The biota have effected profound changes on the environment of the surface of the earth. At thesame time, that environment has imposed constraints on the biota, so that life and theenvironment may be considered as two parts of a coupled system. Unfortunately, the system istoo complex and too little known for us to model it adequately. To investigate the propertieswhich this close-coupling might confer on the system, we chose to develop a model of animaginary planet having a very simple biosphere. It consisted of just two species of daisy ofdifferent colours and was first described by Lovelock (1982). The growth rate of the daisiesdepends on only one environmental variable, temperature, which the daisies in turn modifybecause they absorb different amounts of radiation. Regardless of the details of the interaction, the effect of the daisies is to stabilize the temperature. The result arises because of the peakedshape of the growth-temperature curve and is independent of the mechanics by which the biotaare assumed to modify the temperature. We sketch out the elements of a biological feedbacksystem which might help regulate the temperature of the earth.