THE ORIGIN OF LIFE 79 



instance, the want of colouring matters, are mere 

 physical consequences of the absence of the chemical 

 action of light, and may be induced in the lifetime 

 of an individual, just as a plant may be blanched. 

 Though there is scope for animal life in these caverns, 

 nothing has originated to take advantage of it. Only 

 certain common animals of the daylight, better 

 adapted than others for such conditions, have 

 colonised these recesses, and have undergone certain 

 changes in consequence, which no one can reasonably 

 pretend to be more than varietal. The changes are 

 no more specific than those which certain Arctic 

 animals experience on the approach of winter, and 

 which disappear on the return of spring. 



To understand this, let us suppose that at some 

 point in geological time the light of the sun had been 

 gradually extinguished without the entire loss of heat, 

 so that a period of darkness supervened. Under such 

 circumstances many species, both animal and vege 

 table, might perish. Others, like the cave animals, 

 might survive, and adapt themselves to their new 

 circumstances, becoming colourless, losing their now 

 useless eyes, or portions of them, and improving in 

 delicacy of touch. For generations the whole earth 

 might thus be tenanted by animals like those of the 

 caves. But let us suppose that light was again 

 gradually restored, and that these blind animals 

 recovered the powers and properties they had lost, so 

 that the survivors would present the same appearance 



