J40 LIFE : OUTLINES OF GENERAL BIOLOGY 



many other insects can certainly distinguish and remember colours, 

 and that the colours of flowers are of real significance to their 

 insect visitors; though the insect's range of colour sensations is not 

 the same as oius. All the vertebrates appear to have some power of 

 seeing colours, yet it has proved very difficult or impossible to 

 teach even the intelligent dog to associate any meaning with a 

 given colour, so that we must admit at least that colour-sensations 

 play only a small part in the dog's mental life. 



The axes of the eyes of a man, or of a cat, or of an owl, are almost 

 parallel, and the two visual fields overlap very greatly; nearly 

 everything that is seen is seen with both eyes. But many mammals 

 and most birds have their eyes set on the sides of the head, the 

 axes widely divergent, and only in a narrow angle directly in front 

 of the head (in some birds, directly behind the head also) can objects 

 be seen with both eyes simultaneously. They have lost something 

 in accuracy of vision, especially in appreciation of distance, but they 

 gain by being able to keep constant watch on all sides. It is interesting 

 that it is typically the hunted animals, such as the antelope and the 

 hare, which are best protected against stealthy attack; while the 

 hunters, such as the carnivores and the birds of prey, can focus both 

 eyes upon the quarry. But all animals, even those with eyes farthest 

 apart, and even those which, like the chameleon and the flat- 

 fishes, can move their eyes quite independently of each other, 

 appreciate the advantages of binocular vision, and will bring both 

 eyes to bear on any object that attracts their attention. 



THE EVOLUTION OF SIGHT.— When one thinks of the human 

 eye or the eagle's eye, and of the vision that both enjoy, it is difficult 

 at first to silence the feeling that these structures and functions are 

 much too wonderful to have been evolved. But this is a fallacious 

 impression due to unacquaintance with the graduated series of 

 stages that lead up to the end-results. In regard to human inventions, 

 the final apj)roximations to perfection would often seem magical, 

 if one did not know something about the long series of antecedent 

 tentatives. So is it with what may be called Nature's many inven- 

 tions. The most evolved eyes are miracles of adaptiveness, but the 

 first light-sense organs are mere pigment spots; and the stages are 

 many and gradual that lead from these to the compound eyes of 

 the butterfly, along one line of evolution, to the single-lens eyes of 

 cuttlefishes on another, and to the eyes of backboned animals on a 

 third. 



Two other general points should be noted — (i) that the stages 

 in the evolution of the optical instrument must be correlated with 

 the gradual inii>rovenients in the brain to which it sends tidings; 

 and (2) that the eye had many functions before it became an image- 

 forming or a picture-making organ. Just as the ear was for millions 



