46 EVOLUTION OF THE HUMAN EYE 



be compared with the phototropic reactions met with in 

 plant life, in hydra and in some small worms. 



It would be a simple way of explaining the difference in 

 the mode of stimulation of the rods and cones by light to 

 regard that of the former as a photo-chemical reaction, and 

 that of the latter as a phototropic reaction. Such an expla- 

 nation would serve not only to account for tin* differences 

 observed between rod and cone vision as regards light-sense, 

 but also as regards form-sense, as I shall attempt to show in 

 the consideration of that part of my subject. It is true that 

 the contraction which has been observed in the cones resulted 

 from a strong stimulus of some duration, but so far it is the 

 only change of which we have any evidence as occurring 

 in the cones apart from the rods; it is well, therefore, to 

 see how far such a change will help in explaining visual 

 phenomena. 



The degree of illumination required to excite a photo- 

 tropic reaction would be greater than that required to 

 produce a photo-chemical reaction, so that we should expect 

 rod vision, if it acted by the latter, to come into action with 

 lower degrees of illumination than cone vision, if it acted 

 by the former. Rod vision would fail in high degrees of 

 illumination, due to complete bleaching of the visual purple; 

 while cone vision, being capable of some degree of adaptation 

 due to a varying degree of contraction of the cone-myoid 

 in response to the amount of stimulating light, might con- 

 tinue active after the retina has been completely bleached. 



As already mentioned, the mammals with the* smallest 

 eyes, who only expose themselves to very low degrees of 

 illumination, have only rods in their retinae. Those mammals 

 who, though mainly nocturnal in their habits, enter rather 

 more boldly into the light of day, such as rats, mice and 

 rabbits, are found to have a few cones. Indeed, the amount 



