Human vision like that of other primates has evolved in an arboreal
environment. In the dense complex world of a tropical forest, it is more
important to see as well than to develop an acute sense of smell. In the 41. ______
course of evolution members of the primate line has acquired large 42. ______
eyes while the snout has shrunk to give the eye an unimpeding view. 43. ______
Of mammals only humans and some primates enjoy color vision. The red
flag is black to the bull. Horses live in a monochrome world. Light visible to
human eyes, therefore, occupies only a very narrow band in the whole 44. ______
electromagnetic spectrum. Ultraviolet rays are invisible to humans though
ants and honeybees are insensitive to them. Humans have no direct perception 45. ______
of infrared rays like the rattlesnake that has receptors tuned into 46. ______
wavelengths longer than 0.7 micron. The world would look eerie different 47. ______
if human eyes are sensitive to infrared radiation. Then instead of the darkness 48. ______
of night, we would be able to move easily in a strange shadowless world
where objects glowed with varying degrees of intensity. But human eyes excel
in other way. They are in fact remarkably discerning in color gradation. 49. ______
The color sensitivity of normal human vision is rarely surpassed even 50. ______
sophisticated technical devices.