Vision

Human vision likethat of other primates（灵长目）has evolved in an arboreal（丛林）environment.______(46)

In the course ofevolution,members of the primate line have acquired large eyes while the nosehas shrunk.______ (47) Of mammals（哺乳动物）only humans and some

primates enjoy colorvision.______(48)Horses live in a single-color world.

Light visible tohuman eyes,however,occupies only a very narrow band in the wholeelectromagnetic spectrum（光谱）.Ultraviolet rays（紫外线）are invisible tohumans

though ants andhoneybees are sensitive to them.______(49)The world would look terriblydifferent if human eyes were、 sensitive to infrared radiation（红外线）.Then

instead of thedarkness of night,we would be able to move easily in a strange shadowlessworld.______(50)

But human eyes excelin other ways. They are in fact remarkably discerning in color gradation. Thecolor sensitivity of normal human vision is rarely surpassed even by

complicatedtechnical devices._________（47）

A.Humans have nodirect perception of infrared rays.

B.In the densecomplex world of a tropical forest,it is more important to see well than todevelop an acute sense of smell.

C.That gives the eyea wilder view.

D.The red flag isblack to the bull.

E.There aredifferent mammals in tropical forest.