THE EYE IN EVOLUTION 



may affect the component parts of sessile organisms (phototropism) 

 or may be expressed in translatory movements by motile organisms 

 (phototaxes). In the third place, light acting directly or indirectly is 

 the most potent stimulus for altering the pigmentary distribution in 

 both plants and animals — an understandable reaction since pigment 

 has been evolved specifically for the absorption of light, either to 

 utiHze its energy or as a protection against its excess. 



All these activities have become more complex as evolution has 

 proceeded. The most primitive required no specific organization ; 

 the more complex called for the acquisition of one or more receptor 

 organs, which in their most elementary stages need appreciate only 

 changes in the intensity of the light, but in their more advanced forms 

 must analyse the direction of its incidence and its spatial distribution. 

 Initially, in some unicellular organisms a diffuse reactivity sufficed ; 

 but as multicellular organisms developed, the stimulus must needs be 

 transported to the effector organs, either chemically by hormones or 

 by nervous activity. In this way the effects of light upon metabolism, 

 orientation and pigmentation became correlated through primitive 

 nerve-nets and then became integrated in the ganglia of the central 

 nervous system ; and eventually, when the nervous pathways from the 

 eyes were projected into a head-ganglion and ultimately into the fore- 

 brain, the highly complex faculties of vision and apperception evolved. 



