404 Evolution and Adaptation 



leaflets of Trifolium pratense oscillate in periods of two to four 

 hours, but do so only in the dark ; in the light the leaflets 

 assume a rigid position. There is nothing in the process to 

 suggest that the movement is useful to the plant, and yet it 

 appears to be as definite as are those cases in which the 

 response is of vital importance. Had these movements been 

 of use, their origin would, no doubt, have been explained be- 

 cause of their usefulness, and the conclusion would have been 

 wrong. 



The leaves of the Mimosa respond, when touched, and it 

 cannot be supposed that this is of any great advantage to the 

 plant. The sleep movements of many plants are also due to 

 the effect of light. In some cases the leaflets are brought to- 

 gether with their upper surfaces in contact with one another ; 

 in other cases the lower surfaces are brought together. Dar- 

 win supposed that these sleep movements served to protect 

 the leaves from a too rapid loss of heat through radiation, but 

 it has been pointed out that tropical plants exhibit the same 

 responses. We have here another admirable instance of the 

 danger of concluding that because we can imagine an advan- 

 tage of a certain change, that the change has, therefore, been 

 acquired because of the advantage. In the Mimosa not only 

 do the leaflets close together, but the whole leaf drops down 

 if the stimulus is strong. Other plants also show in a less 

 degree the same movements, Robinia and Oxalis for instance, 

 and certainly in these latter the result does not appear to be 

 of any advantage to the plants. 



The preceding account of some of the tropisms in animals 

 and plants will serve to give an idea of how certain move- 

 ments are direct responses to the environment. Some of 

 the reactions appear to be necessary for the life of the 

 individual, others seem to be of less importance, and a few 

 of no use at all. Yet the latter appear to be as definite 

 and well-marked as are the useful responses. I think the 

 conviction will impress itself on any one who examines 



