoots, such as the climbing shoots of Ivy, which are simi- 

 larly negatively heliotropic. Whenever the heliotropic power is 

 thus reversed in shoots, the change is of a useful kind. Thus the 

 shoots of the Ivy gain the power of clinging closely to a perpen- 

 dicular wall or to some horizontal plane x . In this case, however, 

 it is only the shoot which is negatively heliotropic, its leaves turn 

 towards the light ; and the same is true of the flower-bearing shoots 

 which do not climb. All these are clearly adaptations and not the 

 results of direct influence. The light only provides the stimulus 

 which calls forth the characteristic reaction from each part of the 

 plant, but the cause of each peculiar reaction lies in the specific 

 nature of the part itself which has not been produced by light, but 

 as we believe by processes of natural selection. If this explanation 



1 Compare Sachs, 'Lectures on the Physiology of Plants,' translated by H. 

 Marshall Ward, p. 710. 



