284 Aiiiinal Life and Intelligence, 



credited with strong colour-antipathies, and insect-eating 

 mammals are probably not defective in the colour- sense. 



It is said that nocturnal animals, such as mice, bats, 

 and hedgehogs, have no retinal cones ; and if the cones 

 are associated with colour-vision, they may not improbably 

 be unable to distinguish colours. Some moles are blind 

 {e.g. the Cape golden mole). But the common European 

 mole, though the eyes are exceedingly minute (2^ of an 

 inch in diameter), has the organ fairly developed, and is 

 even said not to be very short-sighted. It is protected by 

 long hairs when the animal is burrowing, and is only used 

 when it comes to the surface of the ground. 



It is probably in birds that vision reaches its maximum 

 of acuteness. A tame jackdaw will show signs of uneasi- 

 ness when seemingly nothing is visible in the sky. Pre- 

 sently, far up, a mere speck in the blue, a hawk will come 

 within the range of far-sighted human vision. Steadily 

 watch the speck as the hawk soars past, until it ceases to be 

 visible ; the jackdaw will still keep casting his eye anxiously 

 upward for some little time. He may be only watching 

 for the possible reappearance of the hawk. But just as 

 he saw it before man could see it, so probably he still 

 watches it after, to man's sight, it has become invisible. 

 So, too, for nearer minute objects, the swift, as it wheels 

 through the summer air, presumably sees the minute 

 insects which constitute its food. And efery one must 

 have noticed how domestic fowls will pick out from among 

 the sand-grains almost infinitesimal crumbs. 



It is probable that the area of acute vision is much 

 more widely diffused over the retina of birds than it is with 

 us. In any case, the cones are more uniformly and more 

 abundantly distributed over the general retinal surface. 



An exceedingly interesting and important peculiarity 

 in the retina of birds, which they share with some reptiles 

 and fishes, is the development, in the cones, of coloured 

 globules. *' The retinae of many birds, especially of the 

 finch, the pigeon, and the domestic fowl, have been care- 

 fully examined by Dr. Waelchli, who finds that near the 



