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 a 
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 
